Case J A, Licho R, King M A, Weaver J P
Department of Nuclear Medicine, University of Massachusetts Medical Center, Worcester 01655, USA.
J Nucl Med. 1999 Apr;40(4):604-13.
Image artifacts from variable self-attenuation are recognized as major sources of diagnostic uncertainty in SPECT. For myocardial perfusion studies, an attenuation map is often obtained from a separate transmission study. However, for many applications such as bone SPECT, it has been believed to be unnecessary to obtain a transmission study to correct for the effects of attenuation. We have had significant success in clinical management of lower spine pain using bone SPECT. This success has led us to consider SPECT for the management of cervical spine pain. Cervical spine reconstructions without attenuation correction are difficult to interpret, because the high attenuation in the mandible and skull tends to decrease estimates of activity of the upper cervical spine, and the lower cervical/upper thoracic vertebrae are obscured by the shoulders. We present a technique that uses downscatter to provide attenuation correction for these acquisitions and compare it with other recognized attenuation correction techniques.
An emission study is acquired using two windows: one for obtaining the photopeak data and another for obtaining the downscattered photons. A body outline is estimated from these datasets using a projection data thresholding method. From this outline, a uniform attenuation map is created using attenuation coefficients appropriate for 99mTc in water (0.154 cm(-1)). These maps are used in SPECT reconstruction using ordered-subset expectation maximization (OSEM). This method is compared with (a) no attenuation correction (NC), (b) conventional Chang attenuation correction based on the interactive determination of the body outline from the 99mTc emission photopeak data (ChangAC) and (c) OSEM correction using attenuation maps estimated with a line source and fanbeam collimators (transAC).
Patient studies using scatterAC demonstrated a significant improvement in the uniformity of estimated cervical spine uptake in normal patients, compared with either NC or ChangAC. Results using scatterAC were similar to those of transAC. We also observed significant improvement in uniformity using scatterAC in SPECT of the lower back in obese patients, as well as the relative limitations of scatterAC versus nonuniform, transmission-based attenuation correction.
Comparisons with reconstructions using transmission data for estimating attenuation demonstrate that reasonable quantitative accuracy can be obtained in SPECT of the cervical spine using this simple attenuation estimate. Both scatterAC and transAC appear to provide consistent and expected uniform spine uptake in the cervical spines of normal patients.
可变自衰减产生的图像伪影被认为是单光子发射计算机断层扫描（SPECT）中诊断不确定性的主要来源。对于心肌灌注研究，衰减图通常是通过单独的透射研究获得的。然而，对于许多应用，如骨SPECT，人们一直认为没有必要进行透射研究来校正衰减的影响。我们在使用骨SPECT对下背部疼痛进行临床管理方面取得了显著成功。这一成功促使我们考虑将SPECT用于颈椎疼痛的管理。未经衰减校正的颈椎重建图像很难解读，因为下颌骨和颅骨的高衰减往往会降低对上颈椎活性的估计，而下颈椎/上胸椎则被肩部遮挡。我们提出了一种利用散射来对这些采集进行衰减校正的技术，并将其与其他公认的衰减校正技术进行比较。
使用两个窗口进行发射研究：一个用于获取光电峰数据，另一个用于获取散射光子。使用投影数据阈值法从这些数据集中估计身体轮廓。根据这个轮廓，使用适合水中99m锝的衰减系数（0.154 cm⁻¹）创建一个均匀衰减图。这些图用于使用有序子集期望最大化（OSEM）的SPECT重建。将该方法与（a）不进行衰减校正（NC）、（b）基于从99m锝发射光电峰数据交互式确定身体轮廓的传统张（Chang）衰减校正（ChangAC）以及（c）使用线源和扇形束准直器估计的衰减图进行OSEM校正（transAC）进行比较。
与NC或ChangAC相比，使用散射AC（scatterAC）的患者研究表明，正常患者颈椎摄取估计的均匀性有显著改善。使用散射AC获得的结果与transAC相似。我们还观察到，在肥胖患者的下背部SPECT中，使用散射AC时均匀性也有显著改善，以及散射AC与基于非均匀透射的衰减校正相比的相对局限性。
与使用透射数据估计衰减的重建方法进行比较表明，使用这种简单的衰减估计方法，在颈椎SPECT中可以获得合理的定量准确性。散射AC和transAC似乎都能在正常患者的颈椎中提供一致且预期的均匀脊柱摄取。
