Magnetic Resonance for Evaluation of the Thorax: Technique
In order to obtain adequate images of the structures within the thorax, ECG gating is necessary. The gating techniques allow multi-slice acquisition. In the standard spin echo gated MR technique, multiple levels are acquired from thoracic inlet to diaphragm. Images of the heart are obtained at different phases in the cardiac cycle with this technique. In order to visualize one anatomic level of the heart from end-diastole to end-systole either a standard spin echo sequence, often termed the rotating gated technique, or a gradient echo technique may be utilized. this
In the rotating gated technique, the order of acquisition of a particular slice level is rotated in time such that it is sampled at different phases in the cardiac cycle. The software program will then format these images to display one anatomic level throughout the cardiac cycle. It is also possible to obtain multiple phases in the cardiac cycle at one anatomic level utilizing the gradient echo type of sequences. Cine images of the heart, aorta, and pulmonary arteries may be obtained utilizing either the standard spin echo or gradient echo technique. Each of these techniques allows anatomic and functional data to be obtained.
When comparing MR imaging to that provided by CT there is a different gray scale appearance for the structures within the thorax. On MR the lungs are seen as regions of black signal intensity similar to the black appearance of lungs on CT. However, when evaluating the soft tissues, several differences are noted. The subcutaneous fat on MR has bright signal intensity as does the fat within the mediastinum. On CT, fat is seen as regions of low, or dark attenuation. The muscles on MR are seen as regions of intermediate signal intensity that may change slightly in signal intensity depending upon the pulse sequence utilized. On CT, muscles have a homogeneous intermediate, gray attenuation. On MR, the vascular structures, including the aorta, main and left and right pulmonary arteries are seen as regions of signal void (black) and stand out quite distinctly from the surrounding mediastinal fat.