Comparison of noise equivalent count rate and image quality for two-dimensional and three-dimensional PET scans

The aim of this study was to investigate the correlation between noise equivalent count (NEC) rates and the signal-to-noise ratio (S/N) in reconstructed images. The NEC rates were determined using uniform 20 cm and 70 cm tall, 20 cm diameter cylinders filled with 11C. The phantoms were scanned in both two-dimensional and three-dimensional modes. The reconstructed image noise was evaluated using FBP and OSEM algorithms (4 iterations and 8 subsets). The images were filtered to a final image resolution of 6.5 mm. From the reconstructed image sets, averages and standard deviations of images were generated, from which the average image S/N (=average/standard deviation) was calculated within an 18 cm central ROI. The S/N of a central slice and an end slice was compared with the NEC. The NEC was found to have a linear relationship to the image S/N of all slices, depending on differences in noise properties specific to the reconstruction algorithm. In two-dimensional mode, although the image S/N of the central slice and the edge slice showed a linear relationship with the NEC, in three-dimensional mode, the S/N of the central slice did not show a relationship with the NEC. The linear relationship was also found in both two- and three-dimensional acquisition modes, as well as for the different activity distributions. These results indicate that the NEC is not only a measure for comparing the count rate performance of imaging systems. However, an absolute evaluation is impossible to depend on reconstruction algorithm, slice number, and phantom type.     

Evaluation of PET count rate performance

A proposal is made for the test conditions to evaluate PET count rate performance. This performance depends in a complex manner on the spatial distribution of activity and scattering material. Therefore, a combined body phantom is proposed, which is as simple as possible but which adequately simulates the range of clinical application of a whole body tomograph. Taking into account the special properties of the new block detector design, a comprehensive test procedure is developed. This includes not only the common count rate characteristic, but also checks for the accuracy of randoms estimation and count loss correction schemes, and for the occurrence of pulse pile up. This is done for different source and scatter configurations, simulating brain, cardiac, and abdominal imaging, respectively. Examples are given, based on measurements of the latest generation PET scanners, namely the CT1 PT 931/08-12 and the Scanditronix PC 2048-07WB.     

Evaluation of PET count rate performance

A proposal is made for the test conditions to evaluate PET count rate performance. This performance depends in a complex manner on the spatial distribution of activity and scattering material. Therefore, a combined body phantom is proposed, which is as simple as possible but which adequately simulates the range of clinical application of a whole body tomograph. Taking into account the special properties of the new block detector design, a comprehensive test procedure is developed. This includes not only the common count rate characteristic, but also checks for the accuracy of randoms estimation and count loss correction schemes, and for the occurence of pulse pile up. This is done for different source and scatter configurations, simulating brain, cardiac, and abdominal imaging, respectively. Examples are given, based on measurements of the latest generation PET scanners, namely the CTI PT 931/08-12 and the Scanditronix PC 2048-07WB.This article was presented at the 1st EEC workshop on accuracy determination in PET, January 19–20th. 1989 Pisa, Italy (COMAC-BME Concerted Project Characterization and Standardization of PET Instrumentation)     

Spatially dependent deadtime losses in high count rate cardiac PET.

Cardiac PET scans result in nonhomogeneous distributions of activity within the body, which might lead to great variations in singles rates around the detector ring. Conventional deadtime correction algorithms assume that the singles rates are uniform. This paper investigates singles nonuniformities during several typical cardiac scanning protocols (bolus injections of 15O-water and 82Rb, slow infusion of 18F-FDG and static imaging with FDG) and estimates how such nonuniformities might affect quantitative data. Nonuniformity was observed in all studies and was described by an asymmetry index which increased to 58% during bolus water injection, the most inhomogeneous study. These results are valid for any scanner with a ring diameter of approximately 78 cm and are independent of the amount of activity injected. Deadtime losses depend on the amount of activity and on the scanner type. Nonhomogeneities in singles can be shown to produce spatially dependent deadtime correction factors; for our scanner, these were seen to differ by up to 16% from the mean deadtime correction during bolus water injection. To demonstrate the distortions generated by average deadtime correction, the activity distribution during a clinical cardiac study was simulated using a phantom. A simple local deadtime correction and its implementation on our system are described, and the resulting improvements in both absolute and relative quantitation of the phantom study are shown.     

The effects of time varying intravascular signal intensity and k-space acquisition order on three-dimensional MR angiography image quality

The optimum infusion timing and k-space ordering for obtaining gadolinium-enhanced three-dimensional MR angiograms was determined through computer modeling using temporal contrast characteristics obtained from patient gadolinium infusion data. The effects of bolus timing were evaluated by varying the relationship between peak intravascular gadolinium concentration and the time at which the center of k space was acquired (t_ck) for sequential and centric acquisition techniques. Flow phantom experiments were performed to validate the theoretical computations. Gadolinium concentration at the time of central k-space acquisition determines intravascular signal intensity. Artifacts, including vessel broadening and edge ringing, depend on the order in which k space is collected and on how rapidly the gadolinium concentration changes. Artifacts are greatest when the center of k space is acquired before the intravascular gadolinium peak. Application of the optimal infusion timing results in preferential arterial enhancement with a minimum of artifacts in patients undergoing MR angiography.     

Variation of gamma camera photographic image density with count rate

In recent years there has been a gradual shift in gamma camera image recording toward the use of transparency films. When this occurs one encounters the intermittency effect. This effect causes the image density to increase as the count rate increases. In addition, density may also increase because the light output per count from the cathode ray tube also increases as the count rate increases. If these effects are not taken into account, images obtained at low count rates may be too light and those obtained at high count rates may be too dark. This paper discusses these phenomena and presents experimental data that shows how image density varies with respect to count rate for several commonly used films.     

Investigation of count rate and deadtime characteristics of a high resolution PET system

Count rate and deadtime characteristics were investigated for a whole body positron emission tomography system by measuring prompt, delayed, and multiple (three or more detectors) coincidence rates and single detector rates as a function of input count rate by imaging a variety of short-lived positron emitting sources as a function of time. Data were collected with cylinder, ring, and point sources for a range of energy thresholds and fields of view. The largest source of deadtime loss involved processes that led to multiple coincidences, which are primarily true or accidental events in coincidence with an unrelated event. In measuring the count rate as a function of time for each type of event, components, with decay constants of 1, 2, or 3 times that of the isotope being measured, could be resolved corresponding to 1 (true events), 2 (accidental and some multiple events), or 3 (multiple events) independent nuclear disintegrations, respectively. Analysis of true, accidental, and multiple coincidence and singles count rate data allowed identification and evaluation of the magnitude of the sources of deadtime losses and provided a basis for a deadtime correction from data available from the PET system.     

心率对64层螺旋CT冠状动脉造影图像质量的影响及对策

目的：评价心率对64层螺旋CT冠状动脉造影图像质量的影响及解决方案。方法：：将112例冠心病按心率快慢分为4组：A组≤60次／min，B组（61～74）次／min，C组（75～89）次／min，D组≥90次／min。分别对重建图像进行定量评分，比较组间差异。结果：A组与B组间、C组与D组间无统计学差异，而A组与C、D组及B组与C、D组间有统计学差异。结论：心率是64层螺旋CT冠脉造影成像质量影响因素之一，当心率≥75次／min时对冠状动脉造影图像质量产生影响。     

Influence of secondary radiation on image quality.

On the basis of observations concerning non-screen films and xerox plates a theory is proposed as to how the image quality is impaired by secondary radiation. The theory includes an explanation of why xerox plates and industrial roentgen films of a high silver content are less affected by secondary radiation than conventional roentgen films.     

Influence of radiation dose on image quality.

When the speed of a recording medium is doubled the background quantum mottle is increased by a factor square root 2. However, the signal/noise ratio is changed not in proportion to the square root of the exposure, but in a linear fashion, i.e. by a factor 2. The change in the depiction of objects with a very high attenuation difference in relation to its surroundings appears not to be linear, but proportional to the square root of the exposure. Such objects (metal wire meshes, lead bar grids) should thus be avoided in routine evaluation of image quality since they give incomplete information as to image impairment when high-speed recording media are used.     

Thoracic and abdominal MRA with gadofosveset: Influence of injection rate on vessel signal and image quality

The purpose of this study was to investigate the influence of different injection rates on the maximum signal intensity and the arterio-venous transit time of dynamic gadofosveset-enhanced first pass MR angiography (MRA). Twenty-one healthy male volunteers were examined with a time-resolved echo-shared MRA at 1.5 T. The volunteers were assigned into three groups using injection rates of either 1, 2 or 4 ml/s. The maximal signal enhancement and peak signal-to-noise ratio in the pulmonary trunk, the aortic arch, the abdominal aorta as well as both kidneys and lung parenchyma were analyzed. The arterio-venous transit time was determined. The time between maximal enhancement of the pulmonary trunk and the aortic arch was higher with the slow injection rate of 1 ml/s, while there were no differences in the time between maximal enhancement of the aortic arch and the abdominal aorta above or below the origin of the renal veins with all three injection rates. With the slow injection protocol of 1 ml/s a longer purely arterial phase of 10.5 s was achieved compared to 7.7 s with higher injection rates (p = 0.045). The time between maximal aortic signal intensity and maximal renal enhancement was equal for all injection protocols. Johanna C. Nissen and Ulrike I. Attenberger contributed equally.     

PET/CT图像采集时间对图像质量和病灶检出率的影响

目的探讨在不影响图像质量和病灶检出率前提下缩短PET／CT图像采集时间的可行性。方法18例接受PET／CT检查的患者，在CT扫描后按1、2、3min／床位依次进行3次PET扫描，共得到54帧图像。患者按体重分为以下3组：（1）45～60kg（7例），（2）61～75kg（7例），（3）76～90k（4例）。由2位有经验的医师共同阅片确定每例患者病变的位置和数目，图像质量由2位医师独立判断。结果3min／床位PET／CT图像上共检出127个病灶，其中最小的病灶直径约为3mm，除1min／床位图像漏检3个病灶外，其他所有病灶均能被1min／床位、2min／床位的图像识别。（1）～（3）组患者在3min／床位采集图像上识别的病灶数分别为71、41、15个，第1组患者的所有病灶在3种不同采集时间图像上均能被识别，第2组患者1min／床位采集图像遗漏了3个病灶，而2min／床位图像识别出的病灶数与3min／床位图像相同。第3组患者的所有病灶在3种采集图像上也均能被识别。1、2min／床位的PET图像质量均较3min／床位图像差，1min／床位图像质量最差，主要表现为图像本底噪声高、对比度差。3组患者中，第3组患者图像质量最差。结论对体重低于60瞻患者，1～3min的采集时间，病变的检出率无明显差异；而对体重60k以上患者，建议选择2～3min／床位的采集时间，对病情较重或不合作患者，根据需要可考虑选择1min／床位的采集时间。     

Data acquisition in PET imaging

Data acquisition is substantially different in PET than it is in planar nuclear medicine and SPECT. There is an entirely different set of definitions and considerations. The objective of this paper is to provide the reader with the terminology and understanding of how PET data are acquired and organized as well as the issues involved in choosing one approach over another. Sinograms and projection views will be presented as methods of storing and viewing raw PET data. Different approaches to increase axial sampling while maintaining a sufficient number of counts for each reconstructed slice are discussed, as well as the use of sinogram compression or "mashing" to reduce the storage requirements of raw PET data. The differences between 2-dimensional (2D) and 3-dimensional (3D) PET and how storage of raw 3D data may differ from that of 2D data are described. The concept of "Michelograms" as a means of displaying the nature of the axial sampling in both 2D and 3D PET is discussed. After reading this paper, the reader will be able to describe 2 methods used to store and display raw PET data and compare them with methods used in SPECT. The reader will be able to define the terms "span" and "maximum ring difference" and describe how they relate to the axial sampling in 2D and 3D PET. The reader will also be able to list 3 ways in which 3D PET differs from 2D PET.     

Examination of PET image evaluation experimentation method aiming at improved accuracy of data acquisition

Multiple data evaluation is desirable for data obtained by positron emission tomography (PET), as the data follow the Poisson distribution. Such evaluation, however, tends to be very complicated, since the count- rates change with nuclide decay. To solve this problem, we propose a new data scan protocol in this communication. With this method, the true+scatter coincidence counts were computed during the initial one-minute scanning, which was fixed as the standard. A dynamic scan was then performed with the fixed counts from the high count-rate region. Regions with +/-2.5% of the image noise of the standard image was chosen to provide the data for evaluation. These were found to be the regions of 16.5-25.5 kcps (2D) and 81.1-138.5 kcps. Image quality was found to be affected by noise (2D) and random coincidence. Using this method, multiple data could be obtained by a single experiment, and very reliable image evaluation could be done.     

图像重建条件对PET脑显像SPM分析结果的影响

目的 探讨不同图像重建条件对^18F—FDG PET脑显像统计参数图（SPM）处理结果的影响。方法对6名健康受试者分别实施真、假针刺条件下的^18F-FDG PET脑显像。分别用4种重建条件进行图像重建。对每一重建结果均用SPM软件进行对齐、归一化及平滑处理，并进行配对t检验。结果 （1）SPM校正多重比较（P校正〈0．05）的统计学处理示，4种条件的重建结果中无一像素超阈值激活；（2）SPM未校正多重比较（P未校正〈0．001）的统计学处理示，4种重建条件的SPM结果不一致。2种重建算法（滤波反投影法和有序子集最大期望值迭代法）间差异最明显，不仅激活的像素总数及区域个数有差异，激活区的空间位置也大多不一致。结论 PET图像重建条件对SPM未校正多重比较的统计学处理结果有影响，下结论要慎重。     

放射性肺炎对^18FDG PET在肿瘤放射治疗中作用影响的研究

目的研究放射性肺炎发生的时间规律、^18F-脱氧葡萄糖（FDG）PET图像的特点及对FDG EPT诊断的影响。方法选择胸部肿瘤施行放疗的患者共15例，放疗前后进行系列FDGPET全身检查。图像判断进行视觉分析和半定量分析。结果本组中有5例出现放射性肺炎，其图像特点为：病变为片状比较均匀的摄取增高影，边界与放疗照射野一致，且都在肺内靠后近胸膜处。截至随访结束时，例2及例4完全消失，而例1、例3仍有轻度摄取，例5则至放疗后13个月一直变化不大。放射性肺炎病变部位SLN随时间而减低，一般SUW在放疗后6个月内下降明显，其后变化较小。结论在放射治疗的各个阶段如果需要了解患者情况，FDG PET结果的判读应结合放疗病史及放射性肺炎不同时期的特点加以分析。