softMip: a novel projection algorithm for ultra-low-dose computed tomography

Two projection algorithms are currently available for viewing computed tomography (CT) data sets: average projection (AVG) and maximum intensity projection (MIP). Although AVG images feature good suppression of image noise but reduced edge sharpness, MIP images are characterized by good edge sharpness but also amplify image noise. Ultra-low-dose (ULD) CT has very low radiation exposure but has high image noise. Maximum intensity projection images of ULDCT data sets amplify image noise and are therefore unsuitable for image interpretation in the routine clinical setting. We developed a synthesis of both algorithms that tries to unite the respective advantages. The resulting softMip algorithm was implemented in C++ and installed on a workstation. Depending on the settings used, softMip images can represent any graduation between MIP and AVG. The new softMip algorithm was evaluated and compared with MIP and AVG in terms of image noise and edge sharpness in a series of phantom experiments performed on 7 different CT scanners. Furthermore, image quality of the transition from AVG to MIP by means of softMip was compared with the image quality of simply blending AVG and MIP. Images generated with softMip showed less image noise than MIP images (P < 0.0005) and higher edge sharpness than AVG images (P< 0.0005). The softMip transition from AVG to MIP had a better ratio of edge sharpness and image noise than blending (P < 0.0005). Our results suggest that softMip is a very promising projection procedure for postprocessing cross-sectional image data, especially ULDCT data sets.     

Evaluation of small pulmonary arteries by 16-slice multidetector computed tomography: Optimum slab thickness in condensing transaxial images converted into maximum intensity projection images

OBJECTIVE: The purpose of this study was to determine the optimal slab thickness for condensing transaxial images into maximum intensity projection (MIP) images in the evaluation of small pulmonary arteries using 16-slice multidetector-row computed tomography (MDCT). METHODS: Helical computed tomography (CT) scans were obtained from lung apices to bases using 16-slice MDCT [120 kV(peak), 180 mA, beam width of 10 mm, beam pitch of 1.375, and reconstruction thickness of 1.25 mm] in 29 patients suspected of having a pulmonary embolism. Four kinds of image series (1.25-mm thick original transaxial source images and 3 kinds of reconstructed images using the MIP technique with slab thicknesses of 2.5 mm, 5 mm, and 10 mm) were obtained from each patient and forwarded to monitors of a picture archiving and communication system for analysis by 2 independent observers. The observers recorded the name of the segmental (20 total; 10 in each lung) and subsegmental (40 total; 20 in each lung) arteries that were traceable in each image series. Image quality of the 4 image types were graded into 5 scales based on their degree of vascular opacification, the sharpness of the vascular margins of the contrast-enhanced CT angiograms, and the visibility of lung parenchyma (excellent [5] to nondiagnostic [1]) and compared. RESULTS: In both the 1.25-mm thick original transaxial and 2.5-mm thick MIP images, a higher percentage of subsegmental arteries was traceable (91.3% [2119/2320 observations] and 87.2% [2023/2320 observations], respectively; P <0.05) than in the 5-mm and 10-mm thick MIP images (66.4% [1540/2320] and 40.5% [940/2320], respectively). No statistically significant difference was observed between the 1.25-mm thick transaxial and 2.5-mm thick MIP images in this respect. Image quality of 2.5-mm thick MIP images was superior to that of the 5-mm and 10-mm thick MIP images (P < 0.0001). No statistically significant difference was found between the scores of the image quality of the 1.25-mm thick original transaxial images and the 2.5-mm thick MIP images. CONCLUSION: After reducing the image number by one half, 2.5-mm thick MIP images using 16-slice MDCT are found to provide satisfactory images, which are comparable to 1.25-mm thick transaxial images for the analysis of subsegmental pulmonary arteries in patients suspected of pulmonary embolism.     

Whole-body positron emission tomography: Part I. Methods and performance characteristics.

Methods for whole-body PET imaging have been developed to provide a clinical tool for the detection and evaluation of primary and metastatic cancers. The axial FOV of the PET system is extended by imaging at multiple bed positions to cover the whole body. In typical rectilinear PET scans, only a small fraction of the data is collected to form two-dimensional projection images. In this work, 100% of the projection data was collected to form the two-dimensional projection images. These projection images were generated for continuous angles over 180 degrees by resorting sinogram data. In addition, tomographic images were formed by using filtered backprojection reconstruction without attenuation correction. Coronal and sagittal cuts were then extracted from the three-dimensional data set. The tomographic images were reconstructed to a resolution of 10.8 mm in all dimensions because of statistical limitations of the data. Both methods of image formation resulted in images of high quality with the tomographic reconstruction providing the highest contrast and resolution. An acquisition time of 1-2 min/bed position after a 10-mCi injection of [18F]fluoride ion or [18F]FDG was found to give a sufficient number of counts for producing images of good resolution and contrast, from a total scanning time of 32-64 min.     

Contrast-to-noise ratios in maximum intensity projection images.

We present a statistical analysis of the maximum intensity projection (MIP) algorithm, which is commonly used for MR angiography (MRA). The analysis explains why MIP projection images display as much as a twofold increase in signal- and contrast-to-noise ratios over those of the source image set. This behavior is demonstrated with simulations and in phantom and MRA image sets.     

Whole-body positron emission tomography in clinical oncology: Comparison between attenuation-corrected and uncorrected images

The clinical need for attenuation correction of whole-body positron emission tomography (PET) images is controversial, especially because of the required increase in imaging time. In this study, regional tracer distribution in attenuation-corrected and uncorrected images was compared in order to delineate the potential advantages of attenuation correction for clinical application. An ECAT EXACT scanner and a protocol including five to seven bed positions, emission scans of 9 min and post-injection transmission scans of 10 min per bed position were used. Uncorrected and attenuation-corrected images were reconstructed by filtered backprojection. In total, 109 areas of focal fluorine-18 fluorodeoxyglucose (FDG) uptake in 34 patients undergoing PET for the staging of malignancies were analysed. To measure focus contrast, a ratio of focus (target) to background average countrates (t/b ratio) was obtained from transaxial slices using a region of interest technique. Calculation of focus diameters by a distance measurement tool and visual determination of focus borders were performed. In addition, images of a body phantom with spheres to simulate focal FDG uptake were acquired. Transmission scans with and without radioactivity in the phantom were used with increasing transmission scanning times (2–30 min). The t/b ratios of the spheres were calculated and compared for the different imaging protocols. In patients, the t/b ratio was significantly higher for uncorrected images than for attenuation-corrected images (5.0±3.6 vs 3.1±1.4;P<0.001). This effect was independent of focus localization, tissue type and distance to body surface. Compared with the attenuation-corrected images, foci in uncorrected images showed larger diameters in the anterior-posterior dimension (27±14 vs 23±12 mm;P<0.001) but smaller diameters in the leftright dimension (19±11 vs 21±11 mm;P<0.001). Phantom data confirmed higher contrast in uncorrected images compared with attenuation-corrected images. It is concluded that, although distortion of foci was demonstrated, uncorrected images provided higher contrast for focal FDG uptake independent of tumour localization. In most clinical situations, the main issue of whole-body PET is pure lesion detection with the highest contrast possible, and not quantification of tracer uptake. The present data suggest that attenuation correction may not be necessary for this purpose.     

Three-dimensional projection display of striatum dopamine function measured by PET

[(11)C]CFT and[(11)C]raclopride images obtained by positron emission tomography (PET) are used to evaluate pre-synaptic dopamine transporter availability and post-synaptic dopamine D(2) receptor binding, respectively. A combined study with these tracers is useful for the differential diagnosis of Parkinson's disease. We generated three-dimensional (3D) animations of striatum PET images for the diagnosis of Parkinson's disease. Brain images of a normal subject and a typical Parkinson's disease patient with[(11)C]CFT and[(11)C]raclopride were obtained using a PET camera. Three-dimensional animations were generated from serial maximum intensity projection (MIP) images created by gradually changing the projection angle. Furthermore, the striatum images extracted from brain data were superimposed over a brain surface magnetic resonance (MR) image that was created by the volume-rendering method, and 3D animations were similarly generated. The present 3D animations were clinically useful for the differential diagnosis of brain diseases, because we were able to observe distributions of[(11)C]CFT and[(11)C]raclopride from any angle and to grasp at a glance the regional differences of distributions in reference to anatomical landmarks.     

Quantitative estimation of brain atrophy and function with PET and MRI two-dimensional projection images]

PURPOSE: The purpose of this paper is to estimate the extent of atrophy and the decline in brain function objectively and quantitatively. METHODS: Two-dimensional (2D) projection images of three-dimensional (3D) transaxial images of positron emission tomography (PET) and magnetic resonance imaging (MRI) were made by means of the Mollweide method which keeps the area of the brain surface. A correlation image was generated between 2D projection images of MRI and cerebral blood flow (CBF) or 18F-fluorodeoxyglucose (FDG) PET images and the sulcus was extracted from the correlation image clustered by K-means method. Furthermore, the extent of atrophy was evaluated from the extracted sulcus on 2D-projection MRI and the cerebral cortical function such as blood flow or glucose metabolic rate was assessed in the cortex excluding sulcus on 2D-projection PET image, and then the relationship between the cerebral atrophy and function was evaluated. This method was applied to the two groups, the young and the aged normal subjects, and the relationship between the age and the rate of atrophy or the cerebral blood flow was investigated. This method was also applied to FDG-PET and MRI studies in the normal controls and in patients with corticobasal degeneration. RESULTS: The mean rate of atrophy in the aged group was found to be higher than that in the young. The mean value and the variance of the cerebral blood flow for the young are greater than those of the aged. The sulci were similarly extracted using either CBF or FDG PET images. CONCLUSIONS: The purposed method using 2-D projection images of MRI and PET is clinically useful for quantitative assessment of atrophic change and functional disorder of cerebral cortex.     

Feasibility of a short acquisition protocol for whole-body positron emission tomography with fluorine-18 fluorodeoxyglucose

The conventional protocol for whole-body positron emission tomography (PET) with fluorine-18 fluorodeoxyglucose (FDG) requires a total acquisition time of 40-60 min, which is inconvenient for many oncological patients owing to fatigue and discomfort. This study examined the feasibility of a short protocol for whole-body PET. A phantom containing six "hot" spheres of gradually increasing diameter (10-38 mm) was imaged using a dedicated PET scanner for 20, 40, 60, 80, 120 and 600 s at various count rates. Thirty-four patients with various neoplasms underwent whole-body emission scans for 1 min per bed position 1 h after intravenous injection of 370 MBq of FDG (short protocol). A standard simultaneous transmission-emission acquisition for 10 min per bed position was performed thereafter. The images were reconstructed using an iterative algorithm. At a count rate of 40 kcps, which is close to the average count rate obtained in a whole-body FDG PET study, the 60-s image visualised five spheres, of which the smallest was 13 mm in size. Despite the better image quality, lesion detection was not improved in images acquired for more than 60 s (80-600 s). Only three of the six spheres could be detected in images acquired for less than 60 s. In the patient study, the standard protocol visualised 120 tumour lesions, of which 93 (78%) could be detected using the short protocol. Among the non-visualised lesions, 22 (82%) were Б.5 cm in size and 17 (63%) were lymph nodes. It is concluded that the proposed short protocol for whole-body FDG PET has a reasonably high detection rate and may be suitable for patients who are unable to undergo scanning for a prolonged period. It may also be useful as a pre-scan guide before a standard whole-body acquisition.     

Feasibility of a short acquisition protocol for whole-body positron emission tomography with fluorine-18 fluorodeoxyglucose.

The conventional protocol for whole-body positron emission tomography (PET) with fluorine-18 fluorodeoxyglucose (FDG) requires a total acquisition time of 40-60 min, which is inconvenient for many oncological patients owing to fatigue and discomfort. This study examined the feasibility of a short protocol for whole-body PET. A phantom containing six "hot" spheres of gradually increasing diameter (10-38 mm) was imaged using a dedicated PET scanner for 20, 40, 60, 80, 120 and 600 s at various count rates. Thirty-four patients with various neoplasms underwent whole-body emission scans for 1 min per bed position 1 h after intravenous injection of 370 MBq of FDG (short protocol). A standard simultaneous transmission-emission acquisition for 10 min per bed position was performed thereafter. The images were reconstructed using an iterative algorithm. At a count rate of 40 kcps, which is close to the average count rate obtained in a whole-body FDG PET study, the 60-s image visualised five spheres, of which the smallest was 13 mm in size. Despite the better image quality, lesion detection was not improved in images acquired for more than 60 s (80-600 s). Only three of the six spheres could be detected in images acquired for less than 60 s. In the patient study, the standard protocol visualised 120 tumour lesions, of which 93 (78%) could be detected using the short protocol. Among the non-visualised lesions, 22 (82%) were < or =1.5 cm in size and 17 (63%) were lymph nodes. It is concluded that the proposed short protocol for whole-body FDG PET has a reasonably high detection rate and may be suitable for patients who are unable to undergo scanning for a prolonged period. It may also be useful as a pre-scan guide before a standard whole-body acquisition.     

A fast progressive method of maximum intensity projection.

Real-time processing and visualization of the 3D image data are the most important requirements for medical imaging. Among various 3D visualization methods, maximum intensity projection (MIP) is a useful tool to visualize 3D medical images. However, a large computation amount is a drawback of using the MIP image in clinical diagnosis. The processing time of the MIP depends on the number of voxels of the 3D data. In order to overcome the large amount of computation for the MIP, we have developed a progressive MIP method that can perform the MIP with low-resolution for fast processing, and use the low-resolution MIP image to generate a full-resolution MIP image with a reduced computation time. In this paper, the progressive MIP method is implemented and its computation complexity is analyzed.     

Investigation of the resolution and count recovery coefficients of a whole-body PET scanner (Shimadzu SET-2400W) in 2D and 3D mode image

We measured the resolution and count recovery coefficients (RC) of the SET-2400W whole-body PET scanner (Shimadzu Co., Japan) in the 2D and 3D clinical modes. METHOD: The 3D images were reconstructed by using the full 3D image reconstruction method (3-D reprojection algorithm: 3DRP) and the Fourier rebinning method (FORE). The 2D images were reconstructed with conventional filtered back-projection method (FBP). The measurements of resolution and recovery coefficient were according to JRIA (Japan Radioisotope Association) protocols. RESULTS: The transaxial resolutions of all methods were better than 7 mm FWHM at a radius of 10 cm with 1.25 cm-1 cutoff frequency. The average slice width of 2D FBP, 3DRP and FORE are 5.8 mm, 8.0 mm and 6.8 mm respectively at the center of transaxial field of view. The RC values were measured in a range from 10 mm to 27 mm at 6 cm from the center with the cylindrical and spherical hot area phantoms. In all methods, RC values at 27 mm diameter were nearly 1.0 in both type of hot area. RC values at 10 mm diameter in 2D FBP, 3DRP and FORE of cylindrical hot area were 0.69, 0.72, 0.73 and those of spherical hot area were 0.52, 0.51, 0.53 respectively. CONCLUSION: At the SET-2400W, resolution and recovery coefficient of 3D mode image under the clinical mode showed the value which did not differ from the 2D mode image.     

MSCT不同层厚的最大密度投影在肺动脉栓塞诊断中的价值

目的：比较16层螺旋CT图像后处理中不同层厚的MIP重组图像对于肺动脉栓子的检出率。方法：对于32例临床拟诊为肺动脉栓塞的患者,采用层厚5mm的胸部增强扫描,利用原始数据分别行0.75mm组,2.mm组,5mm组,10mm组MIP重建。显示清楚的叶、段、亚段肺动脉内有无栓子,并行χ^2检验。结果：CT诊断肺动脉栓塞23例,病变共累及肺动脉72支,对于肺动脉主干及各叶肺动脉栓塞,前4组图像检出率分别为100%、100%、95.2%、90.5%。对于肺段及亚段肺动脉栓塞的检出,0.75mm组及2.mm组（检出率分别为96.1%、90.2%）明显高于5mm组,10mm组（检出率分别为82.9%、60.8%）,0.75mm组及2.0mm组在统计学上没有显著差异（P值为0.433）。2.mm组与5.0mm组、10mm组在统计学上有显著差异。（P值分别为0.004,0.001）。结论：多层螺旋CT,2mm层厚MIP重建图像能明显提高段、亚段肺动脉栓子的显示率,又能减少图像数目,有重要的诊断价值,5.0mm组1、0mm层厚MIP重建图像定位准确,直观明了,为重要的补充,MIP后处理技术为检出肺动脉栓子的最佳技术。     

肺癌的MR全身DWI与PET成像的对照研究

目的：探讨MR DWI及PET成像在肺癌诊断及鉴别诊断中的应用价值。方法：26例临床高度怀疑为肺癌且接受了PET检查的患者行MR DWI检查,通过三维图像重组及黑白翻转技术,得到＂类PET＂图像,观察病变的形态、大小和分布,并与PET影像进行比较,在工作站上测量肺部病变的ADC值及SUV值,进行相关性分析。最终的诊断结果依据影像学检查、病理诊断和随访复查共同确定,其中肺癌19例,肺部炎性假瘤1例,慢性炎症4例,淋巴结反应性增生2例。结果：以病理结果为金标准,PET检出肺癌的敏感度为100%,特异度为57.1%;MR DWI的敏感度为94.7%,特异度为71.2%。肺癌与炎性病变的ADC值及SUV值差异均有统计学意义（P〈0.01）,肺癌病灶的ADC值与SUV值没有明显线性相关关系（r=-0.293,P〉0.05）。结论：MR DWI及PET成像对肺癌的诊断有较高的敏感度及特异度,两种方法互为补充,可为肺癌的诊断及鉴别诊断提供更多的信息。     

A basic study on lesion detectability for hot spot imaging of positron emitters with dedicated PET and positron coincidence gamma camera

The aim of this study was to explore the correlations of detectability and the semi-quantification for hot spot imaging with positron emitters in positron emission tomography (PET) and with a positron coincidence detection system (PCD). Phantom study results for the measurement of the lesion-to-background (L/B) ratio ranged from 2.0 to 30.3, and detectability for hot spot lesion of PET and PCD were performed to correspond to clinical conditions. The detectability and semi-quantitative evaluation of hot spots from 4.4 mm to 36.9 mm in diameter were performed from the PET and PCD images. There were strong correlations between the L/B ratios derived from PET and PCD hot spot images and actual L/B ratios; but the L/B ratio derived from PET was higher than that from PCD with a significant difference of 10% to 54.8%. The detectability of hot spot imaging of PCD was lower than that of PET at 64.8% (PCD) versus 77.8% (PET). Even the actual L/B ratio was 8.0, hot spots more than 10.6 mm in diameter could be clearly identified with PCD imaging. The same identification could be achieved with PET imaging even when the actual L/B ratio was 4.0. This detailed investigation indicated that FDG PCD yielded results comparable to FDG PET on visual analysis and semi-quantitative analysis in detecting hot spots in phantoms, but semi-quantitative analysis of the L/B ratio with FDG PCD was inferior to that with FDG PET and the detectability of PCD in smaller hot spots was significantly poor.     

Do short-time SPECT images of bone scintigraphy improve the diagnostic value in the evaluation of solitary lesions in the thoracic spine in patients with extraskeletal malignancies?

OBJECTIVE: Single photon emission computed tomography (SPECT) images provide many details of the anatomical structure. Also about bone scintigraphy, there are many reports of the improvement of diagnosis by SPECT images. Although SPECT is useful, it requires much time. So to perform SPECT for all cases is difficult in the clinical situation. Recently, due to technical improvements in gamma cameras, we can get SPECT images in a short time. We examined diagnosis of solitary hot spots of thoracic spine in cancer patients using short-time SPECT. And we considered whether short-time SPECT contributes to the precise diagnosis of the lesion. MATERIAL AND METHODS: We performed bone scintigraphy image acquisition and both planar and short-time SPECT of the chest. Short-time SPECT was acquired in 6 minutes. We selected 36 cases with malignancy, whose bone scintigraphy demonstrated a solitary accumulation hot spot in the thoracic spine. Three experienced radiologists in nuclear medicine and 4 beginners diagnosed the images. They interpreted planar, short-time SPECT and maximum intensity projection (MIP) view of the chest of each case. The observers' response data were analyzed with receiver operating characteristic (ROC) curve analysis. RESULTS: Of the three types of images, the Az (the area under ROC curve) values of short-time SPECT were the highest in all the observers except for only one beginner. Compared with experienced observers, beginners scored lower Az values of short-time SPECT. MIP images were constructed using SPECT data, but the Az values of MIP images were not higher than those of planar images. As to diagnosis, beginners tended to interprete most of the accumulations as metastatic lesions. CONCLUSION: Short-time SPECT can be helpful to some degree, but to provide greater benefit, the observers require considerable exercise and experience.     

Automated 3-dimensional elastic registration of whole-body PET and CT from separate or combined scanners.

Registration and fusion of whole-body functional PET and anatomic CT is significant for accurate differentiation of viable tumors from benign masses, radiotherapy planning and monitoring treatment response, and cancer staging. Whole-body PET and CT acquired on separate scanners are misregistered because of differences in patient positions and orientations, couch shapes, and breathing protocols. Although a combined PET/CT scanner removes many of these misalignments, breathing-related nonrigid mismatches still persist. METHODS: We have developed a new, fully automated normalized mutual information-based 3-dimensional elastic image registration technique that can accurately align whole-body PET and CT images acquired on stand-alone scanners as well as a combined PET/CT scanner. The algorithm morphs the PET image to align spatially with the CT image by generating an elastic transformation field by interpolating quaternions and translations from multiple 6-parameter rigid-body registrations, each obtained for hierarchically subdivided image subvolumes. Fifteen whole-body (spanning thorax and abdomen) PET/CT image pairs acquired separately and 5 image pairs acquired on a combined scanner were registered. The cases were selected on the basis of the availability of both CT and PET images, without any other screening criteria, such as a specific clinical condition or prognosis. A rigorous quantitative validation was performed by evaluating algorithm performance in the context of variability among 3 clinical experts in the identification of up to 32 homologous anatomic landmarks. RESULTS: The average execution time was 75 and 45 min for images acquired using separate scanners and combined scanner, respectively. Visual inspection indicated improved matching of homologous structures in all cases. The mean registration accuracy (5.5 and 5.9 mm for images from separate scanners and combined scanner, respectively) was found comparable to the mean interexpert difference in landmark identification (5.6 +/- 2.4 and 6.6 +/- 3.4 mm, respectively). The variability in landmark identification did not show statistically significant changes on replacing any expert by the algorithm. CONCLUSION: We have presented a new and automated elastic registration algorithm to correct for nonrigid misalignments in whole-body PET/CT images as well as improve the "mechanical" registration of a combined PET/CT scanner. The algorithm performance was on par with the average opinion of 3 experts.     

Breast mass assessment on chest CT: Axial,sagittal, coronal or maximal intensity projection?

ObjectivesThe goal of this work is to determine the optimal projection to detect breast masses on Chest CT.MethodsInstitutional Review Board (HIPPA compliant) approval was obtained with a waiver of consent. 10 image pairs of Chest CT images containing breast masses were selected for review by 10 chest radiologists: the pairs consisted of axial, sagittal, coronal and axial MIP images (MIP images) with each projection compared to a MIP and with one another. For each pair, the image where the mass was most conspicuous was recorded.ResultsMIPs were preferred to any cross sectional projection 82% of the time; sagittal (63%) or coronal (63%) images were preferred to the axial projection. When sagittal and coronal images were compared there was no preference.ConclusionsMIP images should be obtained and reviewed for breast pathology; sagittal or coronal projections may provide additional information.     

Automated 3-dimensional registration of stand-alone (18)F-FDG whole-body PET with CT.

Image registration and fusion of whole-body (18)F-FDG PET with thoracic CT would allow combination of anatomic detail from CT with functional PET information, which could lead to improved diagnosis or PET-based radiotherapy planning. METHODS: We have designed a practical and fully automated algorithm for the elastic 3-dimensional image registration of whole-body PET and CT images, which compensates for the nonlinear deformation due to breath-hold CT imaging. A set of 18 PET and CT patient datasets has been evaluated by the algorithm. Initially, a 9-parameter linear registration is performed by maximizing the mutual information (MI)-based cost function, between the CT and the combination of emission and transmission PET volumes, using progressively increased matrix sizes to increase speed and provide better convergence. Subsequently, lung contours on transmission maps and corresponding contours on CT volumes are automatically detected. A large number (few hundreds) of corresponding point pairs are automatically derived, defining a thin-plate-spline (TPS) elastic transformation of PET emission and transmission scans to match the CT scan. RESULTS: In all 18 patients the automatic linear registration with multiresolution converged close to the final alignment, but, in 10 cases, the nonlinear differences in the diaphragm position and chest wall were still clearly visible. The nonlinear adjustment, which was in the order of 40-75 mm, significantly improved the alignment between breath-hold CT and PET, especially in the areas of the diaphragm. Lung volumes measured from transmission and CT scans match closely after the warping has been applied. The average computation time is <40 s for the linear component and <30 s for the nonlinear component for a typical PET scan with 4-6 bed positions. CONCLUSION: We have developed a technique for automatic nonlinear registration of CT and PET whole-body images to common spatial coordinates. This technique may be applied for automatic fusion of PET with CT acquired on stand-alone scanners during normal breathing or breath-hold data acquisition.     

Interactive GPU-based maximum intensity projection of large medical data sets using visibility culling based on the initial occluder and the visible block classification

Maximum intensity projection (MIP) is an important visualization method that has been widely used for the diagnosis of enhanced vessels or bones by rotating or zooming MIP images. With the rapid spread of multidetector-row computed tomography (MDCT) scanners, MDCT scans of a patient generate a large data set. However, previous acceleration methods for MIP rendering of such a data set failed to generate MIP images at interactive rates. In this paper, we propose novel culling methods in both object and image space for interactive MIP rendering of large medical data sets. In object space, for the visibility test of a block, we propose the initial occluder resulting from a preceding image to utilize temporal coherence and increase the block culling ratio a lot. In addition, we propose the hole filling method using the mesh generation and rendering to improve the culling performance during the generation of the initial occluder. In image space, we find out that there is a trade-off between the block culling ratio in object space and the culling efficiency in image space. In this paper, we classify the visible blocks into two types by their visibility. And we propose a balanced culling method by applying a different culling algorithm in image space for each type to utilize the trade-off and improve the rendering speed. Experimental results on twenty CT data sets showed that our method achieved 3.85 times speed up in average without any loss of image quality comparing with conventional bricking method. Using our visibility culling method, we achieved interactive GPU-based MIP rendering of large medical data sets.     

Quantitative evaluation of block-iterative reconstruction image in PET: comparison of the dynamic RAMLA algorithm and OSEM algorithm

PURPOSE: Iterative reconstruction has been successfully used in whole-body PET imaging because of reductions in noise and scanning time. However, there are plural algorithms for image reconstruction such as OSEM, RAMLA and Dynamic RAMLA. Dynamic RAMLA (DRAMA) is an iterative algorithm similar to RAMLA, but the relaxation parameter is controlled in such a way that the propagation of noise from projection data to the reconstructed image. The purpose of this study was to investigate differences in the DRAMA and OSEM algorithms, in terms of real space and frequency space. METHOD: A whole-body torso phantom (NEMA image-quality phantom) filled with F-18 was scanned with a dedicated PET scanner. The smallest four spheres, with internal diameters of 10, 13, 17, and 22 mm, were filled with F-18 at an 8:1 concentration with respect to the background. The two largest spheres, with diameters of 28 and 37 mm, were not filled with water (empty). The emission scan for a 50 cm axial range varied from 5-20 minutes, to examine the effect of the count statistics on the quality of the reconstructed images. The images were reconstructed with OSEM by changing the number of subsets from 1 to 128 in each study. As to the number of iterations, both iterative reconstruction algorithms were one. As a reference standard, images with maximum counts (60 minutes) were reconstructed with a filtered-back projection method. The quality of the reconstructed images was evaluated in terms of contrast, background variability, and two-dimensional power spectrum analysis. RESULTS: As for the real space results, OSEM with more than 32 subsets decreased contrast because of images with checkerboard noise. The DRAMA algorithm provided stable contrast even when count statistics were poor. A two-dimensional power spectrum analysis also revealed that the OSEM algorithm enhanced noise components in reconstruction with more than 32 subsets. CONCLUSION: Our preliminary data suggested that the OSEM algorithm requires few iterations with a small number of subsets for whole-body imaging. However, the DRAMA algorithm provides a reasonable signal-to-noise ratio with satisfactory spatial resolution even with one iteration. Especially in clinical whole-body PET, DRAMA was most useful because of its fast convergence and small computer burden.