Parathyroid Scintigraphy - European Association of Nuclear Medicine

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• • • • windows for 99m Tc (-10% to +5% about the peak at 140 keV) and 123 I (-5% to +10% about the 159-keV peak). Acquire 2-min frames for 20 min using a zoom of 4.0 and a matrix of 128×128. A dynamic acquisition is preferable to a static one as movement correction, provided it is in the x- or y-direction, can be applied to the images. The large zoom is chosen in order to increase spatial resolution. However, this will increase noise in the image and so the acquisition must be of sufficient duration to compensate for this. Pure iodide images may be acquired for 10 min. As well as being critical for processing, these pure iodide images can be beneficial in reducing false-positive cases due to thyroid disease. Without any patient movement, inject 99m Tcsestamibi followed by a 10-ml saline flush, between the 11th and 12th minute and continue the acquisition for 30 min. As a large zoom has been used, it is advisable at the end of the dynamic acquisition to carry out an unzoomed image to include the salivary glands and the heart. This will ensure the detection of any ectopic parathyroid glands, which can occur in the region of the unzoomed image. Acquire this image on the same dual-isotope settings for 300 s onto a matrix of 256×256. 0 Processing In order to detect the increased uptake of 99m Tc-sestamibi in the parathyroid tissue it is necessary to subtract the 123 I image from the 99m Tc-sestamibi image. The precise computer protocol will vary from centre to centre and even from camera system to camera system. However, all protocols will follow the same basic steps. Movement correction As the images have been acquired simultaneously, there will be no need to match the positions by shifting either image, but the images should be checked for movement and any correction algorithms applied before commencing. This can be as simple as checking all the frames and rejecting any with blurring before the frames are summed. This will not help if the patient has moved to a different position rather than having coughed or swallowed deeply and returned to the original position. If overall movement has occurred, the situation may be rescued if the subsequent frames can be shifted by reference to some standard point (sometimes the hottest pixel) or even by eye. The subtraction Both sets of frames (corrected if necessary) are summed to make one 99m Tc and one 123 I image.
There will be more counts in the thyroid on the 123 I image, so it must be matched to the 99m Tc image before subtraction and a scaling factor is used so that the counts in the subtraction image are reduced by this factor, pixel by pixel. The simplest technique reduces the image to be subtracted to, for example, 30%, 40%, 50%, 60% and 70% of its original values, and these images are subtracted in turn from the 99m Tc-sestamibi image, that which gives the best result for eliminating the thyroid tissue being chosen by eye. A more automated system will draw a region of interest around the normal thyroid on the 123 I image by allowing the operator to choose the count contour line which best represents its edges. The counts in this region are then compared against the counts in the same region on the 99m Tc image. The scaling factor is calculated from the ratio of these two values. This adjusted image is then subtracted from the 99m Tc image and the results displayed as a new image. Again, there are usually two or three options offered for the operator to choose the best result. SPECT imaging Additional SPECT imaging gives increased sensitivity and more precise anatomical localisation. Acquiring both early and delayed SPECT can be a useful addition to either the dual-phase 99m Tc-sestamibi method or the dual-isotope 123 I/ 99m Tc-sestamibi subtraction method. 1 • • • • • • • • Chapter 5: Imaging protocols Early SPECT should be acquired 10–30 min after the 99m Tc-sestamibi injection and delayed SPECT at around 3 h post injection. The camera is peaked for both 99m Tc and 123 I as before. An 180 o acquisition optimises the time close to the area of interest and attenuation is not a problem with structures so close to the body surface. Acquisition should start with the camera head at 270 o ; proceed in a clockwise rotation and stop at 90 o . If the acquisition is carried out on a doubleheaded camera, a 90 o L-Mode SPECT will be useful and more counts will be collected as both heads are used for the acquisition. Contouring can be used if available, although the patient should be warned if the camera will move closer during the acquisition. It should be ensured that the zoom chosen allows adequate coverage of the mediastinum to locate any ectopic glands there. Using a zoom of 2 will ensure better spatial resolution than no zoom, and there should be enough coverage of the mediastinum. • A 64×64 matrix is sufficient for the expected EANM
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Page 3 and 4: Contents Foreword 4 Sylviane Prévo
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Page 13 and 14: Radiopharmaceuticals Linda Tutty Ra
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Page 29: Imaging protocols Nish Fernando and
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