Signal vs Time Curves in Follow-Up Studies
After surgery and/or radiation therapy, precise discrimination between scar tissue and recurrent/persistent tumor is often a hard task. An easy and reliable guide for differential diagnosis may be obtained by comparing signal vs time curves of suspected lesions vs normal mucosa, muscle, or vascular structures. This approach requires a dynamic sequence (GE T1), acquiring the same section with a frame rate of 1/s, for 1 min. Signal curves are then calculated by placing regions of interest on the suspect lesion and on adjacent normal tissues (vessels, muscles, mucosa) (Fig. 1.2). Helpful information can be obtained by comparing both the average of maximum signal values and the steepness of the curve, in suspect and presumably normal tissues. The main drawback of this application is represented by its limited coverage: the dynamic sequence acquired allows just a single section to be selected after the acquisition of TSE T2 and plain SE T1 sequence.
Fig. 1.2. Time versus signal curve. Follow-up MR after radical maxillectomy for adenocarcinoma. Dynamic acquisition (single slice, one scan/s, acquisition time 60 s) after bolus injection of Gd-DTPA. By placing regions of interest on the internal carotid artery (red), suspect recurrent tumor (yellow), and presumably normal muscle (green) and mucosa (blue), a time vs signal intensity curve is calculated. Compared to normal tissue, the suspect lesion shows steeper and higher curve. The finding was confirmed at aspiration cytology (true positive)
Fig. 1.3. Parametric image obtained from the whole stack of 60 frames. Intensity is related to the slope of the time-enhancement curve. Arrows point to the confirmed recurrent tumor
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2 CT and MR Anatomy of Paranasal Sinuses: Key Elements
Roberto Maroldi, AnDREA BoRghEsi, Patrizia Maculotti
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