The Science behind Magnetic Resonance Imaging (MRI)

The nucleus of an atom must contain an uneven number of protons in order to be affected by a magnetic field. The simplest atom to contain an odd number of protons is hydrogen, which contains just one proton. Presently, all clinical applications of MRI utilize the hydrogen nucleus. Hydrogen protons are abundant in the body. When located in a random direction, each proton's magnetic field will cancel each other out. However, when a patient is placed in a magnetic field, the proton becomes orientated either parallel or anti-parallel to the magnetic field. Parallel creates a low energy state and anti-parallel creates a high-energy state. When stimulated by a pulsation of radiofrequency waves from a transmitter coil, the protons flip their orientation 90° and start to rotate or resonate at a characteristic frequency, the resonant frequency. The magnetic fields of the resonating protons induce a voltage in a receiver coil, producing a signal. Immediately after the pulse, the spin starts to move out of the transverse position and start rotating out of phase, causing the signal to decay. Because the hydrogen protons of various tissues in the body are held together differently, the rates of relaxation will be different and characteristic for that tissue. These differences can then be measured and exploited to provide tissue contrast to the image.

The unique advantage of MRI is the ability to image in different planes. These planes are defined by using the three-dimensional Cartesian Coordinate System (x, y & z axes). Generally the operating factors controlling the appearance of a regular x-ray or CT (as well as ultrasound or nuclear medicine) are limited. With MRI, however, there are literally hundreds of ways to perform an exam. Depending on which pulse sequence is used, tissues will show up as black, white, and everything in between. For example, pure water such as CSF will appear black on the T1 image and white on the T2 image.

T1-Weighted Images

A radio frequency (RF) is sent in and displaces the longitudinally aligned proton by 90 degrees in the transverse plane. When the RF pulse is turned off, the protons will want to straighten themselves in the longitudinal direction, where they were in the first place. The faster they return to their original position aligned with the main magnet, the stronger the signal and the brighter the visualized structure. It is crucial to measure the emitted signal early so as to differentiate it from those sent out by the various tissues as eventually the protons will all realign and show no differentiation. The sampling time is known as TR and to maximise T1 contrast one must use a short TR sampling time.

T2 Weighted images

T2 contrast relates to transverse magnetization. In addition to the realignment in the longitudinal plane, turning off the RF signal results in what is called a 'dephasing' in the transverse plane, i.e., while in the transverse plane when the RF pulse was on, the protons were all resonating in phase. When the RF pulse was turned off they started to dephase at different rates. At this time, a follow up180° RF pulse is given which puts the dephasing protons back in phase. As time passes, these protons again become out of phase and the signal decreases. Tissues which have a long T2 remain in phase for a lengthy period of time and emit a stronger signal. Since all tissues are initially in phase, maximum T2 contrast can be obtained by delaying the sampling time in the transverse plane (a time designated as TE). Therefore for maximum T2 contrast, a long TE is desired. If we negate the T1 contrast by using a long TR and then use a short TE to negate the T2 contrast, we will be left with an image that has a contrast only different in the proton density. This image is called the proton density image.

Tissues that have minimal hydrogen protons (air, bone, and calcification) will have no signal, and therefore will appear as a signal void or black. Because the spins of flowing blood do not stay in a slice long enough to be affected by the 180° pulse, they rapidly dephase and lose signal; thus they also appear as a signal void on spin echo images. Because fats have short T1 and T2 relaxation times (i.e., they recover and decay quickly), they will appear relatively hyperintense with T1 weighting (short TR) and hypointense with T2 weighting (long TE) on spin echo images. Because pure liquids such as CSF have long relaxation times, they appear hyperintense with T2 weighting and hypointense with T1 weighting. Impure liquids such as the brain have intermediate intensity normally, but have a tendency to become more hyperintense with T2 weighting and either hypointense or isointense with T1 weighting when affected by inflammation or neoplasia.

Contrast Enhancement with MRI

On MR images, distribution of a paramagnetic agent (e.g., gadolinium-DTPA (Gd-DTPA)) will appear bright on T1 because of its ability to shorten the relaxation time of nearby hydrogen protons. Contrast medium is used to identify blood vessels, to monitor blood flow, and to enhance lesions and normal tissue such as the parenchyma of organs. With brain imaging, contrast medium normally enhances the meninges and choroid plexus and the pituitary gland because their capillaries are fenestrated and permit passive diffusion of contrast medium into their interstitium. Contrast medium does not normally enter the brain parenchyma because of the blood-brain-barrier. When this barrier breaks down, substantial leakage of proteinaceous plasma filtrate into the extracellular space of the brain from the diseased or damaged capillaries results in vasogenic oedema. This oedema migrates along the white matter fire tracts and usually does not enter the tightly integrated cortical grey matter. The abnormal signals from lesions can be differentiated from the vasogenic fluid that initially saturates the perilesional space by the process of contrast enhancement. On T1-weighted images, the Gd-enhanced lesion will appear hyperintense and may be surrounded by hypointense perilesional edema.

Spinal MRI

The spatial resolution of MRI generally is not as good as CT for spinal imaging but the greater contrast resolution that is provided by MRI ensures great anatomic detail. This especially pertains to the structure of the disc and the spinal cord itself. The image sequencing used determines the appearance of the normal spine. With

T1-weighted images (T1W), intervertebral discs are of nearly uniform medium signal intensity, slightly greater than that of the spinal cord. The spinal cord and nerve roots are isointense, with slightly less intensity than the disc. Epidural fat has a short T1 relaxation time and so is hyperintense providing great contrast with the surrounding structures. Cortical bone of the vertebrae appears as a black shell in all imaging sequences owing to its relative lack of hydrogen. It is difficult to define the longitudinal and interarcuate ligaments except over the disc spaces.

On T2-weighted (T2W), normal intervertebral discs are characterized by a high-signal central area surrounded by a medium-signal area. This is due to the varying content of ground substance within the structure of the disc. The ground substance is composed of hyaluronic acid and glycosaminoglycans that have a strong negative charge therefore attract and hold water. Epidural fat has medium signal intensity, considerably lower than that seen on T1W images. Heavily T2W images show an area of high signal intensity surrounding the spinal cord, creating a natural myelogram effect. As with the T1 images, cortical bone and the ligaments of the spine are of low signal intensity and cannot be resolved.

Disc Disease

MRI is becoming the imaging modality of choice for the evaluation of degenerative spinal disease in centers that have this facility. This is because MRI has a high sensitivity for the evaluation of disc degeneration. Most reports in veterinary medicine are limited to the study of LS disease in dogs. Disc degeneration is best seen on sagittal T2W images as partial or complete loss of the normal high signal within the nucleus pulposus and inner annular portions of the disc. This again is due to the variations in the content of ground substance in the disc (decreased in the dehydrated degenerative disc). The presence of a normal signal within a disc on T2W images can help rule out disc degeneration.

Anatomic detail is best seen on T1W images because of high signal-to-noise ratio and sharp contrast between high signal epidural fat. The intervertebral disc protrusion can be seen as a dorsal displacement of the disc into the spinal canal, loss of the normal shape of the disc and displacement of the epidural fat on sagittal images. The images can also define a lateralized disc by virtue of its occlusion of the intervertebral foramen and its subsequent affect on the periradicular fat. Transverse images can be helpful with the assessment of the degree of narrowing of the vertebral canal by permitting the visualization of the cross-sectional area of the vertebral canal.

T1W contrast medium such as gadolinium-DTPA has two major applications in the evaluation of degenerative spinal disease. Firstly, the differentiation of a lateral disc herniation from a nerve root tumour, the latter enhancing with contrast administration. This is only possible if the images are taken immediately after the contrast administration as the disc material will contrast enhance after time. Secondly, the use of contrast to differentiate recurrent disc herniation from scar formation at the site of a previous surgery has been described. A scar will typically enhance uniformly, whereas herniated disc material will not do this immediately.