Relaxometry is the measurement of relaxation times from MR images. T1, T2 and T2* can be estimated using the appropriate pulse sequence and parameters. T2 relaxometry has been found to be useful for quantitating signal changes on T2-weighted images, for example evaluating mesial temporal sclerosis.
Details
T2 relaxometry involves measuring the T2 relaxation rate in the area of interest. The relaxation rate is best described by a combination of exponential decay curves in part due to different compartments of water in tissue but for simplicity a first approximation using a single curve is used. The minimum number of points needed to define an exponential curve is three though more is ideal. This can be done for each pixel and the T2 rates averaged or the signal from pixels in the selected volume can be averaged and the calculation performed from this.
T2 relaxation is described elsewhere. The relaxation curve was initially generated by a series of single spin echo "Hahn" sequences repeating the sequence to generate each new point on the curve. To save time the Carr-Purcell multi-echo sequence was proposed resulting in a shortening of the time as occurs in imaging. An accumulation of slight errors in the length of the 180o RF pulse can cause an artificial decrease in the T2 time so a modification of this sequence by Meiboom and Gill changing the phase of alternating 180o pulses by 90o results in correction of the error on the even echoes. A relatively long repetition rate is needed to prevent T1 effects from causing errors. Also, the echo time should be kept to a minimum to limit diffusion effects that can also falsely reduce the T2 time.
T1 and T2 relaxometry were initial used with hopes of differentiating tumors from normal tissue using these parameters alone but overlap was too large to be useful. T1 relaxometry involves measuring T1 relaxation times; best performed with an inversion recovery sequence. T2* is also discussed elsewhere.
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