Magnetic Resonance Imaging

(For a printable version without graphics, please click here)

Learning Objective - To better understand how images are produced with magnetic resonance and to review the limitations and advantages of that technique.

Medical images produced with magnetic resonance are tomographic images based on a matrix of numbers using a gray tone display in which each gray level represents the physical property of the tissue voxel (volume element). The numerical value assigned to each voxel is proportional to the intensity of the radiowave signal emanating from the tissue in which nuclei (usually hydrogen) have been perturbed by a characteristic radio frequency energy input. The contrast depicted on MR images is directly related to this radiowave signal intensity radiated from the tissue. On the image, it has arbitrarily been decided to display high signal areas as white. Areas of reduced signal are displayed as a darker gray tone.

The magnetic resonance machine is somewhat similar to the CT machine in configuration. It consists of a large frame with a center aperture in which a patient is placed on a table which can be precisely controlled. In the magnetic resonance machine, the patient actually is in the bore of a large magnet which produces the background magnetic field. Most of the magnets in clinical use are superconducting magnets although permanent magnets and resistive electromagnets are also used. The images below show the appearance of the magnetic resonance machine in the picture on the left and a diagram of the major components of the equipment on the right. In addition to the magnet producing the background magnetic field, there are gradient coils and radio frequency coils which are used to transmit energy and to encode spatial positioning. Of course, there is a digital computer to manage the information.

Safety

One of the advantages of magnetic resonance imaging is the absence of exposure to ionizing radiation. That does not mean that the patient is not exposed to any radiation. When safety is considered, most knowledgeable people believe that MR imaging is safer than techniques which are based on ionizing radiation, however, the biologic effect of exposure to an intense magnetic field is not completely certain and there is also exposure to radio-frequency radiation. Just to refresh your memory about the relationship of radio-frequency waves to x-rays on the electromagnetic spectrum. Part of that spectrum is shown below.

Imaging

The imaging situation is developed by aligning nuclei which have magnetic moment (an odd number of nucleons) in an intense background magnetic field. Many clinical MRI machines use magnetic fields as high as 1.5 tesla. Do you have some standard for comparison of field strength? Do you know the field strength of the earth? Answer #1 You probably already know that the magnetic field of the MRI machine is so intense that it must be isolated to a very controlled environment in which no ferromagnetic material is allowed. This will disqualify some patients from being studied by magnetic resonance because ferromagnetic material has been implanted in the body during a previous surgical procedure. Other patients are disqualified because they require physiologic monitoring and it is not possible to get the necessary equipment in close proximity to the magnet. Another group of patients will disqualify themselves because of the configuration and operation of the machine. The tunnel in which the patient is placed is very tight and the machine is quite noisy. Individuals who are prone to feelings of claustrophobia will find it difficult to remain in the machine for the imaging period.

When the nuclei are aligned with the background magnetic field, a pulse of radio-frequency energy causes the nuclei to move from alignment with the magnetic field so that their spin causes them to precess. With time, the angle of precession is reduced as the nuclei return to alignment with the magnetic field and, in doing this, a radio frequency signal is emitted.

 The timing of the RF pulse and signal sampling can be varied so that images (or numerical matrixes relative to signal) can be produced showing different characteristics of the tissue. Commonly used methods for medical images include signals which are weighted for, (1) proton density, (2) T1 relaxation, and (3) T2 relaxation. T1 weighted images typically have a TR (repetition time) less than 1000 milliseconds and a TE (echo time) less than 50 milliseconds. T1 images demonstrate overall signal intensity and are more "anatomical". T2 weighted images typically have TR (time of repetition) times greater than 1500 milliseconds and TE (time of echo) greater than 50 milliseconds. T2 images have high signal return from areas with increased water and this is fortunate because most pathologic processes have increased water content within the tissue. The para-saggital image below shows T1 and T2 weighted images of the same brain tumor (a high-grade glioma). The image on the left is T2 weighted while the image on the right is a T1 weighted image.

Some advantages of magnetic resonance imaging include:

1. Superior contrast resolution
2. Superior tissue characterization
3. Superior multiplanar capabilities
4. No exposure to ionizing radiation
5. Possibly safer intravenous contrast agents
6. Angiography without using contrast.

Disadvantages include:

1. Expense
2. Sensitive to motion artifacts
3. Many "technical" artifacts are possible
4. Some patients cannot be examined
5. It is difficult to monitor patients during the study
6. Prolonged imaging times

The table below shows the usefulness of MR imaging in various clinical situations.

CLINICAL USES FOR MRI

(more + ^s = more useful)

1. CNS

A. Cranial

Congenital Malformations	+ + + +
Neoplasms	+ + + +
Infarction	+ + + +
Inflammation/infection	+ + + +
Trauma/Hemorrhage	+
Seizure W/U	+ + + +
Pituitary	+ + + +
IAC W/U	+ + + +

B. Spine

Degenerative Disc
Myelopathy	+ + + +
Radiculopathy	+ + + +
Neoplasms	+ + + +
Trauma	+
Infection/inflammatory	+ + + +

2. MUSCULOSKELETAL

A. Joints

Knee	+ + + +
Hip	+ + + +
Ankle	+ + + +
Shoulder	+ + + +
Wrist	+ + + +
TMJ	+ + + +

C. Tumor Imaging                                                                                                                                                            + +

3. BODY IMAGING

Magnetic resonance imaging is not a completely mature imaging method and innovations are occurring rapidly. In the future, it is likely that very specialized equipment will be available for unique applications such as mammography. If you would like to learn more about this imaging technique and the variety of image patterns which can be produced, the URLs listed below have detailed discussions as well as many clinical images.

Issues

1) What are the possible biological effects of magnetic field and radio frequency exposure?

2) What is the cost of MR relative to CT?

3) Why are most MR uses related to the CNS or the orthopedic study of bones and joints?

Reference Citation

1. Benedetti PF. MR Imaging in Emergency Medicine. RadioGraphics July 1996;16:953-962.

This article is a good discussion of MR imaging in the acute illness/injury situation. Although this is a good general discussion of the clinical usefulness of magnetic resonance imaging, it is an optional reading and no questions from this reference will be used on the examination.

Send us comments: Dr. David Adcock, DAVID@MED.SC.EDU.

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