Feasibility of Improved Attenuation Correction for SPECT Reconstruction in the Presence of Dense Materials Using Dual-Energy Virtual Monochromatic CT: A Phantom Study

Objective: Computed tomography (CT)-based attenuation correction (CTAC) offers the clear benefit of reliable reconstruction of single-photon emission computed tomography (SPECT) images through its ability to achieve object-specific attenuation maps, but artifacts from dense materials often deteriorate CTAC performance. Therefore, we investigate the feasibility of CTAC in the presence of dense materials using dual-energy virtual monochromatic CT data. Methods: A sodium pertechnetate-filled cylindrical uniform phantom, with a pair of undiluted iodine syringes attached, is scanned with a dual-source CT scanner to obtain both single-energy (120 kVp) polychromatic and dual-energy (80 kVp/140 kVp with tin filtering) virtual monochromatic CT images. The single-energy and the dual-energy CT images are then converted to attenuation maps at 141 keV. SPECT images are reconstructed from 99mTc emission data of the phantom using each single-energy and dual-energy attenuation map and incorporating CTAC procedure. A region-of-in- terest analysis is performed to quantitatively compare the attenuation maps between the single-energy and the dual-energy techniques, each at an iodine-free position and a position adjacent to the iodine solutions. Results: At the iodine-free position, the phantom provides a uniform distribution of attenuation maps in both the single-energy and the dual-energy techniques. In the presence of adjacent iodine solutions, however, severe artifacts appeare in the single-energy CT images. These artifacts make attenuation values fluctuate, resulting in erroneous pixel values in the CTAC SPECT images. In contrast, dual-energy CT strongly suppresses the artifacts and hence improves the uniformity of the attenuation maps and the resultant SPECT images. Conclusions: Dual-energy CT with virtual monochromatic reconstruction has the potential to substantially reduce artifacts arising from dense materials. It has the potential to improve the accuracy of attenuation maps and the resultant CTAC SPECT images.

Paradoxical reduction of cerebral blood flow after acetazolamide loading:a hemodynamic and metabolic study with ^(15)O PET

Paradoxical reduction of cerebral blood flow （CBF） after administration of the vasodilator acetazolamide is the most severe stage of cerebrovascular reactivity failure and is often associated with an increased oxygen extraction fraction （OEF）. In this study, we aimed to reveal the mechanism underlying this phenomenon by focusing on the ratio of CBF to cerebral blood volume （CBV） as a marker of regional cerebral perfusion pressure （CPP）. In 37 patients with unilateral internal carotid or middle cerebral arterial （MCA） steno-occlusive disease and 8 normal controls, the baseline CBF （CBFb）, CBV, OEF, cerebral oxygen metabolic rate （CMRO2）, and CBF after acetazolamide loading in the anterior and posterior MCA territories were measured by （15）~O positron emission tomography. Paradoxical CBF reduction was found in 28 of 74 regions （18 of 37 patients） in the ipsilateral hemisphere. High CBFb （〉47.6 mL/100 mL/min, n = 7） was associated with normal CBFb/CBV, increased CBV, decreased OEF, and normal CMRO2. Low CBFb （〈31.8 mL/100 mL/min, n = 9） was associated with decreased CBFb/CBV, increased CBV, increased OEF, and decreased CMRO2. These findings demonstrated that paradoxical CBF reduction is not always associated with reduction of CPP, but partly includes high- CBFb regions with normal CPP, which has not been described in previous studies.

CBF/CBV maps in normal volunteers studied with ^(15)O PET:a possible index of cerebral perfusion pressure

Local cerebral perfusion pressure （CPP） is a primary factor controlling cerebral circulation and previous studies have indicated that the ratio of cerebral blood flow （CBF） to cerebral blood volume （CBV） can be used as an index of the local CPP. In this study, we investigated whether the CBF/CBV ratio differs among different brain structures under physiological conditions, by means of （15）O positron emission tomography. Nine healthy volunteers （5 men and 4 women; mean age, 47.0 ± 1.2 years） were studied by H2（15）O bolus injection for CBF measurement and by C（15）O inhalation for CBV measurement. The CBF/CBV ratio maps were created by dividing the CBF images by the CBV images after anatomical normalization. Regions of interest were placed on the CBF/CBV maps and comparing the regions. The mean CBF/ CBV ratio was highest in the cerebellum （19.3 ＋ 5.2/ min）, followed by the putamen （18.2 ± 3.9）, pons （16.4 ±4.6）, thalamus （14.5：1： 3.3）, cerebral cortices （13.2 ±2.4）, and centrum semiovale （1115 ± 2.1）. The cerebellum and putamen showed significantly higher CBF/CBV ratios than the cerebral cortices and centrum semiovale. We created maps of the CBF/ CBV ratio in normal volunteers and demonstrated higher CBF/CBV ratios in the cerebellum and putamen than in the cerebral cortices and deep cerebral white matter. The CBF/CBV may reflect the local CPP and should be studied in hemodynamicallycompromised patients and in patients with risk factors for small-artery diseases of the brain. Keywords： cerebral perfusion pressure; cerebra blood flow; cerebral blood volume; H2（15）O; C（15）O

^15O-PET定量显像评价默认模式神经网络:脑血流与氧代谢的比较研究

目的静息态功能磁共振成像(rs-fMRI)通过基于血氧水平依赖(BOLD)信号的自发振荡发现了默认模式神经网络(DMN)的存在。BOLD信号反映了脱氧血红蛋白的浓度,而该浓度依赖于局部脑血流(CBF)和脑氧代谢率(CMRO2)的关系。但是,这两者在rs-fMRI时无法区分。研究者尝试通过15O标记气体和水的定量PET显像分析DMN的功能相关区域,并比较CBF和CMRO2在发现DMN功能相关区域中的作用。方法9名正常志愿者[男5名,女4名,年龄(47.0±1.2)岁]行^15O-O2、^15O-CO气体和^15O-H2O的PET显像。通过自动显影法并转换成蒙特利尔神经病学研究所(MNI)标准化脑模板获得定量CBF和CMRO2图像。根据已完成的rs-fMRI图像于标化PET图像上设置感兴趣区(ROI)。在DMN功能相关区域分析中,对脑内所有成对的区域计算其Pearson相关系数(r),同时获得CBF和CMRO2各自发现的功能相关区域的分布图。本研究定义r>0.7为显著正相关,比较CBF和CMRO2各自的功能相关区域的分布图。结果CBF显像时在24对局部脑区域中发现显著正相关(r>0.7),而CMRO2显像时发现22对,两者间有12对神经网络区域是重复的。