Magnetic Resonance Imaging and Radio Frequency. Part 1. Produced on behalf of Mid Sussex Amateur Radio Society by M5BTB

Transcription

1 Magnetic Resonance Imaging and Radio Frequency Part 1 Produced on behalf of Mid Sussex Amateur Radio Society by M5BTB

2 Why Now? During 2011 my physical health was deteriorating, and a brain tumour was diagnosed by MRI. This made me remember what I did in NMR (MRI) cardiac gating, when working for Picker International some 30 years ago...

3 MRI EXAMPLE

4 MRI EXAMPLE

5 My Experience in MRI, during 18 Months 1983/84 Appreciation of theoretical physics Test imaging using phantom, or human subjects RF calibration, EMC Head coil design Production support Cardiac gating (2 US trips) one of the first uses of MRI for stable imaging of the human heart (Part 2) Discovery of what became known as the Magneto-Hydro-Dynamic effect (Part 2)

6 Transverse The Results (3 Axis)

7 Saggital The Results

8 Supine The Results

9 French MRI/IRM (Larrouse)

10 Advantages over X-Ray X-rays are good for detecting broken bones, but the emerging bugbear to human longevity is cancer. MRI can be adjusted to pick-out a change in tissue density, which could be a tumour. Tumour size changes can be monitored accurately. Contrast agents can be used. MRI has helped surgery to be more efficient and successful for the patient. New surgical techniques have also evolved.

11 Comparison with X-Ray Radiology

12 Computed Tomography (CT) The ordinary (flat) X-ray has now been replaced by CT scans which operate in a similar multiple view Rθ (R theta) scan, as in MRI. The equipment is simpler, scans are cheaper; but X-rays do not image tissue. Good on bones, though.

13 History of MRI What we now know as MRI was developed in the UK by EMI (1972) and two UK universities. This led to the terminology of computed tomography or CT, as used in both MRI and X-ray systems. Many technologies are used. Direct plug-in mains cylindrical magnets were inherently noisy and low field (0.15T). Cryogenic magnets were typically 0.5 Tesla 30 years ago, and now 1.5 Tesla is used (ref. PRH, 2 scanners). Scanners up to 3 Teslas are now available. The original physics of the Larmor effect was known as zeugmatography, which became known as Nuclear Magnetic Resonance (NMR), which became known as Magnetic Resonance Imaging (MRI).

14 Extract from A Level Physics Revision Guide (Letts)

15 Larmor Precession In physics, Larmor precession was discovered by Joseph Larmor in northern Ireland over 100 years ago. This was manipulated to produce single directional section views of the subject, which when combined produce what is known as a scan or slice. Multiple slices give 3D representation. Precession of the magnetic moments of electrons, atomic nuclei and atoms about an external magnetic field (field strength measured in Teslas). This frequency is 42.58MHz/Tesla, and based on the H 1 hydrogen nuclei The magnetic field exerts a torque on the magnetic moment, which precesses when externally stimulated.

16 What an MRI System consists of (1): Cryogenic (cylindrical style) superconducting magnet with Z coil. Main field between 0.5 and 1.5 Tesla. The imaging area magnetic field is maintained +/- 1 ppm using a special 6 section solenoid magnet Shaped pulse RF power amplifier The patient is manoeuvred inside the solenoid magnet using the Patient Handling System, and is fitted with electric motors. The PHS is also fitted with RF coils and tuning capacitors. X, Y, Z (gradient) coils fitted close to patient within magnet interior.

17 What an MRI System consists of (2): Reservoirs for liquid nitrogen and helium. Cryogenic coil kept in boiling bath of Helium (~ C or 4 0 K), outer bath containing boiling nitrogen (~ C or 77 0 K) providing cold insulation. Note : Absolute Zero (0 0 K) is C. Computer system to digitally process, reconstruct and handle images. A single operator can handle both the patient, scan control and image processing.

18 Preparation for an MRI Scan Remove everything metallic from your person; and place in security box provided which may be removed from magnet vicinity. Credit card magnetic bands will be wiped clean, add to security box. Own up to any shrapnel in/on body, artificial joints, metal plates or even non-standard metallic tooth fillings. Wear ear plugs or headphones during scan.

19 Magnetically Selecting a Slice The only permanent field inside the magnet is in the Z axis The horizontal change in the Z field is achieved by the X gradient coil The vertical change in the Z field is achieved by the Y gradient coil The Z horizontal change in the Z field is achieved by the Z gradient coil By changing the proportion of current through each gradient coil, any angle of view can be achieved Because of the changing X, Y and Z fields, a signal frequency range (F0 - δf to F0 + δf) is produced for each view. Many views produce a slice Multiple slices give 3D representation.

20 X, Y and Z Directions Y X Z Solenoid Magnet

21 Variation of Z field in X Direction Y X F0 + δf More Z field F0 Z F0 δf Less Z field

22 Variation of Z field in Y Direction Y F0 + δf More Z field X F0 Z F0 δf Less Z field

23 Variation of Z field in X and Y Directions - to form a Programmable View Y View Direction F0 + δf X F0 Z F0 δf

24 RF Sinc Pulse In amateur radio you know that a distorted RF pulse will generate harmonics, sideband splatter, etc. The MRI RF pulse is pre-distorted (Sinc) to give a pulsed frequency from F0 δf to F0 + δf, that is a width of 2δF. Gradient coils are active at this time. This can be several kilohertz wide, and is sufficient to excite material for one view (as set by the X, Y and possibly Z gradient coils).

25 RF Sinc Pulse Waveform The RF pulse envelope is shaped by the Sinc pulse waveform The RF shaped Sinc pulse produces a frequency excitation from F 0 δf to F 0 + δf. Exactly what is required for one view.

26 After RF Sinc Pulse After the RF pulse, the system Tx/Rx coil picks up the precession signal. This contains all the frequency information on one view. X, Y (and possibly Z) imaging coils are set for the next view. All views are added together (100+) to provide a single scan. X, Y (and possibly Z) imaging coils are reset for an offset distance for the next scan.

27 Single View Scan RF response F0 δf Lower Side Band F0 Upper Side Band F0 + δf

28 Two MRI Views (Rθ) Scan Simulation

29 4 MRI Views Scan Simulation with Image Reconstruction Maximum Responses

30 After ~180 Views and Processing: Back to where we started!

31 MRI Sequences There are several types of MRI sequences that are designed to take advantage of blood flow and flesh characteristics to get the best image. Spin Echo and Inversion Recovery pulse sequences are used here. A patient, in a deep coma, has been able to communicate (yes/no) by activating different parts of the brain as detected by an appropriate MRI sequence..

32 MRI EMC Problems Cryogenic magnetic fields extend many metres from the magnet. CRTs of controlling minicomputers need to be placed in neutral field areas. The magnet internal field is prone to distortion from external ferrous material. Shimming with magnetic rods pulls the field back into shape. The Tx/Rx coils are not completely shielded. The Larmor frequency is not particularly strong, and local interference can swamp the needed signal.

33 Magnetic EMC Solutions MRI cylindrical magnet S N Magnet placed in shielded room Neutral space for CRT equipment N S MRI cylindrical magnet Steel plates fitted to wall Rod shim Brick Wall TV Manufacturing Facility

34 Patient Handling System This is necessary to push the patient into the imaging plane of the magnet. It also includes the RF coil which radiates over the patients body. For multiple slices, the PHS moves the patient across the imaging plane for new slices. In order to accurately image the body, a close proximity body coil is used. A head coil is similar, but smaller.

35 Vane tuning capacitors PHS Trolley

36 RF Body Coil

37 RF Coil Assembly - as on PHS

38 MRI 3D Brain Function Scan the latest in MRI Techniques The world is the limit!

39 Health and Safety Strange ambience in magnet room with equipment fan noise. Can walk into cabinets No-one should walk around a cryogenic magnet with keys in pockets. This can be painful! Perfect images require the patient not to move for a long period of time. May not be practicable. Problems with children. Patient claustrophobia. Some say that magnetic fields are good for you. In 1983 pregnant women were banned from MRI scans. (Almost immediately, scan of pregnant nurse was obtained.) Cryogenic magnets are prone to quenching when the coil suddenly becomes resistive, magnetic field is lost and heat is dissipated which heats up both the liquid helium inner jacket and liquid nitrogen outer jacket. Loss of the gases which escape through external venting. Liquid nitrogen is dangerous as piping can cause condensation of liquid oxygen a fire risk.

40 Relative Permeability (RP) Relative permeability relates to how a magnetic field is concentrated or repelled by a physical object. An object with RP may distort an MRI magnetic field, and must be avoided. Ferromagnetism : This is the strongest type of magnetism, and concentrates magnetic fields. RP could be several thousands. See next slide. Paramagnetism : Weakly magnetic, with relative permeability close to 1. Diamagnetism : Opposite to ferromagnetism. Repels magnetic fields. Low negative RP. Para and diamagnetism is allowable in the proximity of MRI systems, but nothing ferromagnetic.

41 Relative Permeability Medium Permeability μ [H/m] Relative Permeability μ/μ 0 Electrical steel ,000 Ferrite (nickel zinc) Steel Nickel Aluminum Wood Air Concrete 1 Vacuum (μ 0 ) 1 Hydrogen Copper Water

42 A Tale of Two Pfennigs During a visit by two German doctors, who were assessing the MRI system for installation into their surgery, they were dismayed by the poor quality of image, so were we. The production manager had the magnetic field switched off, and the patient handling system stripped. Two 1 pfennig coins were found. They had distorted the magnetic field.

43 Lenz s Law The direction of an induced electro-motive force is always in opposition to the change that causes it. This what happens when you use a modern cycling machine in your local gym, where settings are changed to make cycling more difficult. Pulsed gradient coils are used in imaging which interact with the main field, giving clunking noises. This is normal.

44 MRI Scan (pre op.)

45 CT Scan (post op.)

46 Part 2 (tba) MRI and cardiac gating (one of the first heart imaging systems) Magneto-cardiac effect (now called Magneto- Hydro-Dynamic effect) Research paper abstract Corruption of ECG T wave by MRI magnet Noise Figure measurement of coil pre-amplifier Data clocking problems