3D volume rendering using multislice CT for dental implants

OBJECTIVES: To determine the precision and accuracy of three-dimensional (3D) volume rendering spiral multislice computed tomography (CT)-based linear measurements of the mental foramen for dental implants, in vitro, and their precision, in vivo. METHODS: Five cadaver heads were imaged by multislice spiral CT (Toshiba Aquilion) with 0.5 mm thick axial slices (0.5 mm/0.5 s of table feed) at 0.5 mm interval reconstructions. The image data sets were transferred to a networked computer workstation. Using computer graphics the data were analysed with a 3D volume rendering technique using Vitrea software. Two oral and maxillofacial radiologists, independently, made electronic linear measurements from the superior border of the mental foramen to the crest of the alveolar process. The soft tissues were removed and physical measurements made using a 3 Space (Polhemus, Colchester, VT, USA) electromagnetic digitizer with a personal computer running Windows 98. The same linear measurements of 15 patients using the same imaging methodology were performed and the precision was analysed. RESULTS: The findings showed no statistically significant inter- or intra-observer differences in vitro and in vivo, or between imaging and physical measurements in vitro (P>0.05). CONCLUSIONS: 3D multislice spiral CT imaging allows highly accurate measurements for dental implant placement in proximity to the mental foramen. Computer graphics software, using volume rendering is suitable for implant planning.     

Clinical evaluation of 2D versus 3D whole-body PET image quality using a dedicated BGO PET scanner

Purpose Three-dimensional positron emission tomography (3D PET) results in higher system sensitivity, with an associated increase in the detection of scatter and random coincidences. The objective of this work was to compare, from a clinical perspective, 3D and two-dimensional (2D) acquisitions in terms of whole-body (WB) PET image quality with a dedicated BGO PET system.Methods 2D and 3D WB emission acquisitions were carried out in 70 patients. Variable acquisition parameters in terms of time of emission acquisition per axial field of view (aFOV) and slice overlap between sequential aFOVs were used during the 3D acquisitions. 3D and 2D images were reconstructed using FORE+WLS and OSEM respectively. Scatter correction was performed by convolution subtraction and a model-based scatter correction in 2D and 3D respectively. All WB images were attenuation corrected using segmented transmission scans. Images were blindly assessed by three observers for the presence of artefacts, confidence in lesion detection and overall image quality using a scoring system.Results Statistically significant differences between 2D and 3D image quality were only obtained for 3D emission acquisitions of 3 min. No statistically significant differences were observed for image artefacts or lesion detectability scores. Image quality correlated significantly with patient weight for both modes of operation. Finally, no differences were seen in image artefact scores for the different axial slice overlaps considered, suggesting the use of five slice overlaps in 3D WB acquisitions.Conclusion 3D WB imaging using a dedicated BGO-based PET scanner offers similar image quality to that obtained in 2D considering similar overall times of acquisitions.     

Accuracy of three-dimensional (3D) craniofacial cephalometric landmarks on a low-dose 3D computed tomograph

OBJECTIVES: The aim of this paper is to compare the accuracy of cephalometric landmark identification using three-dimensional CT (3D-CT) surface rendering with "high-dose" (200 mAs) and "low-dose" (35 mAs) CT protocols. The absorbed dose levels for radiosensitive organs in the maxillofacial region during the exposure for both 3D-CT protocols were also measured. METHODS: The study population consisted of 15 human dry skulls examined with spiral 3D-CT. 12 cephalometric anatomical landmarks at 7 sites were identified on the 3D-CT surface renderings by 2 observers independently, twice each, using high-dose and low-dose protocols. In total, 1440 imaging measurements were made. Thermoluminescent dosemeters (TLDs) were placed at ten sites around the thyroid and submandibular glands and the eyes in an Alderson phantom for measuring the absorbed dose levels. RESULTS: The intraobserver mean distances between 3D landmarks were smaller for all sites with the high-dose protocol (P = 0.37). There was a significant difference among the observers (P = 0.000004). Interobserver mean distances between 3D landmarks were smaller for four of the seven sites with the low-dose protocol. However, the global interobserver mean distances between 3D landmarks for all sites were smaller with the high-dose protocol (P = 0.028). The low-dose protocol reduced the radiation dose to the thyroid by 6.12, to the submandibular salivary glands by 5.91 and to the eye by 5.44, resulting in a global reduction factor of 5.71. CONCLUSIONS: The accuracy in the landmark's identification was maintained when the milliampere-second values were reduced from 200 mAs to 35 mAs. We recommend use of the low-dose protocol for clinical 3D-CT cephalometric applications.     

Accuracy of 3D acquisition mode for myocardial FDG PET studies using a BGO-based scanner

Purpose The aim of the present study was to evaluate the quantitative and qualitative accuracy of 3D PET acquisitions for myocardial FDG studies. Methods Phantom studies were performed with both a homogeneous and an inhomogeneous phantom. Activity profiles were generated along the phantoms using 2D and several 3D reconstructions, varying the 3D scaling value to adjust the scatter correction algorithm. Furthermore, ten patients underwent a dynamic myocardial FDG PET scan, using an interleaved protocol consisting of frames with alternating 2D and 3D acquisition. For each myocardial study, 13 volumes of interest were defined, representing 13 myocardial segments. First, the optimal scaling value for the scatter correction algorithm was determined using data from the phantom and four patient studies. This scaling value was then applied to all ten patients. 2D and 3D acquisitions were compared for both static (i.e. activity concentrations in the last 2D and 3D frames) and dynamic imaging (calculation of the metabolic rate of glucose). Results For both phantom and patient studies, suboptimal results were obtained when the default scaling value for the scatter correction algorithm was used. After adjusting the scaling value, for all ten myocardial FDG studies, a very good correlation (r ² = 0.99) was obtained between 2D and 3D data. With the present protocol no significant differences were observed in qualitative interpretation. Conclusion The 3D FDG acquisition mode is accurate and has clear advantages over the 2D mode for myocardial FDG studies. A prerequisite is, however, optimisation of the 3D scatter correction algorithm.     

Accuracy of linear measurements using dental cone beam and conventional multislice computed tomography

OBJECTIVES: The aim of this study was to evaluate the accuracy of linear measurements obtained with dental cone beam CT (CBCT) and multislice CT (MSCT) by altering radiation doses using pre-operative planning of the placement of oral implants as a model. METHODS: A human cadaver mandible was examined in two edentulous areas and one dentate area using CBCT and MSCT. The mandible was examined both dry and immersed in sucrose solution isointense with soft tissue. Two readers measured four linear distances twice from each section. The mandible was cut into 4 mm thick slices at three marked places. These slices were microradiographed and used as the gold standard for measurements from each section. RESULTS: The intraclass correlations between the intra- and interobserver readings obtained with the different methods showed almost perfect matches. The measurement error (ME) showed significant differences between the methods studied (P = 0.022): the mean ME was 4.7% for CBCT and 8.8% for MSCT of the dry mandible, 2.3% and 6.6%, respectively, for the mandible immersed in sucrose solution and 5.4% for low-dose MSCT. Lowering the MSCT radiation dose to less than a quarter of its conventional original value did not significantly affect the ME. CONCLUSIONS: CBCT is a reliable tool for implant-planning measurements when compared with MSCT. In this study, a considerable radiation dose reduction could be achieved with low-dose MSCT examinations without a major loss of measurement accuracy.     

Use of a fast EM algorithm for 3D image reconstruction with the YAP-PET tomograph.

OBJECTIVE: We would like to improve the image reconstructions for both signal-to-noise ratio (SNR) and spatial resolution characteristics for the small animal positron emission tomograph YAP-PET, built at the Department of Physics of Ferrara University. The three-dimensional (3D) filtered backprojection (FBP) algorithm, usually used for image reconstruction, has a limited angle restriction due to the tomograph geometry, which causes a serious loss in sensitivity. METHODS: We implemented a 3D iterative reconstruction program using the symmetry and sparse properties of the 'probability matrix', which correlates the emission from each voxel to the detector within a coincidence tube. A fraction only of matrix elements are calculated before the reconstruction and stored on disk: this allows us to avoid on-line computation. A depth dependent function differentiates the voxels in a coincidence tube. Three experimental phantoms with no background were reconstructed by using the program, in comparison with traditionally used FBP. RESULTS: The adopted method allowed us to reduce the computation time significantly. Furthermore, the simple depth dependent function improved the spatial resolution. With 64 x 64 x 20 voxels of 0.625 x 0.625 x 2.0 mm(3) in the field of view, the computation time was less than 4 min per iteration on a Sparc Ultra 450 Workstation, and less than 6 min per iteration on a Mac-PPC G3 300 MHz: the spatial resolution measured with a 0.8 mm diameter 18F-FDG filled capillary reconstructed in this way was 2.0 mm FWHM. By decreasing the voxel size to 0.3125 x 0.3125 x 2.0 mm(3) per voxel the transaxial FWHM was 1.7 mm with a computation time of 15 min per iteration on a Sparc Ultra 450. By using all the acquired data, the SNR improves from 1.3 to 6.0 in the worst measured case, a pair of 0.8mm diameter 18F-FDG filled capillaries, which are 2.5 mm apart each other. CONCLUSION: The adoption of iterative reconstruction allowed us to overcome the loss in sensitivity of previously used FBP: this improved the SNR. The studies of symmetry and sparse properties avoided a severe increase of the reconstruction time and of storing space on disk. This fast EM Algorithm is now routinely used for the image reconstruction with the YAP-PET tomograph.     

2D multislice and 3D MRI sequences are often equally sensitive.

A simple theoretical model was developed to compare the sensitivities (i.e., signal-to-noise ratios per unit imaging time) of two-dimensional (2D) multislice and 3D imaging sequences. The model shows that the sensitivities of 3D and 2D multislice MRI sequences are usually similar. Sensitivities are identical in T2-weighted sequences when the T(R)s of the two sequences are the same. In T1-weighted gradient-echo sequences, sensitivities are very similar when Ernst angle excitation is used and the T(R) of the 2D sequence is less than T1. The predictions of the model are confirmed in phantom and animal experiments.     

Comparative evaluation of scatter correction in 3D PET using different scatter-level approximations

Objective

In 3D PET, scatter of the gamma photons is one of the most significant physical factors which degrades not only image quality but also quantification. The currently most used scatter estimation method is the analytic single scatter simulation (SSS) which usually accommodates for multiple scattering by scaling the single scatter estimation. However, it has not been clear yet how accurate this approximation is for cases where multiple scatter is significant, raising the question: “How important is correction for multiple scattered photons, and how accurately do we need to simulate all scattered events by appropriate scaling?” This study answers these questions and evaluates the accuracy of SSS implementation in the open-source library STIR.

Methods

Different scatter orders approximations are evaluated including different levels of scattering and different scaling approaches using Monte Carlo (i.e. SimSET) data. SimSET simulations of a large anthropomorphic phantom were reconstructed with iterative reconstruction algorithms. Images reconstructed with 3D filtered back-projection reprojection algorithm have been compared quantitatively in order to clarify the errors due to different scatter order approximations.

Results

Quantification in regions has improved by scatter correction. For example, in the heart the ideal value was 3, whereas before scatter correction the standard uptake value (SUV) was 4.0, after single scatter correction was 3.3 and after single and double scatter correction was 3.0. After correction by scaling single scatter with tail-fit, the SUV was 3.1, whereas with total-fit it was 3.0. Similarly, for the SSS correction methodology implemented in STIR using tail-fit the heart SUV was 3.1 whereas using total-fit it was 3.0.

Conclusions

The results demonstrate that correction for double scatter improves image contrast and therefore it is required for the accurate estimation of activity distribution in PET imaging. However, it has been also shown that scaling the single scatter distribution is a reasonable approximation to compensate for total scatter. Finally, scatter correction with STIR has shown excellent agreement with Monte Carlo simulations.

     

Improving specificity of breast MRI using prone PET and fused MRI and PET 3D volume datasets.

MRI is a sensitive method for detecting invasive breast cancer, but it lacks specificity. To examine the effect of combining PET with MRI on breast lesion characterization, a prototype positioning device was fabricated to allow PET scans to be acquired in the same position as MRI scans--that is, prone. METHODS: To test the hypothesis that fusion of (18)F-FDG PET and MRI scans improves detection of breast cancer, 23 patients with suspected recurrent or new breast cancer underwent a routine whole-body PET scan, a prone PET scan of the chest, and a routine breast MRI scan. The attenuation-corrected prone PET and MRI datasets were registered twice by different operators. The fusion results were judged for quality by visual inspection and statistical analysis. A joint reading of the MRI and PET scans side by side and integrated images was performed by a nuclear medicine physician and a radiologist. Sensitivity and specificity of MRI and combined MRI and PET scans were calculated on the basis of pathology reports or at least 1 y of clinical and radiologic follow-up. RESULTS: All fusions were verified to be well matched using specific anatomic criteria. A total of 45 lesions was assessed. Lesion size range was 0.6 to 10.0 cm. Of the 44 breasts examined, 29 were suspicious for cancer, of which 15 were found to be positive on surgical excision. In lesion-by-lesion analysis, sensitivity and specificity of MRI alone were 92% and 52%, respectively; after MRI and PET fusion, they were 63% and 95%, respectively. The positive predictive value and the negative predictive value for MRI alone were 69% and 85%, respectively; after MRI and PET fusion, they were 94% and 69%, respectively. CONCLUSION: Acquisition of prone PET scans using the new positioning device permitted acquisition of prone scans suitable for fusion with breast MRI scans. Fused PET and MRI scans increased the specificity of MRI but decreased the sensitivity in this small group of patients. Additional data are needed to confirm the statistical significance of these preliminary findings.     

Performance characteristics of a newly developed PET/CT scanner using NEMA standards in 2D and 3D modes.

This study evaluates the 2-dimensional (2D) and 3-dimensional (3D) performance characteristics of a newly developed PET/CT scanner using the National Electrical Manufacturers Association (NEMA) NU 2-1994 (NU94) and NEMA NU 2-2001 (NU01) standards. The PET detector array consists of 10,080 individual bismuth germanate crystals arranged in 24 rings of 420 crystals each. The size of each crystal is 6.3 x 6.3 x 30 mm in the axial, transaxial, and radial dimensions, respectively. The PET detector ring diameter is 88.6 cm with axial and transaxial fields of view (FOVs) of 15.7 and 70 cm, respectively. The scanner has a uniform patient port of 70 cm throughout the PET and CT FOV, and the PET scanner is equipped with retractable septa to allow 2D and 3D imaging. METHODS: Spatial resolution, scatter fraction, sensitivity, counting rate, image quality, and accuracy as defined by the NEMA protocols of NU94 and NU01 for 2D and 3D modes are evaluated. The 2D mode data were acquired with a maximum ring difference of 5, whereas the 3D mode acquisition used ring differences of 23. Both 2D and 3D mode data were acquired with an energy window of 375-650 keV. Random estimation from singles counting rate was applied to all relevant analysis. In addition, images from 2 clinical whole-body oncology studies acquired in 2D and 3D modes are shown to demonstrate the image quality obtained from this scanner. RESULTS: The 2D NU94 transaxial resolution is 6.1-mm full width at half maximum (FWHM) 1 cm off center and increases to 6.9 mm tangential and 8.1 mm radial at a radius (R) of 20 cm. NU01 2D average transaxial (axial) FWHM resolution measured 6.1 (5.2) mm at R = 1 cm and 6.7 (6.1) mm at R = 10 cm. The NU94 scatter fraction for 2D (3D) was 13% (29%), whereas the NU01 scatter fraction gave 19% (45%). NU01 peak 2D (3D) noise equivalent counting rate (T(2)/[T + R + S]) was 90.2 (67.8) kilocount per second (kcps) at 52.5 (12) kBq/mL. Total 2D (3D) system sensitivity for true events is 8 (32.9) kcps/kBq/mL for NU94 and 1.95 (9.2) kcps/Bq for NU01. CONCLUSION: The results show excellent system sensitivity with relatively uniform resolution throughout the FOV, making this scanner highly suitable for whole-body studies.     

Simultaneous acquisition of multislice PET and MR images: initial results with a MR-compatible PET scanner.

PET and MRI are powerful imaging techniques that are largely complementary in the information they provide. We have designed and built a MR-compatible PET scanner based on avalanche photodiode technology that allows simultaneous acquisition of PET and MR images in small animals. METHODS: The PET scanner insert uses magnetic field-insensitive, position-sensitive avalanche photodiode (PSAPD) detectors coupled, via short lengths of optical fibers, to arrays of lutetium oxyorthosilicate (LSO) scintillator crystals. The optical fibers are used to minimize electromagnetic interference between the radiofrequency and gradient coils and the PET detector system. The PET detector module components and the complete PET insert assembly are described. PET data were acquired with and without MR sequences running, and detector flood histograms were compared with the ones generated from the data acquired outside the magnet. A uniform MR phantom was also imaged to assess the effect of the PET detector on the MR data acquisition. Simultaneous PET and MRI studies of a mouse were performed ex vivo. RESULTS: PSAPDs can be successfully used to read out large numbers of scintillator crystals coupled through optical fibers with acceptable performance in terms of energy and timing resolution and crystal identification. The PSAPD-LSO detector performs well in the 7-T magnet, and no visible artifacts are detected in the MR images using standard pulse sequences. CONCLUSION: The first images from the complete system have been successfully acquired and reconstructed, demonstrating that simultaneous PET and MRI studies are feasible and opening up interesting possibilities for dual-modality molecular imaging studies.     

Phase-scrambled RF excitation for 3D volume-selective multislice NMR imaging.

An RF excitation technique with which one can improve the effective dynamic range of the receiver and reduce the interference between slices for 3D volume-selective multislice MRI is described. The basic idea of the technique is to use phase scrambling in conjunction with slice encoding through the use of RF pulses. The spins in each slice are encoded by RF pulses which have the scrambled as well as slice-encoded phase components along the slice-selection direction. The scrambled, or randomly distributed, phase reduces the peak signal intensity, thereby reducing the dynamic range of the signal. Since the proposed technique utilizes RF slice encoding together with phase scrambling, interslice image interference is greatly reduced and the dynamic range is improved. In addition, the method has several advantages such as a reduction in the power requirement for the RF pulses and the elimination of the necessity for hardware such as nonlinear gradient coils.     

多层螺旋CT三维重建技术在颅骨修补术中的应用

目的探讨螺旋CT三维容积重建技术在颅骨缺损患者中的使用技巧和计算机个性化设计钛网在临床的应用。方法30例颅骨缺损患者行螺旋CT扫描,数据传至工作站行三维重建,应用计算机辅助设计（CAD）和制造技术（CAM）进行个性化的颅骨缺损修复体的预制,并应用于临床。结果根据CT数据制作的三维模型能精确地显示颅骨缺损区域的结构,修复体嵌合满意,术后无并发症。结论多层螺旋CT三维重建技术在颅骨缺损修复术中具有重要的临床实用价值。     

Brain PET motion correction using 3D face-shape model: the first clinical study

Annals of Nuclear Medicine - Head motions during brain PET scan cause degradation of brain images, but head fixation or external-maker attachment become burdensome on patients. Therefore, we have...     

Improving 3D PET imaging by restoration: a phantom study.

The main objective of our work is to improve 3D PET imaging. Compared with 2D PET, 3D PET imaging has slightly worse axial resolution and a significantly higher contribution of scatter and randoms, but 3D PET has much better sensitivity than 2D PET imaging. A Jaszczak deluxe phantom was acquired in 3D mode on our GE Advance PET system. Activity of 333 MBq of 18F was uniformly distributed. Prior to the emission scan, blank and transmission scans had been acquired. They were used for attenuation correction. The duration of the emission scan was 20 min, transmission 10 min, and blank 20 min. Standard FBP reconstruction software provided by the vendor was used to obtain slice images. Point spread function was also acquired in a 21 cm diameter cylinder phantom filled with water 6.0 cm from the center and used to create restoration filters. Two restoration filters were applied, medium and sharp. Results showed significant improvement in resolution, contrast and detectability of the cold rods. The artifacts outside the phantom were also significantly reduced. For 11.1 mm rods, average contrast was 0.49+/-0.02 in the original image, 0.52+/-0.04 in the medium restored image, and in the sharply restored image 0.75+/-0.05. For 7.9 mm rods, average contrast was 0.07+/-0.01 in the original image, 0.21+/-0.03 in the medium restored image, and 0.50+/-0.04 in the sharply restored image. The amount of noise in the uniform slices, measured as the coefficient of variation (COV), was 5.5, 7.1 and 10.8% in the original image and in the images restored with medium and sharp filters, respectively. In conclusion, restoration can significantly improve the resolution and contrast of 3D PET imaging.