Atlas of Human Anatomy on MRI Spine Extremities Joints Hariqbal Singh, Parvez Sheik
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Physical Principle of Magnetic Resonance ImagingCHAPTER 1

 
Magnetic resonance imaging (MRI) is based on the principle of electromagnetic character of atomic nuclei which was first described by physicist Felix Bloch and Edward Purcell in 1946. They received a Noble prize for this in 1952. However, it was long after this that nuclear magnetic resonance was used for imaging. In 1973, Lauterbur showed that images of human body could be acquired by placing a magnetic field around it. First human images were published by Damadian et al in 1977. Since then use of MRI for medical imaging has seen an exponential growth and now it is a mainstay in the field of medical diagnostics.
Electromagnetism is at the core of MRI physics. When current is passed through a wire, a magnetic field is created around it. Similarly, in a nucleus with odd number of protons or neutrons, the electrons rotating around the nucleus produce a field around them. This gives a “charge” to the nucleus, also called as the spinning charge or “the spin”. Thus these nuclei behave as tiny magnets. Hydrogen proton is the most favorable nucleus for MRI as it is widely available in the water molecules present in the body.
When these nuclei are placed in an external magnetic field (Bo), they either align along the magnetic field or against it. When the number of nuclei along the magnetic field is more as compared to those against the field, a net magnetization is created in the direction of the field.
In order to generate a signal from these spinning nuclei they have to be tipped out of alignment with B0 (i.e. out of the longitudinal plane and towards the transverse plane). The signal generated by each rotating nucleus is much stronger if the nuclei precess in unison with each other at 90 degrees to the main magnetic field. For this a second magnetic field is introduced and it is referred to as B1. This B1 should be applied perpendicular to B0, and it has to be at the resonant frequency. Radiofrequency (RF) coils are used to transmit B1. If sufficient RF pulse is applied the spins are flipped into the transverse plane. This is the 90° RF pulse and it generates the strongest signal. However, as this is a high energy state, the signal starts decaying quickly and is called free induction decay (FID). This decay or relaxation is of two types:
T1 relaxation is the relaxation in the longitudinal plane due to the spins returning to the normal equilibrium state and aligning with the main magnetic field. In T2 relaxation there is dephasing in the transverse plane (90 degree plane). Each individual proton precesses at slightly different speed. After a while, the signal from protons in transverse plane degenerates as protons start precessing out of phase with each other. This is T2 relaxation.
In human tissue T1 is usually 10 times longer than T2 which means that T2 decay occurs before T1 recovery. In actual practice the T2 dephasing time is much quicker than the ‘natural’ T2 due to inhomogenities in the magnetic field B0. This reduced T2 is called T2*.
T1W and T2W images result by manipulating the manner and frequency in which RF pulses are applied (Repetition to Time), and by changing time to start signal acquisition after RF has been applied (Time to Echo), T1-weighted or T2-weighted images can be obtained.
Pulse sequences: (1) Partial saturation (PS): It is also known as gradient echo or field echo and it uses a 90° RF pulse, (2) Spin echo (SE): A 90° pulse is followed by 180° refocusing RF pulse. (3) Inversion recovery (IR): 180° pulse is followed by a 90° pulse.
In a typical image acquisition, the basic unit of each sequence (i.e. the 90°-180° signal detection) is repeated hundreds of 2times. By altering the time to echo (TE) or time to repetition (TR), i.e. the time between successive 90° pulses, the signal contrast can be altered or weighted. For example, if a long TE is used, inherent differences in T2 times of tissues will become apparent. Tissues with a long T2 (e.g. water) will take longer to decay and their signal will be greater (or appear brighter in the image) than the signal from tissue with a short T2 (e.g. fat). In a similar manner TR governs T1 contrast. Tissue, with a long TR (water) will take a long time to recover back to the equilibrium magnetization value, therefore, a short TR interval will make this tissue appear dark compared to tissue with a short T1 (fat). When TE and TR are chosen to minimize both these weightings, the signal contrast is only derived from the number or density of spins in a given tissue. This image is said to be proton density weighted (PDW).
Air is black in all sequences because of very few protons and cortical bone is always black due to no mobility of protons.
Each volume element in the body has a different resonant frequency which depends on the protons present within it. This produces a signal which is specific to the resonant frequency of that volume element. This signal is analyzed by the computers using a mathematical technique called as Fourier analysis.
Magnet forms the main component of the MRI, it is of two types: (1) Permanent or resistive magnets used in low field scanners and are usually referred to as open MRI, (2) Superconducting magnet are used in all scanners above 1.0 Tesla. It is wound from an alloy (usually Nb-Ti) that has zero electrical ­resistance below a critical temperature. To maintain this temperature the magnet is enclosed and cooled by a cryogen containing liquid helium which has to be topped-up on a regular basis.
RF coils are needed to transmit and/or receive the MR signal. The RF coil should cover only the volume of interest. This gives an optimal signal-to-noise ratio (SNR). To achieve this there are various types of RF coils with trade-offs in terms of coverage and sensitivity, e.g. head coil being smaller in size provides better SNR. Body coil is integrated into the scanner bore and is not seen by the patient. Both these coils act as transceivers, i.e. they transmit and receive. Surface coils are used for imaging anatomy near to the coil. They are simple loop designs and have excellent SNR close to the coil but the sensitivity drops off rapidly with distance from the coil. These are only used as receivers, the body coil acting as the transmitter. Quadrature or circularly-polarized coils comprise of two coils 90° apart to improve SNR by a factor of 2½.
Advanced applications include diffusion imaging, perfusion imaging, functional MRI, spectroscopy, interventional MRI.
Possible adverse effects of MRI can be due to static magnetic field, gradients, RF heating, noise and claustrophobia.
Caution needs to be exercised while selecting patients for MRI. Patients with pacemakers, metallic implants, aneurysm clips should be excluded. Metallic objects should not be taken near the magnet as they can be injurious to the patient, personnel and equipment.
 
Special Sequences
 
Short Tau Inversion Recovery (STIR) Sequence
It is heavily T2 weighted imaging, as a result the fluid and edema return high signal intensity and it annuls out the signal from fat. The resultant images show the areas of pathology clearly. The sequence is useful in musculoskeletal imaging as it annuls the signal from normal fatty bone marrow.
 
Fluid Attenuated Inversion Recovery (FLAIR)
This is an inversion-recovery pulse sequence that suppresses or annuls out the signal from fluid/CSF. The sequence is useful to show subtle lesions in the brain and spinal cord as it annuls the signal from CSF. It is useful to bring out the periventricular hyperintense lesions, e.g. in multiple sclerosis.
Table 1.1   Time to echo and time to repetition for MR sequences
Time to Echo TE
Time to Repetition TR
T1 weighted or T1W
Short TE
Short TR
T2 weighted or T2W
Long TE
Long TR
Proton density weighted or PDW
Short TE
Long TR
Table 1.2   Signal intensity of various tissues at T1, T2 and proton density imaging
Tissue
T1
T2
Proton Density
Fat
Bright
Bright (less than T1)
Bright
Water
Dark
Bright
Intermediate bright
Cerebral gray matter
Gray
Gray
Gray
Cerebral white matter
White
Dark
Dark
TR values
TR < 500
TR > 1500
TR > 1500
TE values
TE 50 to 100
TE > 80
TE < 50
3
 
Gradient Echo Sequence
This sequence reduces the scan times. This is achieved by giving a shorter RF pulse leading to a lesser amount of disruption to the magnetic vectors. The sequence is useful in identifying calcification and blood degradation products.
 
Diffusion-Weighted Imaging
‘Diffusion’ portrays the movement of molecules due to random motion. It enables to distinguish between rapid diffusion of protons (unrestricted diffusion) and slow diffusion of protons (restricted diffusion). GRE pulse sequence has been devised to image the diffusion of water through tissues. It is a sensitive way of detecting acute brain infarcts, where diffusion is reduced or restricted.
 
MR Angiography
The most common MR angiographic techniques are time-of-flight imaging and phase contrast. In these sequences, multiple RF pulses are applied with short TRs saturate the spins in stationary tissues. This results in suppression of the signal from stationary tissues in the imaging slab. In-flowing blood is unaffected by the repetitive RF pulses, as a result, as it enters the imaging slab, its signal is not suppressed and appears hyperintense compared with that of stationary tissue. Time-of-flight imaging may be 2D, with section-by-section acquisition, or 3D, with acquisition of a larger volume. Dynamic MRA can also be performed with intravenous gadolinium when in the vascular phase of enhancement.