5 Key MRI FactorsBy Jim McKee on December 2, 2018 | comments
I was fortunate to meet Mark Piper in 1990 when MRI imaging was in its infancy in dentistry. Mark has been the leading pioneer in TM joint imaging and provided much needed clarity to the role of the TM joint in not only patients who present with pain but, more commonly, in patients who present with occlusal abnormalities. He has developed the Piper MRI protocol, which can be given to your imaging center to provide the center with specific directions for obtaining diagnostic MRI scans. Mark has discussed five key factors to better understand MRIs.
Step One: Magnetization
In step one of MR imaging, the body part to be studied is placed into a strong external magnetic field. The magnetic fields vary from one machine to another. You should understand that the external magnetic field changes the internal magnetic fields of the tissues. Clinically, you should know the field strength of the scanner that is used by your radiologist. 1
Step Two: External Radiofrequency Signal
The second step of MR imaging utilizes a second, weaker, oscillating magnetic field to generate an external radiofrequency signal that is conducted through the tissues. This signal orients transversely to the strong external magnetic field to energize the tissues to emit their own magnetic fields or radiofrequency pulses. Nothing in Step Two is included in the MR order. 1
Step Three: Production of Images
The third step of MR image production is to vary the timing of the externally-applied radiofrequency signal and the time interval for the antenna coil to discern the emitted signals from the tissues. You should know that TR is the time interval between radiofrequency pulses, and the TE is a realignment interval for protons. By varying TR and TE intervals, scans can differentiate between lipid and water. These specific intervals are not part of the MR order.1
Step Four: Types of Spin-Echo Sequences
Step four of MR image production is to select the specific image values from the TR and TE that give so-called weighting to the contrast of lipid, water, or both. For TMJ MR, there are four sequences that are useful. You should understand that T1-weighted sequences show lipid, T2-weighted and STIR are useful to visualize water content, and Proton-Density scans improve contrast between tissues. Specification of the types of spin-echo sequences is a part of the MR order.1
Step Five: Improving Signal and Decreasing Noise
Step Five includes parameters to improve scan quality. Alternatively, if these parameters are not followed, the image quality will suffer. There is nothing in this section that is placed on the MR scan order, but if image quality is poor, look to this section for an explanation. You should find a scanning center with a dedicated TMJ surface coil.1
Understanding these five key factors will aid you in obtaining diagnostic scans for your patients. The MRI will help you to assess disk position, disk condition, condyle position and condyle condition. Understanding the anatomy of the TM joint will increase your ability to assess the risk of occlusal or pain issues which will increase your ability to have a realistic prognosis discussion during the treatment planning process.
1. Dawson, PE, Cranham JC. The Complete Dentist Manual: The Essential Guide to Being a Complete Care Dentist St. Petersburg, Widiom 2017, 217-238.