MRI

Radiographic Anatomy of the Skeleton

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 or urine in the bladder will appear black on the T1 image and white on the T2 image and this will be further complicated as new pulse sequences come into use. However, one basic task is paramount and that is to identify the normal anatomy and then to classify the abnormality (usually in morphology or physiology). The final image will be very operator-dependent.

Of the variety of factors affecting the appearance of an MR image we will focus on T1 and T2 contrast, and proton density and flow. These are basic MR tenets and eventually will become part of every referring physician's vocabulary.

T1

A radio frequency is sent in and displaces the longitudinally aligned proton by 90 degrees in the transverse plane. When the RF (radio frequency) 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. This sampling time is known as TR and to maximize T1 contrast one must use a short TR sampling time.

T2

T2 contrast relates to transverse magnetization. Remember that in the T1 contrast example, we first gave a 90 degree RF signal that realigned the protons in the transverse plane (transaxial). 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.

To explain this further, while in the transverse plane when the RF pulse was on, the protons were all precessing in phase. When the RF pulse was turned off they started to dephase at different rates. At this time, a follow-up 180 degree RF pulse is given which puts the dephasing protons back in phase. This signal is now measured in the transverse plane. 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.

Understanding these basic concepts will provide a framework on which we can begin to analyze and interpret an MR image. We see that the T1 contrast is essentially controlled by TR and a short TR is necessary to achieve maximum T1 contrast. Conversely, T2 contrast is controlled by TE and for maximum results, a long TE is desirable.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, or spin density, image.

IMAGE INTERPRETATION

Generally, the majority of tumors, inflammation and pathologic foci increase the tissue's free-water content and radiate a low signal on a T1-weighted image.

Certain tissues, such as bone, fibrous tissue, and calcification have little or no free water. These are characterized by a low signal on both T1- and T2-weighted images.

Blood has a variable appearance, depending on whether it is hyperacute, acute, subacute, or chronic. In some cases it is a combination. Subacute will have a high signal on both T1- and T2-weighted images. Acute or chronic will produce a signal variable on a T1-weighted image and high on a T2-weighted image.

If the fluid is proteinaceous, the signal on the T1-weighted image will be indeterminate or high, and on the T2-weighted image, it will be high.

OVERVIEW

There are three basic types of images. Those with a short TR and a short TE are heavily T1 contrast weighted. Those with a long TR and long TE are heavily T2 contrast weighted. And finally images obtained with a long TR but a short TE derive their contrast from a difference in proton density.

What we have just reviewed is called the Spin Echo or SE pulse sequence. Other common pulse sequences are the PS or partial saturation sequence and the STIR or Short T1 Inversion Recovery.

The PS sequence is highly complex and seldom used. The STIR is marked by a decrease in signal of fatty tissue which makes water- containing tissue more conspicuous. The tissue and the resulting images are very sensitive to T1 differences.

Over the next few years we will be inundated with an alphabet soup of acronyms of the latest pulse sequences--newly-minted fast scans. These scans will virtually eliminate two of the primary roadblocks to a successful MRI:

  1. The difficulty that many patients experience laying still for the length of time presently needed to do a study.
  2. Unavoidable body motion such as heartbeat, respiration, and peristalsis. The new fast sequences will bring the process down to as fast as 10 milliseconds and use flip angles that are less than 90 degrees.

Some of the names and acronyms which you will be seeing include: FLASH (Fast Low Angle Shot), GRASS (Gradient Recalled Acquisition at Steady Rate), TURBO-FLASH and ECHO PLANAR.

Gadolinium or Magnavist, produced by Berlex Laboratories, is the contrast material currently in use. Magnavist has proven valuable in numerous clinical settings and other clinical uses are now being studied.



Special thanks to:
Paul Rodregues, M.D. for information that contributed to this web site.

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