Iida H, Narita Y, Kado H, Kashikura A, Sugawara S, Shoji Y, Kinoshita T, Ogawa T, Eberl S
Department of Radiology, Research Institute for Brain and Blood Vessels, Akita, Japan.
J Nucl Med. 1998 Jan;39(1):181-9.
Appropriate corrections for scatter and attenuation correction are prerequisites for quantitative SPECT studies. However, in most cerebral SPECT studies, uniform attenuation in the head is assumed, and scatter is usually neglected. This study evaluated the effect of attenuation correction and scatter correction on quantitative values and image contrast.
Studies were performed in six normal volunteers (ages 22-26 yr) following intravenous 123I-IMP administration using a rotating, dual-head gamma camera. A transmission scan was acquired with a 99mTc rod source (74 MBq) placed at the focus of a symmetrical fanbeam collimator. Data were reconstructed using two attenuation coefficient (mu) maps: quantitative mu map from the transmission scan and a uniform mu map generated by edge detection of the reconstructed images. Narrow and broad beam mu values were used with and without scatter correction, respectively. Scatter was corrected with transmission-dependent convolution subtraction and triple-energy window techniques. Quantitative rCBF images were calculated by the previously validated IMP-autoradiographic technique, and they were compared with those obtained by (15)O-water and PET. SPECT and PET images were registered to MRI studies, and rCBF values were compared in 39 ROIs selected on MRI.
Clear differences were observed in rCBF images between the measured and constant mu maps in the lower slices due to the airways and in the higher slices due to increased skull attenuation. However, differences were < 5% in all cerebral tissue regions, thus assumption of uniform mu introduces little bias. The scatter correction was found to increase the image contrast significantly, i.e., rCBF increased by 20%-30% in gray matter and decreased in white matter regions by 10%-20% after scatter correction, increasing gray-to-white ratio to be close to that of PET measurement. The rCBF values from the two scatter correction were not significantly different, but the triple-energy window technique suffered from increased noise. After scatter correction, rCBF values were in good agreement with those measured by PET.
This study shows little loss in accuracy results from assuming uniform mu map. However, scatter correction is required for the quantitative rCBF values and gray-to-white ratios to approach those of PET.
对散射和衰减校正进行适当校正是定量单光子发射计算机断层扫描（SPECT）研究的前提条件。然而，在大多数脑SPECT研究中，假定头部的衰减是均匀的，并且通常忽略散射。本研究评估了衰减校正和散射校正对定量值和图像对比度的影响。
对6名正常志愿者（年龄22 - 26岁）静脉注射123I - 异丁基腈（IMP）后，使用旋转式双头伽马相机进行研究。用置于对称扇形准直器焦点处的99mTc棒源（74 MBq）进行透射扫描。使用两种衰减系数（μ）图重建数据：来自透射扫描的定量μ图和通过重建图像的边缘检测生成的均匀μ图。分别使用窄束和宽束μ值进行有无散射校正的情况。采用与透射相关的卷积减法和三能量窗技术校正散射。通过先前验证的IMP - 放射自显影技术计算定量局部脑血流量（rCBF）图像，并将其与通过（15）O - 水和正电子发射断层扫描（PET）获得的图像进行比较。将SPECT和PET图像与磁共振成像（MRI）研究进行配准，并在MRI上选择的39个感兴趣区域（ROI）中比较rCBF值。
由于气道原因，在下层面的rCBF图像中，测量的μ图和恒定μ图之间观察到明显差异；在上层面，由于颅骨衰减增加也观察到差异。然而，在所有脑组织区域差异均<5%，因此假定均匀μ引入的偏差很小。发现散射校正显著提高了图像对比度，即散射校正后，灰质中的rCBF增加20% - 30%，白质区域减少10% - 20%，使灰质与白质比值接近PET测量值。两种散射校正方法得到的rCBF值无显著差异，但三能量窗技术噪声增加。散射校正后，rCBF值与PET测量值吻合良好。
本研究表明，假定均匀μ图导致的准确性损失很小。然而，为使定量rCBF值和灰质与白质比值接近PET值，需要进行散射校正。
