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MR Basics: Module 8 Image Quality 1. MR Basics Image Quality Welcome to Module 8 of MR Basics Image Quality. This module was written by Carolyn Roth, R.T.(R)(MR)(CT)(M)(CV), FSMRT. 2. License Agreement and Disclaimer 3. Objectives After completing this module, you will be able to: List the imaging parameters that determine image contrast. Describe how imaging parameters determine spatial resolution on magnetic resonance (MR) images. Name the imaging parameters involved in MR image formation. Explain parameters and imaging options to obtain diagnostic MR images with minimal image artifacts. 4. Introduction to MR Image Quality MR image quality is related to a number of image characteristics. Spatial resolution is the smallest space between two points that can be distinguished on an image, and pertains to the detail displayed on MR images. Signal-to-noise ratio is the relationship of the desired, or true, signal and unwanted signal, or noise. Contrast-to-noise ratio is the relationship of the signal-tonoise ratio of one structure and the signal-to-noise ratio of an adjacent structure. Scan time is the time it takes to acquire a set of MR images. Temporal resolution is resolution in time, and is significant during cine acquisitions, dynamic enhanced imaging and enhanced MR angiography (MRA) acquisitions. Artifacts are false features on images that are not related to the patient s anatomy or pathology, but are associated with the imaging process. Although an artifact generally is an unwanted characteristic, there are times when the artifact provides diagnostic information that would otherwise be unavailable to the radiologist. We ll discuss artifacts in greater detail later in this module. 5. Intrinsic and Extrinsic Parameters The image characteristics that affect MR image quality can be influenced by a number of intrinsic and extrinsic parameters and imaging options. Intrinsic parameters are relative to the tissues being imaged, and typically cannot be changed. For example, proton-density, T1 and T2 relaxation times are considered intrinsic parameters. There are, however, exceptions to this rule. For example, T1 recovery time and T2 decay time in MR can be altered by administering gadolinium contrast agents or by changing field strength. Generally speaking, intrinsic 2012 ASRT. All rights reserved. 1 MR Basics: Module 8
parameters are those factors that the technologist does not set as part of a protocol selection. Extrinsic parameters are the user-selected parameters and options that technologists choose during MR imaging. 6. MR Parameters and Options MR technologists can modify a number of extrinsic parameters to improve image quality. These parameters include spatial resolution, signal-to-noise ratio, contrast-to-noise ratio, scan time and temporal resolution. In some cases, artifacts also can be manipulated to enhance image quality. Several MR parameters can affect a given image characteristic. For example, parameters that influence voxel size have a significant impact on spatial resolution. These parameters include field of view, slice thickness and the imaging matrix (the number of phase and frequency encodings). Although these are the determining factors for voxel size and therefore resolution, they also affect signal-to-noise ratio, contrast-to-noise ratio, artifacts and, in some cases, scan time. 7. Which Image Is Better? When discussing image quality in MR, judging an image as good or bad usually is not appropriate. There is no perfect MR image. In fact, if you compare the images on this slide and ask, Which image is better, the answer to this question should be, What are we looking for? For example, if the patient s diagnosis is sensory neural hearing loss and the referring physician suspects lesions such as acoustic neuromas in the internal auditory canals, then the image on the right is better because the resolution and the contrast-to-noise ratio are higher. However, for a typical brain scan performed to assess a patient with headaches, the image on the left is acceptable because the signal-to-noise ratio is higher, which in some cases reduces the scan time. 8. Definitions Signal-to-noise ratio is the relationship between the signal from the tissues to be imaged and the unwanted background signal, or noise. Let s look again at the previous images. The signal from the posterior fossa is measured and the noise in the air outside the head is measured. Dividing the signal by the noise gives you the signal-to-noise ratio. Signal-to-noise ratio is merely a guide to image quality. Many MR scanners display the signal-to-noise ratio for a given protocol on the console. This display helps the MR technologist observe the increase or decrease in signal-to-noise ratio when modifying imaging parameters. Contrast-to-noise ratio describes the relationship between the signal-to-noise ratio of one structure and the signal-to-noise ratio of an adjacent structure. On the right image, a measurement was made in the white matter (signal-to-noise ratio one) and another 2012 ASRT. All rights reserved. 2 MR Basics: Module 8
measurement was made in the gray matter (signal-to-noise ratio two). When the first measurement is divided by the second, the result is the contrast-to-noise ratio. Technically speaking, signal-to-noise ratio is a characteristic that is measured and contrast-to-noise ratio is perceived (or seen) on the MR image. 9. MR Artifacts MR imaging artifacts can be caused by a number of phenomena, including the physics of the imaging process, the sampling process by which images are created, or the instrumentation or hardware used to create MR images. The technologist can select certain parameters and options to reduce the occurrence of artifacts or to enhance the display of desirable artifacts on MR images. 10. Physics Artifacts Physics artifacts are inherent to the science behind magnetic resonance. They include chemical shift, magnetic susceptibility, cross-talk and partial volume averaging. Chemical shift occurs because the hydrogen protons in different substances (for example, fat and water) resonate at slightly different frequencies. This difference in resonant frequencies from adjacent structures causes misregistration when converting MR signals from the frequency to the spatial domain. Magnetic susceptibility is a measure of the ability of a substance to become magnetized. Susceptibility artifacts occur near the interfaces of substances with different magnetic susceptibilities. Cross-talk artifacts occur when imaging sequential slices. If the slices are too close together, the excitation pulses for adjacent slices can interfere with each other. A partial volume averaging artifact is caused when an imaging voxel contains the average of the signal for multiple tissue types. 11. Sampling Artifacts Sampling artifacts occur during data acquisition and include aliasing, truncation and phase ghosting (motion artifact). Sampling artifacts are caused by technologist error or patient movement. For example, aliasing occurs when the field of view is smaller than the body part being imaged. Anatomy outside the field of view is seen on the image at the edge of the anatomy being scanned. 12. Instrumentation Artifacts Instrumentation artifacts can be caused by B 0 inhomogeneity or field strength, B 1 inhomogeneity or radiofrequency (RF) coil malfunctions, data reconstruction errors and gradient malfunctions. Instrumentation artifacts are the result of equipment problems or defective equipment rather than technologist error. For example, a Gibbs artifact appears as dark or bright lines next to or parallel to the borders of large intensity changes. The artifact is related to data acquisition and image reconstruction. 13. Technical Error Artifacts 2012 ASRT. All rights reserved. 3 MR Basics: Module 8
Many technical errors can cause artifacts. Among these are improper positioning, improper patient data entry and poor technique choices. For example, if the body part being imaged is not at isocenter or centered to the coil, there can be a loss of signal. 14. Beneficial Artifacts Typically, artifacts are considered undesirable or problematic characteristics on diagnostic images. There are occasions, however, when an artifact can improve the diagnostic quality of the image. For example, the presence of metal in the patient or in the scanner can cause a magnetic susceptibility artifact. If the patient has a brain hemorrhage, a susceptibility artifact would appear on the image. In this case, the artifact is caused by the iron in the blood of the hemorrhage. The MR technologist can select certain technical factors to enhance this artifact, such as using a gradient-echo pulse sequence, and increasing voxel size and echo time (TE). This image is an example of a hemorrhagic lesion in the posterior fossa of the brain. 15. Nonbeneficial Artifacts In other situations, the susceptibility artifact is an undesirable characteristic. For example, when a patient has metallic braces on his or her teeth, the MR technologist should select parameters to reduce the susceptibility artifact. These actions are the reverse of the suggested parameters for increasing beneficial artifacts. For example, the technologist should select a spin-echo acquisition, smaller voxel size or shorter TE. Magnetic susceptibility is more pronounced at higher field strengths. Therefore, the patient should be imaged using the lowest field strength scanner available at the facility. 16. Knowledge Check Answer the following question. 17. Knowledge Check Answer the following question. 18. Knowledge Check Answer the following question. 19. Knowledge Check Answer the following question. 20. Scan Parameters It might seem there are unlimited parameters or combinations available for optimizing MR image quality. The number of selections can be overwhelming for the MR technologist. This table divides the parameters and imaging options into categories. These categories include the 2012 ASRT. All rights reserved. 4 MR Basics: Module 8
hardware components that directly affect image quality, voxel size, sampling, image contrast and imaging options. Hardware determines the static magnetic field B 0 and radiofrequency (RF) field B 1. Voxel size is influenced by field of view, slice thickness and matrix. Sampling depends on the number of signals averaged, bandwidth, number of phase encodings and number of slices in 3-D. Image contrast is affected by repetition time, echo time, inversion time and flip angle. Finally, imaging options that reduce certain artifacts include spatial presaturation, gradient moment nulling, gating and other techniques. Remember that MR terminology often is vendor specific. 21. Scan Parameters Hardware parameters, such as field strength (B 0 ) and coil configuration (B 1 ), can directly affect signal-to-noise ratio, contrast-to-noise ratio, artifacts and other characteristics. The image on the left was acquired with a 1.5-T magnet, and the image on the right was acquired with a 3.0-T scanner. We ll discuss these extrinsic parameters as they pertain to image quality associated with signalto-noise ratio, contrast-to-noise ratio, spatial resolution, scan time, temporal resolution and artifacts. 22. Hardware Field Strength The first hardware component we ll discuss is field strength. Field strength refers to the strength of the main magnetic field, also known as the static field, the primary magnetic field or B 0. The signal-to-noise ratio and contrast-to-noise ratio are directly proportional to the field strength. As field strength increases, the signal-to-noise ratio increases. This slide shows sagittal, high-resolution images of the breast with contrast enhancement, fat suppression and subtraction techniques. The image on the left was acquired at 1.5 T and the image on the right was obtained at 3.0 T. You can see that the appearance of the lesion and overall image quality is improved on the image acquired at 3.0 T. Increasing the field strength from 1.5 to 3.0 T doubles the signal-to-noise ratio and increases the contrast-to-noise ratio, resulting in improved image quality. 23. Hardware Field Strength and Noise As signal-to-noise ratio increases, contrast-to-noise ratio also increases. Remember that the contrast-to-noise ratio is the difference in intensities between the signals detected from adjacent structures on MR images. Signal-to-noise ratio is measured and contrast-to-noise ratio is observed. These axial, 3-D MRA images of the head were acquired to evaluate the circle of Willis. The image on the left was obtained at 1.5 T and the image on the right was acquired at 3.0 T. The vasculature, particularly the smaller vessels, is better displayed on the 3.0-T image. 2012 ASRT. All rights reserved. 5 MR Basics: Module 8
24. Susceptibility Artifact The consequence for the increase in B 0 includes safety considerations because of the stronger magnetic field and higher RF, increased susceptibility artifacts, increased chemical shift artifacts, increased T1 recovery time, reduced T2 decay time and the increased cost of the 3.0-T scanner. As field strength increases, susceptibility artifacts increase. The sagittal, T2-weighted image of the lumbar spine on the left was acquired using a low-field 0.2-T magnet; the image on the right was obtained using a high-field 1.5-T scanner. This patient has metallic hardware from a prior lumbar surgery. The susceptibility artifact is significantly greater on the high-field image on the right than on the low-field image on the left. The image contrast also is slightly different, even though the contrast parameters (repetition time and echo time) are similar. This is because of decreased T2 decay time and increased T1 recovery time at higher field strengths. Susceptibility effects also can occur at the interfaces of tissues with dissimilar characteristics, such as air/bone/soft tissue interfaces. An example is the region of the posterior fossa, which contains mastoid air cells within temporal bone near brain tissue. MR technologists can reduce susceptibility artifacts by selecting a spin-echo acquisition, smaller voxel size or shorter TE. Patients with known implanted metallic hardware should be imaged using a lower strength scanner if available to reduce metallic susceptibility. 25. Hardware RF Next, we ll discuss B 1, the RF source. During MR image acquisition, signals are transmitted to the patient during a period known as excitation and received from the patient during a period known as relaxation. The RF source for MR imaging is the RF coil. RF coils can transmit the RF signal, receive the MR signal, or transmit and receive the signals. Coils that transmit the MR signal during image acquisition are known as transmitters and those that receive the MR signal are known as receivers. Most scanners contain a transmit/receive coil known as the body coil. Although other MR transmitter coils are available, the body coil usually transmits the MR signal. Some head coils and extremity coils, however, are both transmitter and receiver coils; these devices are known as transceivers. From a safety perspective, it s very important to know the characteristics of each type of coil because some implanted devices are MR conditional and can only be scanned using a particular type of coil. 26. RF Coils In MR imaging, signal voltage is induced in the receiver coil during image acquisition. The smaller the RF coil, the better the signal-to-noise ratio. The trade-off for smaller coils, however, is coverage. Smaller coils provide signal information from the diameter of the coil (across) and the radius of the coil (deep). For example, a 5-inch diameter coil displays signal information from 5 inches across the body and 2.5 inches into the body. 2012 ASRT. All rights reserved. 6 MR Basics: Module 8
In addition, more coils provide increased signal-to-noise ratio. Many MR equipment manufacturers have introduced multiple coil arrays, or multichannel coils. In coil arrays, each coil has its own receiver. This configuration provides a significant increase in signal-to-noise and contrast-to-noise ratios and overall image quality. These axial T2-weighted images of the lumbar spine were acquired with a body coil in the image on the left and with a small multicoil configuration in the image on the right. 27. Hardware Positioning Even with a perfect coil configuration, signal quality and overall image quality can be compromised if the coil is not used properly. The coil must be positioned so that the coil field B 1 is perpendicular to the static field B 0. Next, the patient must be positioned appropriately within the sensitive area of the coil. MR technologists also need to consider patient comfort. If the patient is placed in a position that is uncomfortable, he or she tends to move during imaging, which makes the images suboptimal. The use of magnetic field gradients makes the acquisition of various imaging planes possible in MR. For high-quality MR images, the technologist should ensure that the body part being imaged is centered relative to the coil and that the coil is centered relative to the scanner s isocenter. 28. Positioning This slide shows axial T1-weighted images of the foot with contrast enhancement. The left image was acquired with the patient supine and foot positioned in the coil; however, the toes were outside the coil. Note the overall degradation of the image and particularly the loss of signal information from the toes. The image on the right was acquired with the patient supine, the knee bent and the foot positioned flat with toes pointed and well within the coil. You can see the improved image quality when the anatomy of interest is positioned within the sensitive volume of the RF coil. 29. Voxel Size The next parameter critical to image quality is voxel size. Imagine imaging a patient s foot with a new and wonderful coil, but with a 48-cm field of view. In this case, the signal would be excellent, but the resolution would be low as image resolution is determined by voxel size. Voxel size is defined by the field of view, slice thickness and imaging matrix. Image matrix is the number of phase encodings by the number of frequency encodings and will be discussed in more detail later in this module. The next section describes parameters that control voxel size and, therefore, resolution and image quality. 30. Voxel Characteristics The smallest unit of a 2-dimensional digital image is the pixel. MR images are created with a slice thickness, essentially in 3-D. The smallest unit of a 3-dimensional digital image is known as 2012 ASRT. All rights reserved. 7 MR Basics: Module 8
a volume element, or voxel. Voxel size and dimensions can be calculated with user-selected parameters, such as field of view, slice thickness and matrix. The field of view is the amount of the anatomy being imaged. Slice thickness is the volume of tissue in the slice selection direction. The matrix is the number of frequency samples taken and the number of phase encodings performed. 31. Pixel Size Calculating pixel size requires dividing the field of view (the size of the imaging field) by the imaging matrix (the number of pixels along a given direction). For example, if the field of view is 25 cm (250 mm) and the matrix is 250, the pixel size is 1 mm along that direction. If the first calculation was for the right-to-left direction across the image, the next calculation would be anterior to posterior across the image. If the field of view is 25 cm and the matrix is 250, then the pixel size is once again 1 mm along this direction. The final calculation for pixel size is 1 mm x 1 mm. 32. Knowledge Check Using the pixel size formula, complete the following calculation. 33. Knowledge Check The correct answer is 0.0052. 34. Isotropic Voxel An isotropic voxel is a volume element in which all dimensions are equal. For example, in a previous slide we calculated a pixel with dimensions of 1 mm x 1 mm. Selecting a slice thickness of 1 mm would create an isotropic voxel in which the physical dimensions in all three directions are equal. When acquiring a 3-D image, the MR technologist should select acquisition parameters that create an isotropic voxel. When isotropic voxels are used in 3-D imaging, reconstructed images have the same resolution in any plane. 35. Partial Volume Averaging As voxel size increases, signal-to-noise and contrast-to-noise ratios also increase. As the voxel size increases, however, spatial resolution decreases. The reduction in spatial resolution (or detail) is caused by a phenomenon known as partial volume averaging. When the signal within a large voxel is averaged, it decreases image quality of smaller structures. 36. Non-Isotropic Voxels Unlike 3-D isotropic imaging, in nonisotropic 2-D imaging the matrix might not be a perfect square, but rather the length and width could be unequal. For example, a 2-D imaging matrix might be 128 (phase encodings) x 256 (frequency encodings); therefore, the pixels would be rectangular. 37. Voxel Slice Thickness 2012 ASRT. All rights reserved. 8 MR Basics: Module 8
Slice thickness typically is selected based on the anatomy or pathology being imaged. If the examination involves smaller structures, such as the optic nerves, a thin slice thickness creates images with higher resolution. Thinner slice thicknesses provide higher resolution and lower signal-to-noise ratio. Slice thickness is directly proportional to signal-to-noise ratio. As the slice thickness increases, signal-to-noise ratio increases. These axial, T2-weight images of the brain were acquired at the level of the optic nerve and optic chiasm and with slice thicknesses of 5 mm (left) and 2.5 mm (right). As the slice thickness decreases, the optic nerve and optic chiasm are better demonstrated. The reduced resolution is caused by partial volume averaging. 38. Voxel Field of View Like slice thickness, field of view is selected based on the patient s anatomy or the pathology being imaged. If smaller structures such as the internal auditory canals are being studied, a small field of view provides higher resolution and lower signal-to-noise ratio. Field of view is proportional to signal-to-noise ratio. As the field of view increases, signal-to-noise ratio increases. 39. Voxel Field of View The image on the left was acquired with a 24-cm field of view and the image on the right was acquired with 12-cm field of view. In the case of the image on the left, the field of view has been doubled in 2 dimensions (right to left and anterior to posterior). Doubling the field of view yields a 22-mm or four-fold increase in voxel size; this increases signal-to-noise ratio by a factor of 4. The image on the right, acquired with a smaller field of view, has a grainy appearance compared with the 24-cm image on the left. This means that the image has a lower signal-to-noise ratio and a correspondingly lower contrast-to-noise ratio. Much like thickness or any parameter that influences voxel size, an increase in field of view results in less spatial resolution. This reduction in spatial resolution is caused by partial volume averaging. 40. Field of View and Aliasing When the field of view is smaller than the anatomy being imaged, the signal from outside the field of view is undersampled, resulting in an artifact known as aliasing. Aliasing, which is a wraparound or fold-over effect, can occur along the phase-encoding and frequency-encoding directions. Images acquired by today s equipment typically don t demonstrate aliasing in the frequency direction because the MR signal is adequately sampled. However, aliasing artifact is still prominent along the phase direction. Phase encoding typically defaults along the smaller part of the anatomy being imaged. Here s a helpful hint to remember the direction of phase encoding and frequency encoding on MR images: Phase is a smaller word than frequency, and phase defaults along the smaller portion of anatomy. For example, on a sagittal brain image, anterior to posterior is anatomically smaller 2012 ASRT. All rights reserved. 9 MR Basics: Module 8
than superior to inferior. Finally, phase is associated with the smaller part of the matrix. If a matrix is 128 x 256, the 128 represents the number of phase encodings and 256 the number of frequency encodings. Because this artifact results from undersampling, it can be corrected by oversampling. Most MR scanners have imaging options to correct for aliasing. These options are known as antialiasing, no-phase wrap and no-wrap, depending on the equipment manufacturer. 41. Voxel Matrix As with slice thickness and field of view, the matrix is selected based on the patient s anatomy or the pathology being imaged. If the study involves smaller structures, such as ankle tendons or ligaments, a large imaging matrix produces images with higher resolution. A larger matrix (more pixels per inch) provides images with higher resolution and lower signal-to-noise and contrastto-noise ratios. The images on this slide show the effect of matrix size on image quality. The image on the left has a 128 matrix, which produced lower resolution, higher signal-to-noise ratio and a shorter scan time. The center image was acquired with a larger matrix. The image on the right has a 512 matrix, which produced higher resolution, lower signal-to-noise ratio and a longer scan time. 42. Voxel Matrix Matrix size is proportional to signal-to-noise ratio. As the matrix size increases, each pixel and voxel are smaller. The smaller the pixel size, the higher the spatial resolution, but the lower the signal-to-noise and contrast-to-noise ratios. As the matrix increases, the voxel size decreases by a factor of two. If you have twice as many pixels in the same space, the resulting pixels are onehalf the size. When the voxel is reduced by a factor of two, the signal-to-noise ratio is reduced by a factor of two if all other factors remain the same. Matrix is a complex technical factor because it not only relates to voxel size, but also pertains to sampling and therefore scan time. As the matrix size increases, the number of phase encodings or samplings doubles, which in turn doubles scan time. 43. Voxel Matrix In addition to affecting scan time, signal-to-noise ratio, contrast-to-noise ratio and spatial resolution, changing the matrix size by reducing the number of phase encodings affects certain artifacts. The matrix, particularly the number of phase encoding steps, influences an artifact known as truncation, or Gibbs artifact. When MR signals are collected from tissues with low signal, such as cortical bone, that are adjacent to tissues with high signal, such as subcutaneous fat or brain tissue, the signals are sampled low and then high. When a limited number of signals are sampled, signal overshoot or ripples on the images can occur. 44. Voxel Matrix 2012 ASRT. All rights reserved. 10 MR Basics: Module 8
The axial T1-weighted images acquired with a 128 matrix, on the left, and the 256-matrix image on the right both demonstrate a truncation artifact, or rippling. This effect is greater on the 128- matrix image than on the 256-matrix image. As the matrix size (or the number of phase encoding steps) increases, the artifact is reduced. However, increased matrix size adds to scanning time. 45. Sampling Number of Signals Averaged The number of signals averaged is an example of a sampling artifact. This parameter is also known as number of acquisitions or the number of excitations, depending on the MR equipment manufacturer. The generic term, however, is number of signal averages. When signals are sampled during image acquisition, desirable signals and unwanted noise are sampled. As the number of samples increases, the signal-to-noise ratio also increases. Parameters that influence sampling have a square-root relationship with signal-to-noise ratio. As the number of signal averages increases, the signals are added and the noise is canceled, rendering an image with higher signal-to-noise ratio. These coronal oblique T1-weighted images of the shoulder were acquired with one signal average (on the left) and three signal averages (on the right). As the number of signal averages increases from one to three, the signal-to-noise ratio increases by the square root of three. However, scan time also increases by a factor of three, meaning it takes three times longer to complete the scan. Motion artifact, also known as phase ghosting, is merely noise, so the increase in number of signal averages also reduces motion artifacts. On these images, you can see motion artifact superior to the shoulder from respiratory movement. The image with one signal average on the left has more motion artifact than the image with three signal averages on the right. 46. Sampling Bandwidth Signals are sampled during image acquisition. The range of signal frequencies that are sampled is known as the receive bandwidth. Receive bandwidth can be selected to optimize signal-tonoise-ratio, among other image characteristics. Signal-to-noise ratio is approximately equal to one divided by the square root of receive bandwidth. The receive bandwidth is the range of frequencies that are sampled during the TE. For example, a bandwidth of four produces a signal-to-noise ratio of one-half. A default bandwidth generally is about 16 khz, whereas at 1.5 T, signals are sampled at approximately 64 MHz. If signals are acquired with a wide bandwidth, sampling is faster, and the minimum TE can be shorter. Unfortunately, however, wide receiver bandwidth reduces the signal-to-noise ratio. Narrow bandwidth can be on the order of 4 khz. Narrow bandwidth provides higher signal-tonoise and contrast-to-noise ratios. If the signal bandwidth is narrow and sampling time is longer, 2012 ASRT. All rights reserved. 11 MR Basics: Module 8
the gradient applied during sampling is flatter, or at lower amplitude. Applying a low-amplitude gradient creates images with more chemical shift artifact. 47. Sampling Bandwidth The images shown here compare the effect of narrow and wide bandwidths. Narrow bandwidth provides higher signal-to-noise ratio, longer sampling time, longer TE, shorter echo train length and longer scan time for fast spin-echo sequences. Narrow bandwidth often is selected on lowfield imaging systems to increase signal-to-noise ratio. On high-field strength systems, however, wide bandwidth generally is selected for faster sampling, shorter TEs, longer echo train lengths and reduced chemical shift artifact. 48. 2-D vs. 3-D Three-dimensional, or volume, acquisition facilitates reformatting images in multiple planes. Volume imaging is acquired by exciting a slab, or volume, of data instead of a single slice and encoding the slices after the initial excitation. For this reason, scan time for 3-D acquisition is determined by the number of slices and is equal to repetition time (TR) times the number of phase encodings times the number of signal averages times the number of slices. On a 3-D acquisition, signal-to-noise ratio also is related to the square root of the number of slices. If a single slice acquisition is acquired and the signal-to-noise ratio equals one, the same acquisition acquired with 16 slices would yield a signal-to-noise ratio of four. 49. Pulse Sequences Time and Quality A pulse sequence used to produce MR images is a set of RF or gradient pulses and the time between the pulses. The order and timing of the pulse sequence determines image quality. Image contrast, signal-to-noise ratio, contrast-to-noise ratio, resolution and scan time all are affected by pulse sequence selection. The order and timing of the RF pulses typically determine image contrast. The order, timing and amplitude of gradient pulses are responsible for spatial resolution and other quality indicators. Generally speaking, there are three main contrast configurations in MR T1-weighted, T2- weighted and proton-density-weighted imaging. There are two main pulse sequences used to produce these contrast configurations spin-echo and gradient-echo. Spin-echo sequences usually are higher quality acquisitions with longer scan times. Gradient-echo acquisitions usually have lower signal-to-noise and contrast-to-noise ratios but shorter scan times. This table shows the different combinations of the pulse sequences and contrast configurations. Remember that MR terminology is vendor specific. 50. Pulse Sequences and Quality When comparing spin-echo and gradient-echo acquisitions, image quality varies with the pulse sequence selected. For example, spin-echo sequences use a 180-degree RF pulse to refocus the 2012 ASRT. All rights reserved. 12 MR Basics: Module 8
signal and compensate for unwanted effects such as magnetic susceptibility, inhomogeneity and chemical shift. Gradient-echo sequences use gradient pulses to refocus the signals. The axial T2-weighted images on the left demonstrate susceptibility artifact at the level of the temporal bone. In this case, the susceptibility artifact is unwanted. Technologists can reduce susceptibility artifacts by selecting spin-echo sequences, shorter TEs, smaller voxels and using a lower field strength if available. Susceptibility artifacts that occur at interfaces of tissues with dissimilar characteristics, such as air/bone/soft tissue interfaces, can be corrected by choosing from among the same parameters. 51. Pulse Sequences and Quality These axial T2-weighted images acquired at the level of the ventricles demonstrate susceptibility artifact within the area where a stroke occurred. These artifacts are caused by the hemorrhage. In this case, the magnetic susceptibility is a desired artifact. The MR technologist can make the susceptibility artifact more prominent by selecting gradient-echo acquisition, a larger voxel size, longer TE or using a higher field strength if available. 52. Temporal Resolution Temporal resolution is the shortest possible time between two MR events that can be measured. Temporal resolution becomes important in MR imaging when acquiring contrastenhanced images or performing MRA and during cine acquisitions, such as those used in cardiac imaging. Temporal resolution is critical when contrast-enhanced images of the abdomen are acquired. The first set of axial liver images shown on this slide was acquired during contrast injection. The second set was acquired 30 seconds later, and the third set was acquired one minute later. The enhancement of the liver lesions varies considerably. The cancerous liver lesion enhances on the first scan. A hemangioma enhances at the later phase. The timing of the imaging and resulting contrast enhancement allows the radiologist to make an accurate diagnosis of the lesions. 53. Quality and Repetition Time The time between successive excitation pulses is a user-selected parameter known as the repetition time, or TR. TR is selected based on the amount of T1 information desired on the image. For more T1 weighting, the MR technologist selects a shorter TR. If less T1 weighting is desired, the technologist should select a longer TR. TR also influences other facets of overall MR image quality, including scan time, signal-to-noise ratio, contrast-to-noise ratio and the number of available slices. 54. Repetition Time The parameters for these images are the same except for TR. The image on the left was acquired with a longer TR of 2,370 milliseconds. The image to the right was acquired with a 2012 ASRT. All rights reserved. 13 MR Basics: Module 8
shorter TR of 849 milliseconds. The signal within the spinal canal is darker on the shorter TR image. The overall signal is brighter on the image with the longer TR time. 55. Quality and Echo Time The TE is the time from the excitation pulse to the sampled signal. TE is selected based on the amount of T2 weighting desired on the image. For more T2 weighting, the MR technologist selects a longer TE. If the technologist desires less T2 weighting, he or she selects a shorter TE. TE also influences other facets of overall MR image quality, including signal-to-noise ratio, contrast-to-noise ratio, susceptibility artifact and the number of available slices. 56. Echo Time All parameters except TE are similar on these images. The image on the left was acquired with a shorter TE of 15 milliseconds. The image on the right was acquired with a longer TE of 115 milliseconds. The signal within the spinal canal is darker on the shorter TE image on the left. Overall signal (that is, brightness and a less grainy appearance) is increased on the shorter TE image. 57. Quality and T1 Inversion recovery sequences begin with a 180-degree inverting pulse. Typical inversion recovery sequences use the following order: 180-degree inverting pulse followed by a typical spin-echo sequence, such as 90-degree and 180-degree pulses. The time between the 180- degree inverting pulse and the 90-degree excitation pulse is a parameter known as time to inversion, or inversion time. MR technologists select inversion time based on the desired image contrast. For example, if the technologist selects a short inversion time, tissues with short T1 relaxation times such as fat are suppressed. This sequence is known as a short tau inversion recovery, or STIR. STIR sequences are used for musculoskeletal lesions such as bony contusions. 58. Quality and T1 These coronal images of the knee show a bony contusion on the lateral aspect of the tibia. The STIR image on the right suppresses the fat within the bone marrow to better display the bony contusion. 59. Quality and T1 If the MR technologist selects a long inversion time, then fluid is suppressed. This sequence typically is known as a fluid-attenuated inversion recovery (FLAIR) sequence, although the name can vary depending on scanner manufacturer. FLAIR sequences typically are used to image brain lesions, such as periventricular white matter disease. Suppressing the fluid within the ventricles helps display the lesions around the ventricles. 2012 ASRT. All rights reserved. 14 MR Basics: Module 8
60. Quality and T1 The sagittal image of the brain on the left is a T1 spin-echo sequence, and the image on the right was acquired with a FLAIR sequence. The FLAIR image better displays white matter for signs of disease, along with the corpus callosum, superior to the lateral ventricles. 61. Flip Angle Effects In MR imaging, the technologist selects the flip angle and TR to control the amount of T1- weighted information provided on the image. Flip angle is a parameter that is generally selected with gradient-echo sequences. Gradient-echo sequences typically have shorter TR settings to keep scan time to a minimum, and the flip angle is chosen to modify image contrast. Flip angle also affects the signal-to-noise and contrast-to-noise ratios. As the flip angle increases, the signal-to-noise and contrast-to-noise ratios increase. The optimum flip angle for noise and contrast steady states is known as the Ernst angle. 62. Flip Angle and Noise Look at these axial images of the brain. The image on the left was acquired with a 10-degree flip angle. The next image was acquired using a 35-degree flip angle and the third image used a 60- degree flip angle. The final image on the right was acquired using an 85-degree flip angle. Notice the differences in the contrast of the brain tissue as the flip angle is increased. 63. Contrast Enhancement Image contrast also can be affected by intrinsic parameters such as T1 and T2 relaxation times. T1 and T2 can be altered by using contrast media such as gadolinium. Gadolinium shortens the T1 relaxation time, so that lesions demonstrate high signal on T1-weighted images. The axial T1-weighted images of the brain show the effects of contrast administration on T1- weighted images. The image on the left was acquired before contrast administration and the image on the right was obtained postcontrast. Tissues with short T1 times are bright on T1- weighted images. It s important to remember, however, to follow the recommended contrast dose and your facility s protocols for contrast media administration. 64. Flow Motion Compensation Motion artifact can be caused by a number of sources, including periodic motion and aperiodic motion. Periodic motion includes respiration and cardiac movement. Aperiodic motion is random and can occur because of peristalsis or if the patient moves. MR technologists have a number of options to reduce the effects of motion on images. For example, motion can be controlled through methods such as breath hold or sedation, scanning around the motion with gating and triggering, or compensating for the motion with spatial presaturation and gradient moment nulling. 65. Flow Motion Compensation 2012 ASRT. All rights reserved. 15 MR Basics: Module 8
These images are two examples of compensation for flow motion spatial presaturation and gradient moment nulling. In the image on the left, presaturation pulses were applied in the direction of the unwanted blood flow. The spatial presaturation pulses were applied inferior to the axial abdominal image to eliminate venous flow from the inferior vena cava and superior to axial slices to eliminate arterial flow from the abdominal aorta. Presaturation pulses make flowing blood appear black on MR images. Gradient moment nulling applies additional gradient pulses in the direction of the blood flow. This option makes flowing blood bright. To reduce flow motion artifacts, however, gradient moment nulling makes these bright, blood-filled structures repeat across the image in the phase direction. The image of the abdomen on the right has several apparent aortas running from an anterior-to-posterior direction across the image. 66. Respiratory Compensation MR technologists have several methods to compensate for respiratory motion, including breathhold techniques, respiratory triggering and respiratory compensation, and respiratory-ordered phase encoding. Respiratory triggering monitors the respirations using a band around the patient s midsection or a device placed under the patient s nose. The technologist tracks breathing and acquires images during expiration when there is less motion. This technique is known as prospective gating and is similar to cardiac gating. Respiratory-ordered phase encoding compensates for respiratory motion retrospectively. In this option, technologists collect scan data throughout the respiratory cycle. After the data is acquired, signals are sampled and placed in k-space for image formation. Signals collected during periods of more movement are placed along the outer edges, and signals collected during periods with less motion are placed in the center of k-space. 67. Knowledge Check The technologist is able to manipulate a number of parameters that impact scan time, signal-tonoise ratio and spatial resolution. It s important to understand the relationship between these parameters. The following exercise helps demonstrate these relationships. 68. Conclusion This concludes Module 8 of MR Basics Image Quality. Having completed this module, you should now be able to: List the imaging parameters that determine image contrast. Describe the imaging parameters that determine spatial resolution on MR images. Name the imaging parameters involved in MR image formation. Explain parameters and imaging options to obtain diagnostic MR images with minimal image artifacts. 2012 ASRT. All rights reserved. 16 MR Basics: Module 8
69. Bibliography 70. Acknowledgements 71. Development Team 72. Module Completion 2012 ASRT. All rights reserved. 17 MR Basics: Module 8
Bibliography Basic Principles of MRI. Module 1. International Center for Postgraduate Medical Education. Ithaca, NY: 2001. Meachem KS. The MRI Study Guide for Technologists. New York, NY: Springer-Verlag; 1995. 2012 ASRT. All rights reserved. 18 MR Basics: Module 8
MR Parameters Actions and Associated Trade-offs In MR imaging, there are no hard-and-fast rules. MR technologists choose a pulse sequence based on the area of interest, the condition of the patient and other clinical factors or protocols. For the new MR technologist, this aspect can be very frustrating, but in reality it is what makes the modality interesting and challenging. All MR technologists must be well-versed in the ramifications of various choices. The benefits and limitations of these choices are called trade-offs. This chart summarizes MR parameters and their associated trade-offs. MR Parameters Actions and Associated Trade-offs Parameter Action Benefit Limitation TR Increase Increased SNR Increased scan time Increased number of slices Decreased T1 weighting TR Decrease Decreased scan time Decreased SNR Increased T1 weighting Decreased number of slices TE Increase Increased T2 weighting Decreased SNR TE Decrease Increased SNR Decreased T2 weighting NSA/NEX Increase Increased SNR Direct proportional increase in scan time NSA/NEX Decrease Direct proportional decrease in scan time Slice thickness Slice thickness Decreased SNR Decreased signal averaging Increase Increased SNR Decreased spatial resolution Increased coverage of anatomy Decrease Increased spatial resolution Decreased SNR Decreased partial volume averaging Increased partial volume averaging Decreased coverage of anatomy FOV Increase Increased SNR Decreased spatial resolution Increased coverage of anatomy Decreased chance of aliasing FOV Decrease Increased spatial resolution Decreased SNR Increased chance of aliasing Decreased coverage of anatomy Matrix Increase Increased spatial resolution Increased scan time Decreased SNR if pixel is small Matrix Decrease Decreased scan time Decreased spatial resolution Receive bandwidth Receive bandwidth Increased SNR if pixel is large Increase Decreased chemical shift Decreased spatial resolution Decreased minimum TE Decrease Increased SNR Increased chemical shift Increased minimum TE Large coil - Increased area of received signal Decreased SNR Increased aliasing if using small FOV 2012 ASRT. All rights reserved. 19 MR Basics: Module 8
Increased chance of artifacts Small coil - Increased SNR Decreased area of received signal Decreased chance of artifacts Decreased aliasing if using small FOV FOV = field of view; NSA/NEX = number of signal averages/number of excitations; TE = echo time; TR = repetition time; SNR = signal-to-noise ratio. 2012 ASRT. All rights reserved. 20 MR Basics: Module 8
Optimizing Image Quality In MR imaging, the technologist can manipulate a parameter to enhance image quality. However, these actions have consequences and will alter other aspects of the image. This table summarizes how changing a parameter affects other aspects of a scan. Optimizing Image Quality Desired outcome Adjusted Parameter Consequences Maximize SNR Increase NEX Increased scan time Decrease matrix Increase slice thickness Decrease bandwidth Increase FOV Increase TR Decrease TE Decreased scan time Decreased spatial resolution Decreased spatial resolution Increased minimum TE Increased chemical shift Decreased spatial resolution Decreased T1 weighting Increased number of slices Decreased T2 weighting Minimize scan time Decrease TR Increased T1 weighting Maximize spatial resolution (assumes a square FOV) Increase phase encodings Decrease NSA/NEX Decrease slice number in volume averaging Decrease slice thickness Increase matrix Decreased SNR Decreased number of slices Decreased spatial resolution Increased SNR Increased SNR Increased motion artifacts Decreased SNR Decreased SNR Decreased SNR Increased scan time Decrease FOV Decreased SNR FOV = field of view; NSA/NEX = number of signal averages/number of excitations; TE = echo time; TR = repetition time; SNR = signal-to-noise ratio. 2012 ASRT. All rights reserved. 21 MR Basics: Module 8
MRI Fundamentals Glossary B 0 (pronounced B zero ) symbol used to represent the static main magnetic field of the magnetic resonance (MR) imaging system; the strength of the magnetic field is expressed in units of tesla (T). B 1 symbol used to represent the radiofrequency (RF) field in the MR system; the RF coils, or transmitter coils, at the Larmor frequency produce the B 1 field. b-value summarizes the influence of the gradients in diffusion-weighted imaging; the higher the b-value the stronger the diffusion weighting. Chemical shift phenomenon caused by protons resonating at different frequencies in a magnetic environment. Coherence the process of maintaining a constant relationship between the rotations of hydrogen protons; loss of phase coherence of the nuclear spins results in a decrease in transverse magnetization and decrease in MR signal. Coil single or multiple loops of wire that produce a magnetic field when current flows through them, or that detect a changing magnetic field by voltage induced in the wire. Dephasing after a radiofrequency (RF) pulse is applied, phase differences appear between precessing spins; the resulting decay in spin-spin interaction occurs in the transverse plane. Diamagnetic a substance that has a magnetic susceptibility of less than 0 because it has no unpaired orbital electrons; examples include silver, copper and mercury. Dielectric effect the result of radiofrequency (RF) wavelengths shortening inside the body at higher field strengths. Duty cycle interval during the repetition time (TR) that the gradient is permitted to be at maximum amplitude. Echo spacing time period from the middle of one echo to the middle of the next echo. Echo time (TE) time in milliseconds between the 90-degree pulse and the peak of the echo signal; TE is the primary factor controlling T2 relaxation. Eddy current electric current induced in a conductor when that conductor is exposed to a changing magnetic field. 2012 ASRT. All rights reserved. 22 MR Basics: Module 8