Gemini PET/CT137Cs及CT衰减校正对标准摄取值的影响

CT衰减校正(CTAC)在PET／CT系统中的应用缩短了检查时间，但对标准摄取值(SUV)可能存在影响。笔者对Gemini PET／CT系统所配置的^137Cs和CT衰减校正模式进行了SUV差异的比较，现报道如下。     

PET/CT显像中不同密度人体组织及残留钡剂对CT衰减校正的影响

目的探讨^18F-脱氧葡萄糖（FDG）PET／CT显像中不同密度人体组织及残留钡剂对CT衰减校正（CTAC）的影响。方法20例PET／CT显像患者，分别重建CTAC及^137Cs衰减校正（CsAC）图像。选择所有病例全身不同密度的正常组织及残留钡剂处勾画感兴趣区（ROI），分别测量CTAC及CsAC图像相同部位的标准摄取值（SUV）及相应部位的CT值。比较所测部位CTAC和CsAC的平均SUV差异及两者间图像差异。结果在所有非骨组织（脑组织除外）中，CTAC平均SUV较CsAC低17％（t=-5．328，P〈0．001）；在骨组织中，CTAC较CsAC高16％（t=7．960，P〈0．001）；在高密度钡剂中，CTAC较CsAC高98％（t=3．451，P=0．018）。在所有不同密度人体组织中，CTAC和CsAC图像间差异不影响诊断效果；在高密度钡剂中，于CTAC图像上见明显^18F—FDG异常高摄取伪影，而CsAC图像上为正常摄取影。结论不同密度人体组织及残留钡剂在CTAC和CsAC时SUV存在不同程度的差异；CTAC对骨组织及残留钡剂的SUV明显高估，并在残留钡剂区域造成图像伪影。     

不同肠道准备对腹部增强CT图像质量的影响

目前,在临床CT检查前的肠道准备方法有很多,为了探讨不同肠道准备对腹部增强CT图像质量的影响,本文收集2010年9月～2012年2月分别采用3％甘露醇和1％泛影葡胺同时、分次口服对56例腹部增强CT检查患者进行检查前肠道准备,并与采用1％肥皂水清洁灌肠后分次服用1％泛影葡胺的肠道准备方法的对照组患者进行肠道充盈效果、CT图像质量、肠道准备所需时间、不良反应对比研究,取得了良好的临床效果,现将结果报道如下.     

三种对比剂在CT增强扫描使用效果的对比分析

由于增强扫描的给药特点是荆量大、浓度高、注射速度快，在某些患者身上可能出现程度不同的副反立。本文总结我院CT室使用的60％泛影葡胺、优维显和欧乃派克三种对比剂，分别观察其不良反应和图像质量。结果60％泛影葡胺、优维显和欧乃派克的轻中度不良反应表现分别为7．5％，2．3％和2％，三种对比剂均未发生严重不良反应，三种对比剂所获得的CT图像质量均达到诊断要求效果。     

结肠CT扫描方法

由于结肠属于空腔脏器,常规CT扫描对结肠及邻近器官的显像,明确病变所在部位和侵及范围等,均存在着一定困难,为此,结肠CT检查须做好充分的准备。我院从1997年10月至今对疑有结肠病变的患者行CT扫描前,分别用三种不同的方法(即结肠内充盈25％泛影葡胺造影剂、气体充盈及水囊填置直肠行螺旋CT扫描。     

增强CT检查中泛影葡胺过敏反应的预防与处理

目的：探讨增强CT检查中泛影葡胺过敏反应的预防和处理措施。方法：对我们医院CT室使用60％泛影葡胺所做12547例增强扫描中的213例过敏反应进行了分析。结果：213例过敏患者中,轻度反应208例,中度反应4例,重度反应1例。结论：提高对泛影葡胺过敏反应的认识,在增强CT扫描前预防用药,并及时正确处理碘过敏反应,对降低泛影葡胺的毒副作用,保证患者的儿命安全十分重要。     

连续摄片及泛影葡胺行子宫输卵管造影

目的：探讨连续摄片及应用泛影葡胺造影利对下孕症患者行子宫输卵管造影的价值。方法：采用60％泛影茼胺510ml注入宫腔，同时连续摄片，每秒2-4帧，以子宫输卯营完全充盈满意为准。结果：60例子宫输卯管造影均显示满意，图像清晰。28例子宫输卵管通畅，19例患者一侧输卯管未显影，11例患者一侧输卵管通而不畅。2例子宫发育不良，为幼小子宫．结论：连续摄片加泛影葡胺与传统的普通摄片加碘化油相比有明显的优越性，使用更方便安全，图像是清楚。     

口服不同对比剂对PET／CT胃肠道充盈及FDG摄取影响

目的 比较PET／CT显像前口服泛影葡胺、甘露醇和清水对胃肠道显示和脱氧葡萄糖（FDG）摄取的差异。方法将61例无胃肠道疾病行PET／CT显像的患者按随机数字法分成3组,组1（25例）显像前口服质量分数1％泛影葡胺1L;组2（20例）显像前口服质量分数2．5％甘露醇1L;组3（16例）显像前口服清水1L。患者注射18F—FDG（按体质量5．55MBq/kg）后50min应用二维（2D）模式进行PET／CT显像。由3位核医学科医师分别应用目测分析法将胃肠道充盈状态和FDG摄取情况分为4类：无、轻度、中度和重度。应用秩和检验、配对t检验对相关数据进行统计学分析。结果组2患者口服甘露醇前后血糖和血胰岛素水平差异无统计学意义。组2患者胃肠道充盈好于组1;除直肠外,其余部位充盈状态好于组3。组1患者胃、空肠、升结肠和横结肠充盈状态好于组3患者。组3患者胃、空肠和回肠FDG摄取程度高于组2患者（z=-3．192,-3．290,-3．290,P〈0．05）;空肠FDG摄取高于组1（z=-3．603,P〈0．05）,升结肠FDG摄取低于组1（z=-2．706,P〈0．05）,横结肠和降结肠FDG摄取均明显低于其他2组（Z=-3．503,-2．403,-4．223,-4．027,P〈0．05）,直肠FDG摄取明显低于组2（Z=-4．128,P〈0．01）。组1患者胃、空肠、回肠和升结肠最大CT值分别为（132±23）,（191±31）,（313±47）和（374±53）HU,差异有统计学意义（t=-7．088～-1．781,P〈0．01）。结论口服甘露醇作为增强剂胃肠道充盈好,FDG生理性摄取相对较低。     

腹部水模不同管电压、管电流CT扫描的研究

目的通过不同的管电压、管电流(m As)对水模反复扫描,探索最适腹部低剂量图像的扫描条件。方法将碘剂、生理盐水按比例配制成实验水模,固定于CT质量确认模上,置于扫描床中心。选择人体腹部扫描条件,固定扫描条件(层厚、螺距、FOV等)不变,仅改变m As或管电压,对配有不同碘浓度的七只试管进行扫描,测量CT图像的一定感兴趣区(ROI)的CT值(x珋±SD,SD表示噪声值),计算得出SNR,并将常规扫描得到的CT值、图像噪声、SNR与改变扫描条件后的进行比较。观察不同管电压、管电流对CT值、图像噪声值及SNR的影响。结果 1)管电压为120k V时,管电流降到60m As时,图像质量明显下降,达不到临床诊断标准;管电压为100k V,管电流降到120m As时,图像质量明显下降,达不到临床诊断标准;管电压为80k V,管电流为200m As或低于200m As时,图像质量达不到临床诊断标准;管电压为70k V,管电流为200m As或低于200m As时,图像质量达不到临床诊断标准;2)CT值、SNR值均随对比剂碘浓度的升高而升高;CT值随电压升高而降低,而电压为100k V时,SNR值最大。结论腹部低剂量检查时,推荐扫描条件100k V,150m As,既能降低辐射剂量,也能得到接近管电压120k V、200m As图像质量的图像。     

背景因素对模拟结节CT值的影响

目的探讨在相同扫描条件下,背景密度的变化对模拟结节CT值测量的影响。方法建立实验模型:选取直径10cm薄壁聚乙烯材质圆柱形容器作为实验模型的主体,将0.5%的复方泛影葡胺溶液(CT值约50HU),分装入橡胶囊内,制作成直径为1.5cm的球体胶囊,模拟结节灶,固定于容器中心,制成实验模型。选取空气、油脂、蒸馏水及不同密度的复方泛影葡胺溶液等15种物质,依次充填入容器内,作为结节灶的背景介质,每更换一种介质进行一次螺旋扫描。扫描参数:电压120kV,电流50mA,FOV 15cm,准直0.75mm,标准算法重建,重建函数:B40fmedium,重建层厚2mm。采用统一标准在sygo工作站处理图像并测量各组背景介质中结节灶的CT值,用SPSS17.0软件进行统计学处理。结果-1 000～20HU密度背景下,结节灶的测量CT值随背景密度增高而减小,P<0.05;在40～120HU密度背景下,结节灶的测量CT值无显著差异,P>0.05;140～430HU密度背景下,结节灶的测量CT值随背景密度增高而增高,P<0.05。结论不同密度的均质背景对小结节灶的CT值测量具有一定的影响作用;背景密度与结节灶密度差异越大,结节灶测得的CT值越偏高。     

不同衰减校正方法及无衰减校正对PET显像结果的影响

目的 探讨CT衰减校正(CTAC)、137Cs衰减校正(CsAC)及无衰减校正(NOAC)对PET图像质量和标准摄取值(SUV)的影响.方法 对Jaszczak模型及30例患者行PET/CT显像,均分别重建CTAC、CsAC和NOAC图像.30例患者中显像未见异常者9例,肺癌7例,肝癌4例,胰腺癌3例,肠癌7例.目测比较模型显像在CTAC、CsAC和NOAC时图像分辨率、均匀性的差异,计算CrAC、CsAC时模型显像均匀区各层面感兴趣区(ROI)的最大和最小百分比非均匀性(Nut),比较在CTAC和CsAC时平均SUV的差异.在图像上对患者正常软组织和骨组织、18F-脱氧葡萄糖(FDG)高摄取病灶以及高密度残留钡剂区勾画ROI,比较各ROI CTAC和CsAC的平均SUV差异,以及CTAC、CsAC和NOAC的图像差异.采用SPSS 12.0软件,2组数据间比较行配对t检验.结果 目测比较模型显像的图像分辨率,在CTAC和CsAC时无明显差异,NOAC图像中心区域的分辨率明显下降;CTAC和CsAC图像均匀性明显优于NOAC.CTAC图像Nut为(23±2)%(最大)、(19±1)%(最小),均匀性优于CsAC[Nut为(29±3)%(最大)、(23±2)%(最小)],两者平均SUV差异无统计学意义(0.9±0.1和1.0±0.1;t=0.367,P=0.719).患者正常软组织及FDG高摄取病灶CTAC和CsAC时的平均SUV差异无统计学意义(0.71±0.20和0.75±0.23,t=-2.159,P=0.054;5.50±4.80和5.70±5.00,t=-2.032,P=0.0.54);在正常骨组织及高密度残留钡剂区中,CTAC的平均SUV明显高于CsAC(1.37±0.29和1.18±0.36,t=7.960,P=0.001;1.82±0.62和0.92±0.20,t=3.451,P=0.018).正常软组织、骨组织及FDG高摄取病灶的CTAC和CsAC图像差异不影响诊断效果.高密度残留钡剂区在CTAC图像上表现为FDG高摄取伪影,而CsAC及NOAC图像上为正常摄取.结论 CTAC和CsAC的图像分辨率无明显差异,CTAC的图像均匀性优于CsAC,两者图像质量均明显优于NOAC;CTAC对高密度物质的SUV可明显高估,且可出现FDG高摄取伪影.     

Correction for oral contrast artifacts in CT attenuation-corrected PET images obtained by combined PET/CT.

Recent studies have shown increased artifacts in CT attenuation-corrected (CTAC) PET images acquired with oral contrast agents because of misclassification of contrast as bone. We have developed an algorithm, segmented contrast correction (SCC), to properly transform CT numbers in the contrast regions from CT energies (40-140 keV) to PET energy at 511 keV. METHODS: A bilinear transformation, equivalent to that supplied by the PET/CT scanner manufacturer, for the conversion of linear attenuation coefficients of normal tissues from CT to PET energies was optimized for BaSO(4) contrast agent. This transformation was validated by comparison with the linear attenuation coefficients measured for BaSO(4) at concentrations ranging from 0% to 80% at 511 keV for PET transmission images acquired with (68)Ge rod sources. In the CT images, the contrast regions were contoured to exclude bony structures and then segmented on the basis of a minimum threshold CT number (300 Hounsfield units). The CT number in each pixel identified with contrast was transformed into the corresponding effective bone CT number to produce the correct attenuation coefficient when the data were translated by the manufacturer software into PET energy during the process of CT attenuation correction. CT images were then used for attenuation correction of PET emission data. The algorithm was validated with a phantom in which a lesion was simulated within a volume of BaSO(4) contrast and in the presence of a human vertebral bony structure. Regions of interest in the lesion, bone, and contrast on emission PET images reconstructed with and without the SCC algorithm were analyzed. The results were compared with those for images obtained with (68)Ge-based transmission attenuation-corrected PET. RESULTS: The SCC algorithm was able to correct for contrast artifacts in CTAC PET images. In the phantom studies, the use of SCC resulted in an approximate 32% reduction in the apparent activity concentration in the lesion compared with data obtained from PET images without SCC and a <7.6% reduction compared with data obtained from (68)Ge-based attenuation-corrected PET images. In one clinical study, maximum standardized uptake value (SUV(max)) measurements for the lesion, bladder, and bowel were, respectively, 14.52, 13.63, and 13.34 g/mL in CTAC PET images, 59.45, 26.71, and 37.22 g/mL in (68)Ge-based attenuation-corrected PET images, and 11.05, 6.66, and 6.33 g/mL in CTAC PET images with SCC. CONCLUSION: Correction of oral contrast artifacts in PET images obtained by combined PET/CT yielded more accurate quantitation of the lesion and other, normal structures. The algorithm was tested in a clinical case, in which SUV(max) measurements showed discrepancies of 2%, 1.3%, and 5% between (68)Ge-based attenuation-corrected PET images and CTAC PET images with SCC for the lesion, bladder, and bowel, respectively. These values correspond to 6.5%, 62%, and 66% differences between CTAC-based measurements and (68)Ge-based ones.     

Computed tomography-based attenuation correction in neurological positron emission tomography: evaluation of the effect of the X-ray tube voltage on quantitative analysis

BACKGROUND: The advent of dual-modality positron emission tomography/computed tomography (PET/CT) imaging has revolutionized the practice of clinical oncology by improving lesion localization and facilitating treatment planning for radiation therapy. In addition, the use of CT images for CT-based attenuation correction (CTAC) allows the overall scanning time to be decreased and a noise-free attenuation map (micromap) to be created. The most common procedure requires a piecewise linear calibration curve acquired under standard imaging conditions to convert the patient's CT image from low effective CT energy into an attenuation map at 511 keV. AIM: To evaluate the effect of the tube voltage on the accuracy of CTAC. METHODS: As different tube voltages are employed in current PET/CT scanning protocols, depending on the size of the patient and the region under study, the impact of using a single calibration curve on the accuracy of CTAC for images acquired at different tube voltages was investigated through quantitative analysis of the created micromaps, generated attenuation correction factors and reconstructed neurological PET data using anthropomorphic experimental phantom and clinical studies. RESULTS: For CT images acquired at 80 and 140 kVp, average relative differences of -2.9% and 0.7%, respectively, from the images acquired at 120 kVp were observed for the absolute activity concentrations in five regions of the anthropomorphic striatal phantom when CT images were converted using a single calibration curve derived at 120 kVp. Likewise, average relative differences of 1.9% and -0.6% were observed when CT images were acquired at 120 kVp and CTAC used calibration curves derived at 80 and 140 kVp, respectively. CONCLUSION: The use of a single calibration curve acquired under standard imaging conditions does not affect, to a visible or measurable extent, neurological PET images reconstructed using CTAC when CT images are acquired in different conditions.     

Cause and magnitude of the error induced by oral CT contrast agent in CT-based attenuation correction of PET emission studies.

CT images represent essentially noiseless maps of photon attenuation at a range of 40-140 keV. Current dual-modality PET/CT scanners transform them into attenuation coefficients at 511 keV and use these for PET attenuation correction. The proportional scaling algorithms hereby used account for the different properties of soft tissue and bone but are not prepared to handle material with other attenuation characteristics, such as oral CT contrast agents. As a consequence, CT-based attenuation correction in the presence of an oral contrast agent results in erroneous PET standardized uptake values (SUVs). The present study assessed these errors with phantom measurements and patient data. METHODS: Two oral CT contrast agents were imaged at 3 different concentrations in dual-modality CT and PET transmission studies to investigate their attenuation properties. The SUV error due to the presence of contrast agent in CT-based attenuation correction was estimated in 10 patients with gastrointestinal tumors as follows. The PET data were attenuation corrected on the basis of the original contrast-enhanced CT images, resulting in PET images with distorted SUVs. A second reconstruction used modified CT images wherein the CT numbers representing contrast agent had been replaced by CT values producing approximately the right PET attenuation coefficients. These CT values had been derived from the data of 10 patients imaged without a CT contrast agent. The SUV error, defined as the difference between both sets of SUV images, was evaluated in regions with oral CT contrast agent, in tumor, and in reference tissue. RESULTS: The oral CT contrast agents studied increased the attenuation for 511-keV photons minimally, even at the highest concentrations found in the patients. For a CT value of 500 Hounsfield units, the proportional scaling algorithm therefore overestimated the PET attenuation coefficient by 26.2%. The resulting SUV error in the patient studies was highest in regions containing CT contrast agent (4.4% +/- 2.8%; maximum, 11.3%), whereas 1.2% +/- 1.1% (maximum, 4.1%) was found in tumors, and 0.6% +/- 0.7% was found in the reference. CONCLUSION: The use of oral contrast agents in CT has only a small effect on the SUV, and this small effect does not appear to be medically significant.     

Respiratory motion artifacts on PET emission images obtained using CT attenuation correction on PET-CT

PET-CT scanners allow generation of transmission maps from CT. The use of CT attenuation correction (CTAC) instead of germanium-68 attenuation correction (Ge AC) might be expected to cause artifacts on reconstructed emission images if differences in respiratory status exist between the two methods of attenuation correction. The aim of this study was to evaluate for possible respiratory motion artifacts (RMA) in PET images attenuation corrected with CT from PET-CT in clinical patients. PET-CT scans were performed using a Discovery LS PET-CT system in 50 consecutive patients (23 males, 27 females; mean age 58.2 years) with known or suspected malignancy. Both CTAC and Ge AC transmission data obtained during free tidal breathing were used to correct PET emission images. Cold artifacts at the interface of the lungs and diaphragm, believed to be due to respiratory motion (RMA), that were seen on CTAC images but not on the Ge AC images were evaluated qualitatively on a four-point scale (0, no artifact; 1, mild artifact; 2, moderate artifact; 3, severe artifact). RMA was also measured for height. Curvilinear cold artifacts paralleling the dome of the diaphragm at the lung/diaphragm interface were noted on 84% of PET-CT image acquisitions and were not seen on the (68)Ge-corrected images; however, these artifacts were infrequently severe. In conclusion, RMA of varying magnitude were noted in most of our patients as a curvilinear cold area at the lung/diaphragm interface, but were not diagnostically problematic in these patients.     

PET/CT: artifacts caused by bowel motion

BACKGROUND AND AIM: In a combined positron emission tomography (PET) and computed tomography (CT) system, the CT images can be used for attenuation correction as well as for image fusion. However, quantitative and qualitative differences have been reported between CT based attenuation corrected PET and conventional transmission scan corrected PET images. The purpose of this study was to investigate potential differences in PET/CT caused by attenuation differences in bowel due to motion. METHODS: Twelve patients had PET/CT scans performed using 68Ge transmission and CT attenuation correction methods. Three emission imaging datasets were generated including CT corrected PET, Ge corrected PET, and the difference images (CT corrected PET minus Ge corrected PET). PET difference images were used to identify regions of mismatch and to quantify possible discordance between images by using standardized uptake values (SUVs). Using the Ge corrected PET as the standard, differences in emission images were classified as an overestimation (pattern A) or an underestimation (pattern B) in these difference images. RESULTS: One hundred and twenty-three mismatched areas were identified. Among them, overestimated areas in CT corrected image were detected in 36 regions (pattern A), while underestimated areas were evaluated in the remaining 87 regions (pattern B). The mean value of the difference in pattern A (mean +/- standard deviation = 0.84 +/- 0.44) was slightly higher than that in pattern B (0.60 +/- 0.23), and statistically significant. Six of 36 regions in pattern A had an SUV of greater than 2.5 in CT corrected PET but less than 2.5 in Ge corrected PET; two of 87 regions with pattern B demonstrated an SUV greater than 2.5 in Ge corrected PET and less than 2.5 in CT corrected PET. CONCLUSION: Physiological bowel motion may result in attenuation differences and subsequent differences in SUVs. Overestimation of fluorodeoxyglucose uptake should not be misinterpreted as disease.     

Use of segmented CT transmission map to avoid metal artifacts in PET images by a PET-CT device

BACKGROUND: Attenuation correction is generally used to PET images to achieve count rate values independent from tissue densities. The goal of this study was to provide a qualitative comparison of attenuation corrected PET images produced by a PET-CT device (CT, 120 kV, 40 mAs, FOV 600 mm) with and without segmentation of transmission data (ACseg+ and ACseg-respectively). Methods: The reconstructed images were compared to attenuation corrected images obtained with a high-energy transmission source (Cs-137 - 662 keV).Thirty oncologic patients were studied using CT and 137Cs for attenuation correction. All image data were acquired using the Gemini PET-CT scanner (Philips Medical Systems). It is an open PET-CT system that consists of the MX8000 multislice CT and the Allegro PET scanner arranged in a separable configuration. Images with ACseg+ and ACseg- were analyzed simultaneously in coronal, sagittal and transaxial planes. Two nuclear medicine physicians reviewed the image sets. Results: The image quality in the area of metal implants was better with ACseg+ than ACseg-, without metal induced artifacts generally observed in CT corrected images. Further the images with ACseg+ were qualitatively comparable to those obtained with 137Cs attenuation correction. Conclusions: In case of metal implants, PET studies corrected by CT should preferably use the ACseg+ method to avoid the image artifacts.     

Do implanted pacemaker leads and ICD leads cause metal-related artifact in cardiac PET/CT?

Artifacts related to metallic implants are an established limitation of CT-based attenuation correction (CT-AC) in PET/CT. However, the impact of metallic components of pacemaker leads and implantable cardioverter defibrillator (ICD) leads on the accuracy of cardiac PET has not been evaluated. The goal of this study was to investigate the magnitude of artifacts related to pacing and defibrillation leads in both phantom and patient studies. METHODS: Images were acquired on a PET/CT scanner using CT-AC and were compared with those obtained on a dedicated PET scanner using transmission source-based attenuation correction. Phantoms consisting of pacemaker leads and ICD leads submerged in uniform background activity solution were imaged, and regions were analyzed to measure radionuclide concentrations at known lead locations relative to background. In addition, 15 cardiac 18F-FDG patients (having either pacing leads, defibrillation leads, or both) were imaged on both PET/CT and PET scanners. Images were visually and quantitatively assessed to determine whether artifact related to the implanted leads was present and, if so, its severity relative to surrounding myocardium. RESULTS: In phantom studies, artifacts caused by pacing lead electrodes were barely noticeable, but artifacts arising from highly radioopaque ICD shock coil electrodes were clearly apparent. In the patient studies, no artifacts from pacing leads were identified. However, significant artifact was observed in 50% of the patient studies with ICD leads. In the affected areas, local myocardial uptake in PET/CT images using CT-AC was, on average, 30% higher than that in the corresponding PET images. CONCLUSION: Although pacemaker leads do not appear to cause artifact in cardiac PET/CT images, ICD leads frequently do result in artifacts of sufficient magnitude to impact clinical image interpretation. Accordingly, software-based corrections in CT-AC algorithms appear necessary for accurate cardiac imaging with PET/CT.     

CT-based attenuation correction in the calculation of semi-quantitative indices of [<Superscript>18</Superscript>F]FDG uptake in PET

The introduction of combined PET/CT systems has a number of advantages, including the utilisation of CT images for PET attenuation correction (AC). The potential advantage compared with existing methodology is less noisy transmission maps within shorter times of acquisition. The objective of our investigation was to assess the accuracy of CT attenuation correction (CTAC) and to study resulting bias and signal to noise ratio (SNR) in image-derived semi-quantitative uptake indices. A combined PET/CT system (GE Discovery LS) was used. Different size phantoms containing variable density components were used to assess the inherent accuracy of a bilinear transformation in the conversion of CT images to 511 keV attenuation maps. This was followed by a phantom study simulating tumour imaging conditions, with a tumour to background ratio of 5:1. An additional variable was the inclusion of contrast agent at different concentration levels. A CT scan was carried out followed by 5 min emission with 1-h and 3-min transmission frames. Clinical data were acquired in 50 patients, who had a CT scan under normal breathing conditions (CTAC(nb)) or under breath-hold with inspiration (CTAC(insp)) or expiration (CTAC(exp)), followed by a PET scan of 5 and 3 min per bed position for the emission and transmission scans respectively. Phantom and patient studies were reconstructed using segmented AC (SAC) and CTAC. In addition, measured AC (MAC) was performed for the phantom study using the 1-h transmission frame. Comparing the attenuation coefficients obtained using the CT- and the rod source-based attenuation maps, differences of 3% and <6% were recorded before and after segmentation of the measured transmission maps. Differences of up to 6% and 8% were found in the average count density (SUV(avg)) between the phantom images reconstructed with MAC and those reconstructed with CTAC and SAC respectively. In the case of CTAC, the difference increased up to 27% with the presence of contrast agent. The presence of metallic implants led to underestimation in the surrounding SUV(avg) and increasing non-uniformity in the proximity of the implant. The patient study revealed no statistically significant differences in the SUV(avg) between either CTAC(nb) or CTAC(exp) and SAC-reconstructed images. The larger differences were recorded in the lung. Both the phantom and the patient studies revealed an average increase of approximately 25% in the SNR for the CTAC-reconstructed emission images compared with the SAC-reconstructed images. In conclusion, CTAC(nb) or CTAC(exp) is a viable alternative to SAC for whole-body studies. With CTAC, careful consideration should be given to interpretation of images and use of SUVs in the presence of oral contrast and in the proximity of metallic implants.     

Does reducing CT artifacts from dental implants influence the PET interpretation in PET/CT studies of oral cancer and head and neck cancer?

In patients with oral head and neck cancer, the presence of metallic dental implants produces streak artifacts in the CT images. These artifacts negate the utility of CT for the spatial localization of PET findings and may propagate through the CT-based attenuation correction into the PET images. In this study, we evaluated the efficacy of an algorithm that reduces metallic artifacts in CT images and the impact of this approach on the quantification of PET images. METHODS: Fifty-one patients with and 9 without dental implants underwent a PET/CT study. CT images through the patient's dental implants were reconstructed using both standard CT reconstruction and an algorithm that reduces metallic artifacts. Attenuation correction factors were calculated from both sets of CT images and applied to the PET data. The CT images were evaluated for any reduction of the artifacts. The PET images were assessed for any quantitative change introduced by metallic artifact reduction. RESULTS: For each reconstruction, 2 regions of interest were defined in areas where the standard CT reconstruction overestimated the Hounsfield units (HU), 2 were defined in underestimated areas, and 1 was defined in a region unaffected by the artifacts. The 5 regions of interest were transferred to the other 3 reconstructions. Mean HU or mean Bq/cm(3) were obtained for all regions. In the CT reconstructions, metallic artifact reduction decreased the overestimated HUs by approximately 60% and increased the underestimated HUs by approximately 90%. There was no change in quantification in the PET images between the 2 algorithms (Spearman coefficient of rank correlation, 0.99). Although the distribution of attenuation (HU) changed considerably in the CT images, the distribution of activity did not change in the PET images. CONCLUSION: Our study demonstrated that the algorithm can enhance the structural and spatial content of CT images in the presence of metallic artifacts. The CT artifacts do not propagate through the CT-based attenuation correction into the PET images, confirming the robustness of CT-based attenuation correction in the presence of metallic artifacts. The study also demonstrated that considerable changes in CT images do not change the PET images.