MRI ANAIBI MOLINA(R) (RT) (MR) (CT) T2 DEPHASING SUPERCONDUCTIVE MAGNET FREE INDUCTION DECAY ARTIFACTS MRI REGISTRY REVIEW PHYSICAL PRINCIPLES OF IMAGE FORMATION
Mri Registry Review Physical Principles of Image Formation Developmental Editor: Jimmy Vargas Graphics Project Editor: Deborah DeNicola Marielis Abreu Art/Design Coordinators: Jimmy Vargas Graphics Koko Designs Taylor Graphics Printer Services: Top Drawer Media Solutions Copyright 2016 MRI Registry Review 2016, Advanced Imaging Education. All rights reserved. No part of this information may be reproduced or used in any form in written, electronic, graphic and or mechanical including photocopying, taping, recording and or Web distribution without the written permission from the author Anaibi Molina. Disclaimer: The information of this book does not warrant or guarantee passing the MRI Examination. The student should gather the information necessary that he or she needs and review the material as necessary in order to pass the MRI registry examination. The information of this MRI Registry Review should not be considered as a medical advice. Each facility and medical entity has its own regulations and protocols. Please follow those regulations and protocols. Advanced Imaging Education disclaims any liability for the acts of the technologist, individuals, and or Physicians who receives any information from this educational material. 2
MRI REGISTRY REVIEW ANAIBI MOLINA(R) (RT) (MR) (CT) ALL RIGHT RESERVED. No part of this study guide is to be reproduced by any means of copying, recording and/or transmitted electronically without the permission from the author Anaibi Molina. 3
Physical Principles of Image Formation CONTENT CATEGORY Physical Principles of Image Formation ( PART III ) SECTION 1 ELECTROMAGNETISM SECTION 2 RADIOFREQUENCY SYSTEM SECTION 3 SECTION 4 SECTION 5 SECTION 6 GRADIENT SYSTEM NUCLEAR MAGNETISM TISSUE CHARACTERISTICS SPATIAL LOCALIZATION SECTION 7 SECTION 8 ARTIFACTS QUALITY CONTROL The ARRT Content Specifications for the Magnetic Resonance Imaging Examination are reprinted by permission of the ARRT. The ARRT Content Specifications for the Magnetic Resonance Imaging Examination are copyrighted by the ARRT. Notice: Some changes from the author have been made from the ARRT Content Specifications for the Magnetic Resonance Imaging Examination. 4
Contents Section 1 ELECTROMAGNETISM 6 A. FARADAY'S LAW... 6 B. TYPES OF MAGNETS... 6 C. MAGNETIC FIELD STRENGTH... 9 Magnetic fringe fields Section 2 RADIOFREQUENCY SYSTEM D. COIL CONFIGURATION... 11 General positioning E. TRANSMIT AND RECEIVE COILS... 11 General coil characteristics E.1 TRANSMIT AND RECEIVE BANDWIDTH... 17 F. PULSE PROFILE... 19 G. PHASE ARRAY... 19 Section 3 GRADIENT SYSTEM H. COIL CONFIGURATION... 21 Gradient strength or amplitude I. SLEW RATE... 22 J. RISE TIME... 22 K. DUTY CYCLE... 22 Section 4 NUCLEAR MAGNETISM 23 L. LARMOR EQUATION... 23 M. PRECESSION FREQUENCY... 23 N. GYROMAGNETIC RATIO... 24 O. RESONANCE... 25 P. RF PULSE... 25 Q. EQUILIBRIUM MAGNETIZATION... 25 R. ENERGY STATE TRANSITIONS...25 S.PHASE CONHERENCE...26 T. FREE INDUCTION DECAY... 26 11 20 Section 6 SPATIAL LOCALIZATION 30 X. VECTORS... 30 X, Y AND Z COORDINATES Y. PHYSICAL GRADIENT... 33 Slice Select, Phase Encoding and Frequency Encoding Z. K-SPACE(RAW DATA)... 35 Section 7 ARTIFACTS 36 A. CAUSE AND APPEARANCE... 36 Section 8 QUALITY CONTROL 59 A. SLICE THICKNESS... 59 B. SPATIAL RESOLUTION... 61 High Contrast Spatial Resolution C. CONTRAST RESOLUTION... 62 Low contrast object detectability D. SIGNAL TO NOISE RATIO... 63 E. CENTER FREQUENCY...//... 63 F. TRANSMIT GAIN OR ATTENUATION... 64 G. GEOMETRIC ACCURACY... 64 Quality Assurance Section 9 EQUIPMENT HANDLING 66 AND INSPECTION SYSTEM INDICATOR LIGHTS... H. 66 SAFETY LIGHTS AND LIGHT BULBS... I. 66 J. RF ROOM INTEGRITY AND DOOR SEALS... K. EMERGENCY CART... 67 L. COILS AND CABLES... 68 67 Section 5 TISSUE CHARACTERISTICS 27 U. T1, T2, T2*, PROTON DENSITY... 27 Indications V. FLOW... 29 W. DIFFUSION AND PERFUSION... 29 Indications 5
ne Section ELECTROMAGNETISM Electromagnetism is one of the fundamentals forces in nature. It deals with the physical relations between electricity and magnetism. A. FARADAY'S LAW The law of electromagnetic induction states that when a conductor is placed in a magnetic field; an electrical voltage will be induced in the conductor. Also when an electrical voltage is traveling along a conductor, a magnetic field will be produced around it. The higher the strength of the magnetic field, the greater the current flowing in the conductor. B. TYPES OF MAGNETS Divided into: Superconductive, Permanent and Resistive Superconductive Magnet (Electromagnet) Most common types of magnet. High field systems which mainly used in magnetic fields of 1.5 Tesla or higher. 85% of clinical MRI scanners are 1.5 Tesla. SUPERCONDUCTIVE MAGNET 6
Characteristics of superconductive magnets Closed field system Horizontal magnetic field Solenoid configuration= A coil is wound into a tightly packed helix. The superconductive magnet is a cylinder that has 55 to 70cm in diameter in order to accommodate a human The magnetic field is produced by the flow of current induced in the coil. Ohm's law governs the resistance of current along a wire which is directly proportional to the potential difference across two points. Ohm's law formula V (voltage) = I (current) x R (resistance within the wire) Superconductivity is a property of some materials in which there is zero electrical resistance at very low temperatures. Niobium-titanium alloy is used to make the coil windings for superconducting magnets. The coil windings are surrounded by liquid helium that when cooled below 4 K (kelvin) creates virtually zero current resistance and the magnetic field is maintained. Cryogens= Super-cooled substances such as liquid helium (most commonly used) or liquid nitrogen that remove current resistance in the coil windings. This creates superconductivity. Cryostat=A steel cylindrical tank which maintains the low temperature of the liquid helium and or nitrogen and does not allow it to boil off. 7 CRYOSTAT
Permanent magnets Low to mid field strength between 0.3 to 0.5 Tesla but recently there are a number of high field systems of 1.0 Tesla. PERMANENT MAGNET Characteristics Open field systems Vertical magnetic field Heavy weight Do not required electricity to power up the magnet Sensitive to temperature Small fringe field Fewer safety considerations with respect to the fringe field (magnetic field surrounding the magnet) which reduces the chance of projectiles. The use of ferromagnetic materials in permanent magnets such as iron, cobalt or steel will retain its magnetism after being exposed to the magnetic field. Ceramic bricks consisting of magnetic blocks of slabs which possess ferromagnetic properties that can be magnetized can also be used to produce permanent magnets. An option to use for claustrophobic patients 8
Resistive Magnet(Electromagnet) Low field strength less than 0.2 TO 0.3Tesla RESISTIVE MAGNET Characteristics Open field systems In order to maintain the magnetic field; a continuous flow of current must always be available. Magnetic field can be turn off horizontal or vertical magnetic field Light in weight High operating cost to maintain it High power consumption in order to keep the current in order to maintain the magnetic field. C. MAGNETIC FIELD STRENGTH Magnetic field strength is measured in tesla (T) or gauss (G) The Tesla unit is used to measure high field strength (1 tesla =10,000 gauss). The Gauss unit= is used to measures low field strength such as the fringe field (magnetic fields outside the bore of the magnet). Classified as: Low field strength Less than 0.3 Tesla Mid field strength Between 0.3 to 1.0 Tesla High field strength 1.5 Tesla to 3 Tesla 9 Ultra high field strength 14 Tesla used for research, spectroscopy and high resolution studies
Question 1. If 1 Tesla is equal to 10,000 Gauss, How many Gauss are in 0.3T? P Formula 1 Tesla 10,000 Gauss 0.3Tesla x Problem Solving 0.3Tesla x 10,000 Gauss=3000/1 Tesla=3000 Gauss Answer 3,000 gauss 3,000 Magnetic fringe fields The magnetic field outside of the magnet bore. 5 GAUSS LINE FRINGE FIELDS ISO CENTER FRINGE FIELD The magnetic field outside the bore of the magnet has a 5 gauss limit which represents the safety margin level for exposure to the magnetic fringe fields in patients that have a pacemaker. The use of magnetic shielding is used to reduce the area of the fringe field. Homogeneity The uniformity of the main magnetic field which is measured in parts per million (ppm). 10
wo Section.. RADIOFREQUENCY SYSTEM D. COIL CONFIGURATION The radiofrequency coil is the first inner layer component within the gantry. Commonly referred to as the body coil is capable of transmitting and receiving radiofrequencies.. E. TRANSMIT AND RECEIVE COILS Transmit coils= capable of transmitting the signal of an RF pulse. The primary RF transmitter is the body coil which is also capable of receiving the RF pulse. Most of the transmit coils are also capable of receiving the signal as well. MAIN BODY COIL Some other examples include head, knee and foot coils. Receive coils= only capable of receiving the signal of an RF pulse. The received coils send the signal to the MRI magnet in order for the computer to process the signal. These coils are placed on or around the surface of the area of interest. 11 Example: Surface (local) coils, Linear and or Phase array coils which are also referred as multichannel coils.
Surface (local) coils are placed directly on the surface of the area of interest which is usually small in size. These coils provide an excellent SNR but are only sensitive to tissue depths of 50 to 75% the coil's diameter. Example: ENDORECTAL SURFACE (LOCAL) COIL TEMPOROMANDIBULAR JOINT SURFACE (LOCAL) COIL Linear coils consist of a single loop of wire and provide a good signal since a small area of noise is detected. Example: The Helmholtz coil configuration which uses multiple coils with one single receiver. It consists of two circular coils parallel to each other. SENSE FLEX MEDIUM HELMHOLTZ COIL THAT CAN BE USED FOR SHOULDER AND OR WRIST 12
Another Helmholtz coil configuration can be used to obtain signal for the anterior neck and cervical spine. Example Phase Array Coils uses multiple coils with multiple receivers whose signal is combined to create images with an increased in SNR and yet covers a large anatomical area. It is sometimes used in combination with parallel imaging techniques in order to decrease the scan time. Example: brain, vascular neck, abdomen, pelvis and spine coil 8 CHANNEL PHASE ARRAY BRAIN COIL 8 CHANNEL PHASE ARRAY VASCULAR HEAD AND NECK COIL 13
8 CHANNEL PHASE ARRAY ABDOMEN AND PELVIS COIL 8 CHANNEL PHASE ARRAY SPINE COIL Trans-receiver coils= capable of transmitting and receiving the signal of an RF pulse. Notes 14
Example: Knee and foot quadrature coils, volume coils such as head and the body coil (the main magnet). The quadrature coil configura on uses wires that are perpendicular to one another. QUADRATURE KNEE AND FOOT TRANSMIT-RECEIVE COIL Volume coils can be used to accommodate a large volume of tissue but this causes a decrease in SNR. Example 1: HEAD (BIRDCAGE) TRANSMIT-RECEIVE COIL 15
HEAD TRANSMITCOIL HEAD-RECEIVE COIL MAIN BODY TRANSMIT-RECEIVE COIL General coil characteristics Use the smallest coil possible that fits the area of interest to be imaged. The correct coil that fits the area of interest should be used in order to have an increased in SNR. The closer the coil is to the area of interest, the greatest the signal detection. In general; the smaller the radiofrequency coil; the better the signal to noise ratio. In order to get the appropriate signal of the area of interest being imaged, the coil must be positioned perpendicular to the Bo field. 16
Example: COIL POSITIONING WAS PERPENDICULAR TO THE B0 FIELD COIL POSITIONING WAS OBLIQUE TO THE B0 FIELD COIL POSITIONING WAS PARALLEL TO THE B0 FIELD COIL POSITIONING WAS PERPENDICULAR TO THE B0 FIELD E.1 TRANSMIT AND RECEIVE BANDWIDTH Transmit Bandwidth Transmit bandwidth is used to define slice thickness. Once a certain gradient slope is applied, the transmit bandwidth will provide a range of radiofrequencies (RFs), centering about the Larmor frequency, transmitted to excite the slice, which should match the difference in precessional frequency between two points. 17
A narrow transmit bandwidth and/or steep slice select gradient is used to achieve thin slices. A broad transmit bandwidth and/or shallow slice select gradient is used to achieve thick slices. The resonant frequency bandwidth of the hydrogen protons to be visualized is transmitted in order to get resonance. A broad transmit bandwidth will give rise to: A shallow slice select gradient Low amplitude slice select gradient Increase slice thickness Decrease resolution Receive Bandwidth The range of frequencies the receiver can sample accurately, which must be mapped across the FOV. This is determined by the upper and lower limits of frequencies on the either side of the center frequency of the echo. The receive bandwidth is related to the slope of the frequency encoding gradient. Based on the Nyquist theorem Receive bandwidth = 2 x the highest frequency (Nyquist Frequency) An increase in receiver bandwidth will give rise to: Notes Faster sampling time Low minimum TE decrease chemical shift artifacts Decrease SNR 18
Noise Signal Noise Signal Noise Noise Broad Bandwidth Narrow Bandwidth 32 KHz bandwidth 12 KHz bandwidth Decrease Signal Increase Noise Increase Signal Decrease Noise F. PULSE PROFILE A waveform representation of the RF pulses that are received by the MR system. The center of the waveform is sampled during readout. The outer edges of the RF pulse waveform are not sampled but are still present. A gap of 30% of the slice thickness is used in order to prevent the cross excitation artifact. G. PHASE ARRAY A coil that uses from 4 to 32 receivers in order to increase SNR and yet coverage a larger area of anatomy. Please refer to the phase array coil section on page for more information. 19
Three Section. GRADIENT SYSTEM GRADIENTS Small electromagnets that are superimposed over the main magnetic field which uses wires of current to alter the external magnetic field. Gradients are the main source of noise in an MR system. 20
The banging noise that is experience when performing an MRI sequence occurs when rapid pulses of electricity are passed through the gradient coils which cause alterations in the external magnetic field. Gradient magnetic fields are used to spatially encode data in the slice, phase and frequency direction. H. COIL CONFIGURATION The order of the coils in a magnet from the inner to the outer layer of the magnet bore is as follows: The radiofrequency coil, the gradient coils, the shim coils and then the magnet coils. MAGNET SHIM COILS GRADIENT COILS RF COILS 21
Question nd 1. What coil represents the 2 layer component going from the inner to outer part of the magnet bore? A. the shim coil B. the radiofrequency coil C. the main body coil D. the gradient coil Answer D. the gradient coil Gradient strength or amplitude Defines how strong or steep a particular gradient is. It is measured in MilliTesla/ per meter (mt/m) or gauss per centimeter (G/cm). Typical maximum gradient strengths values range from 10-40mT/m. I. SLEW RATE The time it takes for a gradient to achieve maximum amplitude. It describes the rate of change in the speed and strength of the gradient amplitude and is measured in T/m/s. Slew rates can range between 70-200Mt/m/s. It can be calculated by dividing the maximum gradient amplitude by the rise time. Slew rate is the best indicator of gradient performance. J. RISE TIME (SPEED) The time it takes for a gradient to reach maximum amplitude. It is measured in microseconds. The shorter the rise time; the faster the gradients and therefore the smaller the echo spacing. Shorter echo spacing will give the advantage of increase resolution and more slices allowed/tr. K. DUTY CYCLE The time that the gradient is able to work at maximum amplitude. How long a gradient can work at its maximum strength and speed. It is measured in percentage (%). 22
Four Section. NUCLEAR MAGNETISM L. LARMOR EQUATION A mathematical equation which determines the frequency that has to be used in order to create resonance. It provides the precessional frequency that is needed in order to excite the specific spins that are required in order to obtain images of a specific area. The larmor equation describes the relationship between the static magnetic field and the gyromagnetic ratio of the hydrogen protons in specific tissues. In order to obtain the precessional frequency; the Larmor equation has to be applied. Larmor Equation The strength of the static magnetic field is multiply by a constant called gyromagnetic ratio. Wo= Bo x Y M. PRECESSION FREQUENCY It describes the wobbling motion of protons that spin around their axis in the presence of an external magnetic field. In order to obtain an MR image of a specific area; an RF pulse at the precessional frequency of the hydrogen protons to be imaged must be applied in order to get resonance. The precessional frequency of the protons in a magnetic field can be provided by using the Larmor equation. The larmor equation: Wo= Bo x Y Wo represents the precessional frequency (in MHz) Bo represents the strength of the static magnetic field (in Testla) Y represents the gyromagnetic ratio. Mhz/T 23 The precessional frequency of hydrogen at 1.5 is Tesla is 63.86 MHz.
QUESTION 2. If the precessional frequency of the hydrogen proton is approximately 25 MHz, the magnetic field experienced by the patient's protons is. PROBLEM SOLVING 63.86Mhz 25Mhz 1.5Tesla x 25 MHz x 1.5Tesla=37.5/63.86 MHz=0.6Tesla. ANSWER 0.6Tesla QUESTION 3. In a 3.0 Tesla magnet, what is the precessional frequency of the hydrogen protons? PROBLEM SOLVING The gyromagnetic ratio of hydrogen at 1.0 tesla is 42.6 MHz. We just have to apply the Larmor equation (precessional frequency): Multiply the gyromagnetic ratio of the particular proton by the static magnetic field in order to find the precessional frequency. 42.6MHz x 1.0 Tesla =128 MHZ 3.0 Tesla ANSWER 128 MHZ N. GYROMAGNETIC RATIO The rate that a spin will precess when exposed to a magnetic field. This is a constant that defines the precessional frequency of an element at 1 Tesla. This gyromagnetic ratio is specific for each atom. It represents the ratio of the magnetic moment vs. the angular momentum of a particle. Different MR active nuclei have its unique gyromagnetic ratio which is specific for each atom. The hydrogen atom gyromagnetic ratio is equal to 42.6 MHz/T. 24
O. RESONANCE Resonance occurs when a nucleus is exposed to an oscillation that has a frequency close to its own and absorbs the energy from that force. This occurs by an RF pulse being transmitted at the precessional frequency of that particular atom in which its spins will flip into the transverse plane creating phase coherence (a signal induction). In order for energy transfer to occur; the system must resonate at the correct resonant frequency. Depending on the field strength, the higher the field strength, the faster and higher the resonant frequency needed. P. RF PULSE A pulse which is transmitted at exactly the resonant frequency of the hydrogen protons needed in order to obtain images of a specific location. This will tip the hydrogen protons into the transverse plane in order to obtain signal. The greater the amplitude and the longer the pulse is applied, the greater the flip angle. The RF field is oriented perpendicular to the main magnetic field in order to obtain signal. The RF energy used in MRI is an electromagnetic radiation that is non-ionizing. Q. EQUILIBRIUM MAGNETIZATION Thermal equilibrium is a condition in which more spins are aligned parallel than antiparallel to the main magnetic field. During thermal equilibrium, the vector that represents the excess spins aligned in the longitudinal plane is called net magnetization vector. Net magnetization vector is the sum of the contributions of all the magnetic moments of the individual protons. The magnetic field associated with the protons is known as the magnetic moment. R. ENERGY STATE TRANSITIONS When the hydrogen protons are placed in a magnetic field, the majority of them will align parallel in a lower energy state. Some of them but fewer will also be aligned antiparallel in a higher energy state. 25
Only the protons in the parallel energy state will add up to the net magnetization vector and are used to provide the MR signal S. PHASE COHERENCE It occurs when an RF pulse is transmitted at the precessional frequency of hydrogens protons for a specific area to be imaged. These spins tipped into the transverse plane and starts precessing along the same rotational path in phase. T. FREE INDUCTION DECAY(FID) After the RF pulse is turn off; the signal from the hydrogen protons will decay immediately and become out of plane which causes longitudinal magnetization to increase and out of phase which causes transverse magnetization to decreases. This causes a reduction of voltage induced in the receiver coil. The FID is a combination of true T2 decay which is produced by out of phase hydrogen protons and T2* decay dephasing which is produced by magnetic field inhomogeneities and spin-spin relaxation. Relative signal amplitude T2* decay T2 decay Time FID 26
Five Section. TISSUE CHARACTERISTICS During excitation The net magnetization of hydrogen protons is tipped into the transverse plane in which low energy spins enter the high energy state and begins to precess in-phase. During Relaxation The net magnetization of hydrogen protons is returning into the longitudinal plane in which high energy spins return to the low energy state and begin to precess out of phase and out of plane (loss of phase coherence). T. T1, T2, T2* AND PROTON DENSITY T1 Relaxation The process by which the removal of the RF pulse causes spins to relax back into the longitudinal plane. Also known as T1 RECOVERY (spin-lattice relaxation) (longitudinal relaxation) which is the time it takes for 63% of the tissue magnetization to recover in the longitudinal plane. Depending on the type of tissue, for example; fat will relax more quickly due to its molecular tumbling rate. Fat has a slow molecular tumbling rate and its molecules relax faster. Water has a fast molecular tumbling rate and its molecules relax slower. Notes 27
T2 relaxation The process by which the interactions between the magnetic fields of adjacent spins causes dephasing. Also known T2 DECAY (spin-spin relaxation) (transverse relaxation) which is the time it takes for 63% of a tissue magnetization to be lost in the transverse plane. (37% decay). Depending on the type of tissue, for example; fat will relax quicker due to its molecular tumbling rate. Fat has a slow molecular tumbling rate and its molecules relax faster. Water has a fast molecular tumbling rate and its molecules relax slower. T2* susceptibility The dephasing of the precessing spins due to magnetic field in homogeneities. AXIAL SUCEPTIBILITY WEIGHTED VENOGRAM OF THE BRAIN Proton (spin) Density The number of mobile hydrogen protons/unit volume of tissue. Each tissue has a different amount of hydrogen protons. Example: Brain tissue has a high number of mobile hydrogen protons while cortical bone has a low number of mobile hydrogen protons. 28
AXIAL PROTON DENSITY OF THE BRAIN U. FLOW Please refer to flow dynamics in section D. Data acquisition and processing. W. DIFFUSION AND PERFUSION DIFFUSION A functional technique that assesses the movement of molecules in the extracellular space due to random thermal motion. PERFUSION The technique that to assesses the microcirculation of blood in the tissues.. 29
Six Section. SPATIAL LOCALIZATION X. VECTORS Any quantity that has both direction and current and is usually represented by an arrow. The magnetic moment MR active nuclei that have a charge and are spinning acquires a magnetic property and can align with an external magnetic field. The net magnetization vector Produced as a result of the excess hydrogen protons aligned in the parallel direction to the external magnetic field. The NMV increases when the magnetic field strength is increased and results in an improved SNR Notes 30
X, Y, and Z coordinate system It is used in order to spatially localize the signal in 3 spatial dimensions. XY plane Referred to as the transverse plane where we obtain the signal. Closed magnets have a vertical XY plane while open magnets have a horizontal XY plane. Y CLOSED MAGNET Z Bo X OPEN MAGNET Y Z X Bo Z axis Referred to as the longitudinal plane. It is oriented in the same direction as the external magnetic field. 31 Closed magnets have a horizontal Z axis while open magnets have a vertical Z axis.
Isocenter Referred to as the area where the X Y and Z axes intercept. The anatomy of interest to be scanned is placed as close as possible in the isocenter as this area provides the most magnetic field homogeneity. X= Transverse plane(signal detection) Y= B1 field (radiofrequency field) Z= Longitudinal plane-bo (external magnetic field) Notes 32
X. PHYSICAL GRADIENT The actual pieces of hardware that makes up the gradients. Composed of the X, Y and Z gradients. The gradient system is composed of 3 coordinates in order to spatially locate the anatomical area of interest. SLICE SELECT GRADIENT (Z axis) Location of the anatomical slice plane. PHASE ENCODING GRADIENT (Y axis) Location of the signal in the short axis of the anatomy. FREQUENCY ENCODING GRADIENT (X axis) Location of the signal in the long axis of the anatomy. Slice Select Gradient The slice select gradient is turn on during the application of the RF pulse. It locates the signal and location of the particular slice. The slope of the slice select gradient will determine the slice thickness and gap. The slice select gradient determines the anatomical plane that is acquired. It is divided into: X, Y and Z gradient 33
X GRADIENT ALTERS THE MAGNETIC FIELD STRENGTH AND PRECESSIONAL FREQUENCY OF THE HYDROGEN SPINS. SELECTS SAGITTAL SLICES Y GRADIENT ALTERS THE MAGNETIC FIELD STRENGTH AND PRECESSIONAL FREQUENCY OF THE HYDROGEN SPINS. SELECTS CORONAL SLICES Z GRADIENT ALTERS THE MAGNETIC FIELD STRENGTH AND PRECESSIONAL FREQUENCY OF HYDROGEN SPINS. SELECTS AXIAL SLICES EXAMPLE: Y gradient Z gradient bore table X gradient At least two gradients need to be used for oblique imaging. 34
Phase Encoding Gradient The phase encoding gradient is switched on after the RF pulse. This gradient varies with amplitude with each TR. It locates the signal along the short-axis of anatomy. This is because motion artifacts occur along this gradient. Example: The X gradient is used as the phase encoding gradient for the acquisition of an axial slice of the head because it uses the short axis of the anatomy. This results in a decrease in scan time. The Y gradient is used as the phase encoding direction for the acquisition of an axial slice of the pelvis because it uses the short axis of the anatomy. This results in a decrease in scan time. Note: The slope of the phase encoding gradient also determines which line of K-space is filled. Example: The application of a shallow phase encoding gradient (high signal amplitude) will fill the center lines of K-space first and this will store the contrast information which contains the most SNR. The application of a steep phase encoding gradient (low signal amplitude) will fill the outer lines of K-space last and this will store the spatial resolution information. Frequency encoding gradient (readout gradient) It is turn on during the readout of the echo. It is used to collect the echoes. It locates the signal along the long-axis of anatomy. Logical gradients The computer software assigns letters to each physical gradient. Y. K-SPACE (RAW DATA) The spatial frequency domain that serves as a temporary storage space for data collection during image acquisition. K-space is composed of phase and frequency encoding steps. 35
even Section. ARTIFACTS An anomaly seen on an image not normally present that can cause distortion and obscure the anatomy being visualized. It can cause a confusing artifact appearance that may mimic pathology which can be misdiagnosed. Technologists must learn how to recognized artifacts and known what can be done to reduce or eliminate them. A. Cause and Appearance of artifacts 1. ALIASING (BACKFOLDING, PHASE WRAP, WRAP-AROUND) Cause When the FOV acquired is smaller than the area of interest that needs to be visualized; the tissue outside the selected FOV is undersampled. Appearance The tissue outside the selected FOV is undersampled along the phase axis which causes the anatomy outside the FOV to be mapped within the FOV. BRAIN ALIASING Occurs in the phase encoding direction. 36
ABDOMEN ALIASING Solution Use a larger FOV that covers the entire area of interest in the phase direction. Apply pre-saturation pulses to undesirable anatomy. Use no phase wrap (Fold-over suppression, Phase oversampling or anti-fold over). Notes 37
. Gibbs(Truncation) Artifact Cause Under-sampling of data. Appearance The interfaces between high and low signal intensity are incorrectly represented. Occurs in the phase encoding direction. TRUNCATION ARTIFACT (Black or white parallel lines appear adjacent to high contrast interfaces) Notes 38
It can also appear as lines seen along the inside of the spinal cord in sagittal spine imaging which can mimic a syrinx. Solution Increase # of phase encoding steps Increase the NSA (number of acquisitions) Chemical Shift(Type 1) Cause Occurs due to the slight difference in resonant frequency between hydrogen molecules residing in water compared to the resonant frequency of hydrogen molecules residing in fat. The size of the chemical shift artifact can be obtained as follows: A total receiver bandwidth is 20 khz with 256 pixels in the frequency encoding axis; the bandwidth/pixel can be calculated as 20,000/256=78Hz. 39
Since the fat and water frequency difference at 1.5Tesla is 220Hz; the size of the chemical shift artifact can be calculated as: (220Hz) / (78Hz/pixel) =2.8 pixels. The degree of chemical shift between fat and water depends on magnetic field strength, receive bandwidth and pixel size. To calculate the water-fat frequency difference; you multiply the larmor frequency at the specific field strength you are using x 3.5ppm. Example: In a 1.5T MRI scanner that operates at 64 MHz, the frequency difference between fat and water is calculated as follows. 6 6 Δf = (64 MHz)(3.5 ppm) = (64 x 10 Hz)(3.5 x10 ) 220 Hz Magnetic field strength Frequency difference between fat and water 1 Tesla 147 Hz 1.5 Tesla 220 Hz 3 Tesla 440 Hz Factors that increase the Chemical Shift(Type 1) Artifact: Increase magnetic field strength Decrease receive bandwidth Increase Pixel size Increase voxel volume 40
Appearance Black or white band seen at the edge of the interface between fat and water. Occurs in the frequency encoding axis. Example 1: Example 2: Black lines seen at the vertebral end plates SAG T1 41
Solution Decrease the magnetic field strength Increase the Receive bandwidth for a smaller water/fat shift. Decrease pixel size Decrease voxel volume Increase matrix Using a fat suppression technique such as a fat spectral suppression RF pulse Use a STIR sequence Chemical Shift Misregistration (out of phase artifact) (phase cancellation artifact) (Type 2) Cause The inability of the gradient in gradient echo sequences to rephase the spins efficiently. When fat and water are in phase within the same voxel; the signals will add up. However when fat and water are out of phase within the same voxel; the signals will cancel each other allowing chemical misregistration to take place and this produces a signal void. Selecting a TE that is approximately ½ the in-phase TE can generate out of phase images where the signal from fat is suppressed. Appearance Black boundary artifact Can appear in the phase or frequency axis. SIGNAL VOID AX GRE AX GRE 42 SIGNAL VOID
Ring of dark signal seen around certain organs where water and fat resides within the same voxel. SIGNAL VOID AX OUT OF PHASE Solution The use of a spin echo sequence Applying TE intervals of 4.4ms in which fat and water and in-phase with each other. Using a fat suppression technique such as fat spectral suppression RF pulse Using a STIR sequence The in and out of phase process is a function of echo time (TE). When a voxel is imaged with a TE containing fat and water out of phase, the signal will cancel out and produce a signal void in that voxel. To correct for the out of phase black boundary artifact; the operator should select a TE where fat and water in phase.
The following formula should be in order to obtained in-phase or out of phase images: 1 x 3.355 Field strength 1 x 3.355= 2.2msec 1.5 Tesla TE= 0ms in phase TE= 2.2ms out of phase TE=4.4ms in phase In phase images can be obtained at 1.5 Tesla with TE's of 4.4msec, 8.8msec, etc. Out of phase images can be obtained at 1.5T with TE's of 2.2, 6.6, 11.0 msec. WATER FAT 0.0 1.1 2.2 3.3 4.4 The arrow represents the milliseconds after RF pulse is applied. WATER FAT AND WATER FAT BRIGHT SIGNAL DARK SIGNAL Dixon Technique BRIGHT SIGNAL When doing out of phase imaging, a TE of 2.2ms is used to allow chemical misregistration to occur. A dark band will be seen around structures where fat and water resides. 44
Magnetic Susceptibility Refers to the property of a substance to become magnetized. It is used to describe the strength of magnetic forces acting on substances that are exposed to the magnetic field. Classified as: 1. Diamagnetic These substances have paired orbital electrons and display no magnetic field however when exposed to an external field; these substances begin to slightly repel with the external magnetic field. These substances have a low negative magnetic susceptibility and show a small decrease in magnetic field strength. Example: Lead, plastic, wood, copper, bismuth, silver, oxygen. Notes 45
2. Paramagnetic These substances have unpaired orbital electrons and display a small positive magnetic moment. When exposed to an external magnetic field; these substances are slightly attracted to the external magnetic field and align themselves with it. These substances have a small increase in magnetic susceptibility and show a small increase in magnetic field strength. Example: Aluminum, magnesium, tungsten, platinum, gadolinium contrast agents. 3. Super-paramagnetic These substances have an intermediate positive magnetic moment that is greater than paramagnetic but less than ferromagnetic substances. Example: Iron oxide particles and iron oxide contrast agents (generally used as T2 or T2* contrast agents in the liver). 4. Ferromagnetic These substances have half-filled electron shells and display a large magnetic moment. When exposed to an external magnetic field; these substances begin to be attracted to the external magnetic field with increase strength. These substances retain its magnetism even after being removed from the external magnetic field. Example: Iron, cobalt, steel and nickel. Non-magnetic These substances display no magnetic moment. Example: Glass, plastic and wood. 46
Magnetic susceptibility artifacts Cause The presence of metallic objects or naturally occurring iron in patients that have a hemorrhage causes the spins in the area to be imaged not to be affected by the excitation pulse because they precess at a vastly different precessional frequency. Example: Metallic teeth implants, titanium orthopedic implants, Regions of air/bone/soft tissue interfaces, iron particles in mascara. Appearance Regions of signal void or increase signal. SIGNAL VOID IN THE AREA OF THE DENTAL IMPLANT SURROUNDED BY AN INCREASED SIGNAL AX T1 47
Susceptibility artifacts due to iron particles in mascara which causes an increased signal Regions of air/bone soft tissue interfaces causes local gradients distortions due to differences in magnetic field susceptibility which causes an increase in signal. Factors that increase the artifact: Increase in magnetic field strength An increase in TE which allow more time for dephasing The use of gradient echo sequences The use of narrow bandwidth techniques The use of thicker slices Solution The patient must be changed into a gown that has no metallic snaps and ask to remove all types of metals that he or she might have. Avoid using T2*gradient echo sequences as this increases magnetic suceptibity artifacts. : 48
A fast spin echo sequences is helpful as this uses multiple 180 degrees RF pulses which are more efficient at rephasing spins. Example: SIGNAL VOID SIGNAL VOID AX GRE AX T2 FSE The use of parallel imaging techniques. When a patient has a metallic hardware that could not be removed; try to use spin echo imaging sequences. Unfortunately some patients cannot remove certain types of metals for example: dental implants. These patients are to be instructed that any heating or discomfort they might experience; to immediately inform to the technologist. 49
Radiofrequency (Zipper Artifact) Causes The most common cause of RF artifacts is an extraneous noise source which reaches the receiver coil in which the door of the RF shielded scanner room is not fully closed or the seal is defective. RF emissions from anesthesia machines inside the MRI room. RF signals from radios or machine malfunctions. Improper RF transmitter adjustments Stimulated echoes from imperfect slice selection profiles Appearance Band of alternative white and black areas. Can occur in the phase or frequency direction. RF ARTIFACT RF ARTIFACT This Zipper artifact is oriented perpendicular to the frequency direction) 50
RF artifacts oriented in the frequency direction are caused by stimulated echoes from imperfect slice selection profiles or improper RF transmitter adjustments. Example: RF ARTIFACT RF ARTIFACT SAG T1 COR T1 Solution Eliminate the source of noise by calling the service engineer to check for any RF leaks. Replacing any broken RF door seals. Improve the spoiling of transverse magnetization. 51
(Motion and flow artifacts) Phase Mismapping and or Ghosting Can be divided into: Motion and Flow artifacts Motion Artifacts Cause Voluntary motion occurs from patient movement and or breathing. Involuntary motion occurs from the heart beating or due to abdominal peristalsis. Appearance Seen as replications of moving anatomy across the phase axis of an image. Occurs in the phase encoding direction BREATHING ARTIFACTS SEEN AS MULTIPLE REPLICATIONS Breathing motion artifacts seen in the abdomen 52
Solution Voluntary Motion Proper instructions and communication provided to the patient to remain still and or follow breathing instructions during the examination. Making the patient as comfortable as possible. Example: Stabilization of the patient by using the aid of positioning sponges, taping and or supporting pads. Using breath hold techniques The use of pre-saturation pulses Example: In the area of the neck to prevent swallowing artifacts. Claustrophobic patients might benefit from receiving some anxiety medication to be able to help them relax during the exam. Swapping the phase and frequency encoding axes will shift the direction of the motion but will not eliminate them. Solution Involuntary Motion Application of cardiac or peripheral gating when applicable. The use of ultrafast sequences such as (HASTE OR EPI) can be used to acquired rapid images between 2 to 5 seconds without the use of breath hold techniques when necessary Physiologic motion such as peristalsis of the abdomen can be reduced with an appropriate pharmacological agent such as 1mg of glucagon. Increasing the number of signals averaged (NSA, NEX) will reduce artifacts and increase signal-to-noise but at expense of increased imaging time. Flow artifacts (ghosting) Blood flow is measured as the total volume of blood that passes a certain point in the vascular system within a given period of time. Blood flow can be laminar, spiral, vortex or turbulent. 53 Cause Blood flow and pulsations
Appearance Seen as ghosting and or replications of a vessel Solution Use Gradient moment nulling techniques Reducing the TE The use of spatial pre-saturation pulses Swapping the phase and frequency encoding axes will shift the direction of flow artifacts but will not eliminate them. 7. Partial volume averaging Cause The signal intensities from adjacent nuclei within the same voxel average togetherand this prevents a true representation of the individual spins in the tissues. This is cause by the use of large voxels, a large FOV, A coarse matrix and thick slices. As the slice thickness increases, there is an increase of different types of tissues projecting in the slice. When a large FOV is acquired, there is an increase superimposition of different MRI signals. 54
Appearance A loss of spatial resolution Solution Use thinner slices Decrease voxel volume Decrease the FOV Use a large(fine) matrix 8. Crosstalk (CROSS-EXCITATION) An RF pulse of a particular slice excites an adjacent slice that was previously excited therefore intercepting with each other and these extra RF pulse create a saturation of signal. Appearance A black line or band is seen in the intercepting area CROSSTALK Two Intercepting RF pulses causes a signal void CROSSTALK ARTIFACT CREATING A SIGNAL VOID 55
Solution In 2D multi-slice acquisition, use a gap between slices. Most sequences use a minimum 10% gap. Inversion recovery sequences a 20% gap. Avoid using steep angles between slice groups to prevent cross excitation between slices. Careful placement of stacks so they don't cross each other. Use an Interleaved slice acquisition. Example: Acquiring the odd number of slices first followed by the even slices. 10. Moiré Artifact Cause A combination of wrap around artifact (from anatomy that extends outside the FOV) and Inhomogeneity of the main magnetic field from one side of the body to the other (due to the patient's body pressed against the bore of the magnet). Appearance Areas of white and black bands seen along the edge of the FOV Moiré artifact Solution Using spin echo pulse sequences which compensates for magnetic field inhomogeneities. 56
Keep the patient arms away from touching the surface of the bore of the magnet when performing gradient echo sequences. Use of anti-aliasing techniques. 11. Parallel Imaging Artifacts Cause Increasing the acceleration factor results in a reduction in the number of phase encoding lines acquired which results in a small phase FOV. Appearance Anatomy from outside the FOV is folded into the anatomy inside the FOV along the phase encoding direction resulting in a wraparound artifact. WRAP-AROUND ARTIFACT Parallel imaging artifact Solution Reducing the acceleration factor(r factor) Proper use of a calibration scan. The SENSE and GRAPPA technique works either by unwrapping the aliasing in the images or, equivalently, by filling in the missing lines of k-space. 57
Other Artifacts Magic angle artifact Cause When structures containing collagen lies at a 55 degree angle to the main magnetic field; this causes a faster T2 decay. Appearance Areas of high signal intensity near tendons where collagen is usually present. SAG PD SAG T2 FAT SAT Solution Changing the positioning angle of the anatomical structure Lengthening the TE in will result in a slower T2 decay. 58
ight Section QUALITY CONTROL 4. Quality Control The process that evaluates standards of quality and takes corrective action when output doesn't meet standards. It requires the following steps: 1. Acceptance testing. 2. The establishing of baseline performance. 3. Detection and diagnosis of changes in the equipment performance. 4. Correction verification. The ACR requires several quality control tests to be performed in order for facilities to obtain accreditation. The following are quality control tests that are performed: A. SLICE THICKNESS The accuracy with which a prescribed slice achieves the desired thickness. This test is performed annually using an MRI ACR phantom. When doing an AXIAL T1 weighted ACR series; the acceptable ranges are between 5.0mm +- 0.7mm.. 59
Slice thickness plays an important role in obtaining a good MR quality image.. Depending on the area of interest; a thin or thick slice may be chosen. Example: Areas such as the orbits and internal auditory canal require thin 1mm slices in order to visualize the small structures. In this way; lesions will not be missed and there will be an adequate resolution. When the slice thickness is reduced by a factor of 2, the NSA must be increased by a factor of 4 in order to maintain the same SNR. Notes 60
B. SPATIAL RESOLUTION The amount of small visual detail that can be seen in an image. It provides the ability to distinguish two points as separate and distinct. High contrast spatial resolution The test is used to determine the accuracy of the scanner to resolve small objects. It determines the degree to which individuals small bright spots spaced closely together in arrays are discernible. It should be performed weekly using the ACR MRI phantom. When doing an AXIAL T1 weighted ACR series; the acceptable resolution should be 1.0mm or better with four holes in at least 1 row being recognized as separated and distinct How many holes comprise each array row and Column? UL A Single Array Pair UR 61
C. CONTRAST RESOLUTION The subtle differences in signal intensities from tissues being imaged. Low contrast object detectability The ability to assess the extent to which objects of low contrast are detectable in an MR image. This test is to be performed weekly using an ACR MRI phantom. If the number of visible spokes is reduced to more than 3; the service engineer is to be notified. Two complete spokes 1 2 Notes 62
D. SIGNAL TO NOISE RATIO Determines the accuracy of the SNR. E. CENTER FREQUENCY This test ensures that the correct Larmor frequency is transmitted by the RF antennae. It is performed weekly using an ACR MRI accreditation phantom. Ranges for the center frequency should not deviate by more than 1.5ppm between weekly measurements. Center frequency determines: 1. The frequency of the RF pulse used for excitation 2. The frequency of the RF pulse used for signal detection Question 4. If the center frequency of hydrogen in a 1.5 Tesla scanner is 63.86 MHz, what is the center frequency of hydrogen in a 3.0 Tesla scanner? PROBLEM SOLVING 63.86 MHz 1.5Tesla 3.0Tesla 63.86 MHz x 3.0Tesla= 191.58 /1.5Tesla=128 MHz ANSWER 128 MHz 63
F. TRANSMIT GAIN OR ATTENUATION This test measures the amplitude and or dura on of the RF pulse (determines the flip angle) and should be performed weekly using an ACR MRI phantom. G. GEOMETRIC ACCURACY Determines the accuracy in which the image represents the true dimensions of the body part being image. Determine by the gradient system. This test should be performed weekly using an MRI ACR phantom. Acceptable ranges of geometric accuracy should be +-2mm of the true values. st 1 SAGITTAL LOC 148mm + / - 2mm SLICE 1 190mm + / - 2mm Air- bubble Circular ROI SLICE 5 190mm + / - 2mm 64
Quality Assurance The process that aims to prevent quality control problems through planned and systematic activities including documentation which help assure the quality of services. The technologist and the engineer are responsible to do the quality control testing in the MRI machine. Preventive maintenance is to be done by a service engineer Monthly which can include installing any software updates that might be available. Notes 65
ight Section EQUIPMENT HANDLING AND INSPECTION Equipment handling and inspection The purpose of this test is to ensure that MRI system equipment components are working properly and electrically stable. The test should be performed weekly. Several exams include: 1. System indicator lights 2. Safety lights and light bulbs 3. RF room integrity and door deals 4. Emergency cart 5. Coils and cables H. SYSTEM INDICATOR LIGHTS Indicator lights will provide an accurate centering for the examination of interest. I. SAFETY LIGHTS AND LIGHT BULBS Safety lights will provide alertness to the personnel and or anyone regarding the static magnetic field. The flickering of light bulbs can cause RF artifacts that can affect image quality which can cause RF artifacts 66
. J. RF ROOM INTEGRITY AND DOOR SEALS Passive shielding in which iron, copper or steel plates are incorporated into the walls, ceiling, and/or floor of the magnet room known as (Faraday's cage) is used to reduce the magnetic field and provide room integrity by preventing RF artifacts. Patient monitoring devices that are MR conditional can also be the cause of RF artifacts. Door seals It must be shielded as this can prevent RF leak artifacts from entering the room. Copper shielding Missing RF copper shielding K. EMERGENCY CART Only label and inspected MR safe and or appropriate MR conditional emergency carts following appropriate protocols are to be used in the MRI suite. Notes 67
. L. COILS AND CABLES The technologist should inspect coils and cables for any signs of tear and wear as this can cause the possibility of a patient burn and or fire hazard. Any signs of wear or tear must be reported to the supervisor and service engineer. A tag must be placed on defective equipment by the service engineer. Example: Notes 68
69 Notes
References Physical Principles Of Image Formation http://clinical-mri.com/wp-content/uploads/software_hardware_updates/graessner.pdf http://web.mit.edu/viz/em/visualizations/notes/modules/guide10.pdf http://pubs.rsna.org/doi/full/10.1148/rg.261055134 http://www.ncnr.nist.gov/equipment/cryostats/cryogensafety.pdf http://www.ceessentials.net/article7.html http://chickscope.beckman.uiuc.edu/roosts/carl/artifacts.html http://www.ismrm.org/smrt/chapters/southcarolina/sc2011mr05.pdf http://chickscope.beckman.uiuc.edu/roosts/carl/artifacts.html http://www.ets-lindgren.com/pdf/spg-2009.pdf https://www.cis.rit.edu/htbooks/mri/chap-9/chap-9.htm Carolyn Kauth, Roth (2006). Basic and Advanced Principles of MRI: MRI Review Program for Technologists. Boothwyn, Pennsylvania: Imaging Education Associates Elster D, Allen and Burdette H, Jonathan (2001). Questions and Answers in MAGNETIC RESONANCE IMAGING. Second Edition. St Louis, Missouri: Mosby: An affiliate of Elsevier Science. Schering AG Berlin/Bergkamen (1990). MRI made Easy, Germany by Nationales Druckhaus Berlin 70