Welcome to Teaching Corner's Radiology Today.
This months topic centers on
Understanding MRIs.
Radiology Corner
Understanding MRIs
J. Todd Knudsen, D.C.
Magnetic Resonance Imaging (MRI or MR) is an extraordinarily useful tool for the chiropractic physician. MR gives us the capability of displaying anatomy in startling detail and detecting pathological processes at the earliest stages possible. MR is the procedure of choice for imaging of the spine and consequently is fast becoming the primary advanced imaging modality for the chiropractic profession.
Through the medium of this "newszine", I will discuss various important principles with regard to MRI in an attempt to demystify this important modality. I will explain basic MR physics, imaging sequence properties, contrast agents, indications and contraindications for MR studies (by body area), and MRI reporting terminology. Your questions and comments will be appreciated.
BASIC PRINCIPLES OF MRI
MRI, formerly called Nuclear Magnetic Resonance (changed for its obvious negative connotations), uses a strong magnetic field to polarize or align certain atomic nuclei (hydrogen atoms a.k.a. protons -- most commonly) as if they were small bar magnets. To give you an idea how strong that is, the earth’s magnetic field strength is on the order of 0.5 gauss (G) and 10,000 G is equal to 1.0 tesla (T). The direction or orientation of the magnetic filed is then changed, or flipped. When a burst of an appropriate radio frequency strikes these polarized atoms, the result is that these atoms undergo precession (in other words, they "wobble"). The magnetic field is then turned off or the orientation of the field returns to its original state. The energy or signal given off by the atoms as they return to their original orientation, or relax, is detected by the scanner, and thus "signal" is generated. This process is repeated several times. The kind of image we see is dependent (among other things) upon the time allowed to the atoms to relax (TR), the degree of shift of the magnetic field orientation (flip angle), and the time of and between cycles (TE).
T1 (short TR and short TE) pulse sequences:
These images show anatomic detail and "crisp" appearing anatomy. Fat containing tissues will appear brighter (have higher signal) on T1 weighted images. These are sometimes referred to as FAT images. Structures with high H2O content will appear darker (muscle, CSF).
T2 (long TR and long TE) pulse sequences:
T2 weighted images will appear slightly "grainy" or "pixely". This sequence is designed to show fluid collections, can detect tumor infiltration of marrow, infection, acute fractures with hemorrhage, or other pathological conditions that usually have with them associated edema. CSF will be bright on this imaging sequence. Degenerating discs are of lower signal intensity because they become dried out (desiccated) as they degenerate.
Proton Density (long TR and short TE) pulse sequences:
This is an intermediate sequence of images that maintains relatively good, crisp anatomical detail like T1 weighted images, and begins to take on some of the detection properties of T2 weighted images.
Gradient echo pulse sequences:
Because T2 images are very time consuming (long TR and long TE) gradient echo images are a way of gaining much of the information available in T2 weighted images (T2WI’s) in a shorter period of time. These images show joints to good advantage. The major disadvantage is that the content of bone (marrow) is not shown as well.
The purpose of this short article has been to introduce basic concepts in MR imaging. With an understanding of these basic concepts, the doctor of chiropractic will be better able to understand the radiological reports and images they receive from their Imaging Center. Future issues will deal with other important concepts pertaining to Magnetic Resonance Imaging.
Dr. Knudsen is a board certified chiropractic radiologist (D.A.C.B.R.) and is Director of Clinical Diagnostic Imaging at National College of Chiropractic in Lombard, Illinois. For more information about Dr Knudsen’s radiology consulting services, MRI imaging, or quality control issues, please call Dr. Knudsen at (800)469-9729.
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