Image reconstruction algorithm for a spinning strip CZT SPECT camera with a parallel slat collimator and small pixels

This paper discusses the use of small pixels in a spinning CdZnTe single photon emission computed tomography (SPECT) camera that is mounted with a parallel slat collimator. In a conventional slat collimation configuration, there is a detector pixel between two adjacent collimator slats. In our design, the pixel size is halved. That is, there are two smaller pixels to replace a regular pixel between two adjacent slats while the collimator remains unchanged. It has an advantage over our older design that uses tilted slats. In order to acquire a complete data set the tilted-slat collimator must spin 360 degrees at each SPECT view while the proposed design requires only 180 degrees at each SPECT view. Computer simulations and phantom experiments have been carried out to investigate the performance of the small-pixel configuration. It is observed that this design has the potential to increase the spatial resolution of the detector while keeping photon counts the same.     

Image reconstruction algorithm for a rotating slat collimator

A slat collimator in single photon emission computed tomography consists of a set of parallel slats. As the collimator spins, the detector measures a one-dimensional projection data set. A complete data set can be obtained by rotating the detector/collimator assembly around the object (patient) while the collimator spins continuously. The measured projection data are assumed to be weighted planar integrals of the object. This paper describes the development of an approximate three-dimensional image reconstruction algorithm for a rotating/spinning slat collimator. This algorithm is in filtered backprojection form. Computer simulations were performed to verify the effectiveness of the algorithm.     

Characterization of a rotating slat collimator system dedicated to small animal imaging

Some current investigations based on small animal models are dedicated to functional cerebral imaging. They represent a fundamental tool to understand the mechanisms involved in neurodegenerative diseases. In the radiopharmaceutical development approach, the main challenge is to measure the radioactivity distribution in the brain of a subject with good temporal and spatial resolutions. Classical SPECT systems mainly use parallel hole or pinhole collimators. In this paper we investigate the use of a rotating slat collimator system for small animal brain imaging. The proposed prototype consists of a 64-channel multi-anode photomultiplier tube (H8804, Hamamatsu Corp.) coupled to a YAP:Ce crystal highly segmented into 32 strips of 0.575 × 18.4 × 10 mm(3). The parameters of the rotating slat collimator are optimized using GATE Monte Carlo simulations. The performance of the proposed prototype in terms of spatial resolution, detection efficiency and signal-to-noise ratio is compared to that obtained with a gamma camera equipped with a parallel hole collimator. Preliminary experimental results demonstrate that a spatial resolution of 1.54 mm can be achieved with a detection efficiency of 0.012% for a source located at 20 mm, corresponding to the position of the brain in the prototype field of view.     

Performance assessment of a slat gamma camera collimator for 511 keV imaging.

The physical performance of a prototype slat collimator is described for gamma camera planar imaging at 511 keV. Measurements were made of sensitivity, spatial resolution and a septal penetration index at 511 keV. These measurements were repeated with a commercial parallel hole collimator designed for 511 keV imaging. The slat collimator sensitivity was 22.9 times that of the parallel hole collimator with 10 cm tissue equivalent scatter material, and 16.8 times the parallel hole collimator sensitivity in air. Spatial resolution was also better for the slat collimator than the parallel hole collimator (FWHM at 10 cm in air 17.9 mm and 21.2 mm respectively). Septal penetration was compared by a single value for the counts at 120 mm from the point source profile peak, expressed as a percentage of the peak counts, showing less penetration for the slat collimator than the parallel hole collimator (1.9% versus 3.6% respectively). In conclusion, these results show that the slat collimator may have advantages over the parallel hole collimator for 511 keV imaging, though the greater complexity of operation of the slat collimator and potential sources of artefact in slat collimator imaging are recognized.     

3D RBI-EM reconstruction with spherically-symmetric basis function for SPECT rotating slat collimator

A single photon emission computed tomography (SPECT) rotating slat collimator with strip detector acquires distance-weighted plane integral data, along with the attenuation factor and distance-dependent detector response. In order to image a 3D object, the slat collimator device has first to spin around its axis and then rotate around the object to produce 3D projection measurements. Compared to the slice-by-slice 2D reconstruction for the parallel-hole collimator and line integral data, a more complex 3D reconstruction is needed for the slat collimator and plane integral data. In this paper, we propose a 3D RBI-EM reconstruction algorithm with spherically-symmetric basis function, also called 'blobs', for the slat collimator. It has a closed and spherically symmetric analytical expression for the 3D Radon transform, which makes it easier to compute the plane integral than the voxel. It is completely localized in the spatial domain and nearly band-limited in the frequency domain. Its size and shape can be controlled by several parameters to have desired reconstructed image quality. A mathematical lesion phantom study has demonstrated that the blob reconstruction can achieve better contrast-noise trade-offs than the voxel reconstruction without greatly degrading the image resolution. A real lesion phantom study further confirmed this and showed that a slat collimator with CZT detector has better image quality than the conventional parallel-hole collimator with NaI detector. The improvement might be due to both the slat collimation and the better energy resolution of the CZT detector.     

Optimizing the indexing of a resolution-enhancing tertiary collimator for radiotherapy.

A method for improving the resolution of multileaf collimator (MLC) defined radiotherapy fields using a tertiary, slotted grid collimator has been investigated and developed further. The original concept was for each slot to be aligned with each leaf pair of opposing MLC leaves. The total treated area is composed of a series of sub-fields, a pattern of irradiated strips, with the width of each strip defined by the width of the slot and the length by the relative separation of the MLC leaf pair. To complete the field, the patient must be indexed relative to the collimator, with the number of sub-fields required determined by the width of the slots and the spacing between them. Two methods were considered by which this indexing could be achieved: movement of the patient while holding the tertiary collimator fixed, or rotating the grid with the point of rotation defined as the radiation source. Consideration of the movement resolution and precision required for the patient support system for non-cardinal gantry, collimator and table angles cast doubt on the practicality of the use of such a strategy. To assess the effect of divergence on the abutting sub-fields, measurements were also performed to assess the uniformity of single fields generated by the tertiary collimator in planes above and below the isocentre using both methods of indexing. As expected, rotation of the collimator resulted in a similar degree of non-uniformity for any plane chosen, whereas significant dose heterogeneities were introduced to treatment planes within 5 cm above and below the isocentre if the patient support system was used. Therefore, the rotation strategy will be implemented with all future versions of the device.     

Optimizing color reproduction of a topometric measurement system for medical applications

Integration of color information in 3D models acquired with a topometric measurement system opens a wide field of applications in medicine, e.g. prosthetics. To ensure an accurate and reliable color reproduction in a 3D topometric measurement system, different approaches have been evaluated. These techniques focus on the calibration of the CCD camera using color test charts by applying a polynomial regression method as well as numerical methods to compensate an intensity decay of the illumination in 3D datasets which is caused by varying distances and orientations of the measured object with respect to the light source. The methods presented in this contribution have been applied to a topometric measurement system with one-chip color CCD cameras. An enhanced color reproduction of various objects is demonstrated.     

用于人体成分测定的多频复阻抗同步获取系统

目的 设计一种生物组织多频复阻抗同步获取系统,克服多频复阻抗同步测量受栅栏效应影响的问题,用于人体脂肪、肌肉等组成成分的精确测定。方法 首先根据频域阻抗测量原理,基于现场可编程门阵列(field programmable gate array,FPGA)编程实现了一个用于阻抗测量的高精度多频复合激励源,然后设计了适宜于微弱信号的高信噪比调理和采集模块,之后提出一种避免栅栏效应影响的精确离散傅里叶变换(discrete Fourier transform,DFT)分析方法,实现了精准且简便的多频复阻抗同步测量分析系统。最后采用人体RC模型和对比设备分别进行了复阻抗和人体成分测量准确性的验证实验。结果 人体阻抗模型测量表明该系统幅值相对误差不超过1%,相位角最大误差不超过0.5°,人体成分主要测量指标与Tanita MC-180测量结果的相关系数r=0.984~0.994(P0.001)。结论 该系统同MC-180的测量结果高度显著相关,同时,复杂度低使得该系统易于普及推广,为身体成分的大样本人群普查提供了一种有效的手段。     

Dosimetric verification of a commercial 3D treatment planning system for conformal radiotherapy with a dynamic multileaf collimator

The dosimetric accuracy of a 3D treatment planning system (TPS) for conformal radiotherapy with a computer-assisted dynamic multileaf collimator (DMLC) was evaluated. The DMLC and the TPS have been developed for clinical applications where dynamic fields not greater than 10 x 10 cm2 and multiple non-coplanar arcs are required. Dosimetric verifications were performed by simulating conformal treatments of irregularly shaped targets using several arcs of irradiation with 6 MV x-rays and a spherical-shaped, tissue-simulating phantom. The accuracy of the delivered dose at the isocentre was verified using an ionization chamber placed in the centre of the phantom. Isodose distributions in the axial and sagittal planes passing through the centre of the phantom were measured using double-layer radiochromic films. Measured dose at the isocentre as well as isodose distributions were compared to those calculated by the TPS. The maximum percentage difference between measured and prescribed dose was less than 2.5% for all the simulated treatment plans. The mean (+/-SD) displacement between measured and calculated isodoses was, in the axial planes, 1.0 +/- 0.6 mm, 1.2 +/- 0.7 mm and 1.5 +/- 1.1 mm for 80%, 50% and 20% isodose curves, respectively, whereas in the sagittal planes it was 2.0 +/- 1.2 mm and 2.2 +/- 2 mm for 80% and 50% isodose curves, respectively. The results indicate that the accuracy of the 3D treatment planning system used with the DMLC is reasonably acceptable in clinical applications which require treatments with several non-coplanar arcs and small dynamic fields.     

Velocity profile method for time varying resistance in minimal cardiovascular system models

This paper investigates the fluid dynamics governing arterial flow used in lumped parameter cardiovascular system (CVS) models, particularly near the heart where arteries are large. Assumptions made in applying equations conventionally used in lumped parameter models are investigated, specifically that of constant resistance to flow. The Womersley number is used to show that the effects of time varying resistance must be modelled in the pulsatile flow through the large arteries near the heart. It is shown that the equation commonly used to include inertial effects in fluid flow calculations is inappropriate for including time varying resistance. A method of incorporating time varying resistance into a lumped parameter model is developed that uses the Navier-Stokes equations to track the velocity profile. Tests on a single-chamber model show a 17.5% difference in cardiac output for a single-chamber ventricle model when comparing constant resistance models with the velocity profile tracking method modelling time varying resistance. This increase in precision can be achieved using 20 nodes with only twice the computational time required. The method offers a fluid dynamically and physiologically accurate method of calculating large Womersley number pulsatile fluid flows in large arteries around the heart and valves. The proposed velocity profile tracking method can be easily incorporated into existing lumped parameter CVS models, improving their clinical application by increasing their accuracy.     

Communication and sampling rate limitations in IMRT delivery with a dynamic multileaf collimator system

The delivery of an intensity modulated radiation field with a dynamic multileaf collimator (MLC) requires precise correlation between MLC positions and cumulative monitor units (MUs). The purpose of this study is to investigate the precision of this correlation as a function of delivered MUs and dose rate. A semi-Gaussian shaped intensity profile and a simple geometric intensity pattern consisting of four square segments were designed to deliver a total of 1, 4, 16, 64, and 100 MUs at three different dose rates of 100, 400, and 600 MU/min. The semi-Gaussian intensity pattern was delivered using both sliding window and step and shoot techniques. The dose profiles of this intensity pattern were measured with films. The four square intensity pattern was delivered using step and shoot and conventional delivery techniques for comparison. Because of geometrical symmetry, the dose to each segment in this intensity pattern is expected to be the same when the same MU is assigned to each segment. An ionization chamber was used to measure the dose in the center of each of the four square segments. For the semi-Gaussian shaped profile, significant artifacts were observed when the profile was delivered with small MUs and/or at a high dose rate. For the four square intensity pattern, the dose measured in each segment presented a large variation when delivered with small MUs and a high dose rate. The variation increases as the MU/segment decreases and as the dose rate increases. These MU and dose rate dependencies were not observed when the intensity pattern was delivered using a conventional delivery technique. The observed distortion of the semi-Gaussian profile and dose variations among the segments of the four square intensity pattern are explained by considering the sampling rate and the communication time lag between the control systems. Finally, clinical significance is discussed.     

Multileaf collimator tracking integrated with a novel x-ray imaging system and external surrogate monitoring

We have previously developed a tumour tracking system, which adapts the aperture of a Siemens 160 MLC to electromagnetically monitored target motion. In this study, we exploit the use of a novel linac-mounted kilovoltage x-ray imaging system for MLC tracking. The unique in-line geometry of the imaging system allows the detection of target motion perpendicular to the treatment beam (i.e. the directions usually featuring steep dose gradients). We utilized the imaging system either alone or in combination with an external surrogate monitoring system. We equipped a Siemens ARTISTE linac with two flat panel detectors, one directly underneath the linac head for motion monitoring and the other underneath the patient couch for geometric tracking accuracy assessments. A programmable phantom with an embedded metal marker reproduced three patient breathing traces. For MLC tracking based on x-ray imaging alone, marker position was detected at a frame rate of 7.1 Hz. For the combined external and internal motion monitoring system, a total of only 85 x-ray images were acquired prior to or in between the delivery of ten segments of an IMRT beam. External motion was monitored with a potentiometer. A correlation model between external and internal motion was established. The real-time component of the MLC tracking procedure then relied solely on the correlation model estimations of internal motion based on the external signal. Geometric tracking accuracies were 0.6 mm (1.1 mm) and 1.8 mm (1.6 mm) in directions perpendicular and parallel to the leaf travel direction for the x-ray-only (the combined external and internal) motion monitoring system in spite of a total system latency of ~0.62 s (~0.51 s). Dosimetric accuracy for a highly modulated IMRT beam--assessed through radiographic film dosimetry--improved substantially when tracking was applied, but depended strongly on the respective geometric tracking accuracy. In conclusion, we have for the first time integrated MLC tracking with x-ray imaging in the in-line geometry and demonstrated highly accurate respiratory motion tracking.     

两种胶片分析方法比较多叶光栅到位精确度:多中心测量结果的初步研究

【摘 要】 目的:应用两种胶片分析方法分析调强治疗多叶光栅（MLC）到位精确度。 方法:选择4个省共15家医院,其中8家为Varian加速器,MLC型号均为Millenium 120;7家为Elekta加速器,MLC型号为MLCi或MLCi2。胶片放在固体水模体30 cm×30 cm,dmax点处（水下1.5 cm）,SAD=100 cm,6 MV照射,250 MU（监督系数）/栅栏野,应用计划系统,在EBT3胶片上形成5条MLC栅栏野,每条栅栏野射野宽度为6 mm,5条栅栏野射野中心位置相对于中间栅栏野射野中心的位置距离分别为-6、-3、0、3、6 cm。将照射后的胶片用Epson Expression 10000XL扫描,应用Film QATM Pro软件得到栅栏野剂量曲线（profile）,并用两种归一方法即截断部分光密度值区域后归一和归一到局部位置区的光密度值,从射野位置及中心位置偏差、射野宽度及偏差4个方面分析比较MLC到位精确及多中心测量结果。 结果:两种分析方法比较,5条栅栏野实际射野位置相对于计划射野位置偏差,均测得9家医院位置偏差超过国际原子能机构（IAEA）规定偏差限值±0.5 mm;分析每条栅栏野射野中心位置的偏差,分析结果均符合IAEA规定限值±0.5 mm;分析5条栅栏野宽度,并与计划设定宽度6 mm相比较,偏差均符合IAEA规定不超过±1 mm;分析射野宽度最大最小值偏差及标准差,分析结果均符合IAEA规定偏差不超过±0.75 mm,标准差不超过0.30 mm。 结论:两种胶片分析方法测量MLC叶片到位精确度,结果相近,差别较小,在此实验中两种归一方法均可被用。     

Optimizing measurement of self-reported adherence with the ACTG Adherence Questionnaire: a cross-protocol analysis

BACKGROUND/OBJECTIVE: The AIDS Clinical Trials Group (ACTG) Adherence Questionnaire is used extensively, but investigators frequently only use the first item of the questionnaire (4-day recall). DESIGN/METHODS: A secondary analysis was conducted to (1) estimate the validity and reliability of each of the 5 scale items and (2) compare the approach commonly used to summarize adherence data collected with the instrument (average 4-day recall) with alternate approaches derived using principal component (PC) analysis and the full questionnaire. We hypothesized that an estimate of adherence taking all items of the questionnaire into account would provide a stronger measure of adherence. RESULTS: Logistic regression analyses showed that the first PC identified (PC1) was significantly correlated with plasma HIV RNA outcome (P < 0.0001 for ACTG 370 data and P = 0.006 for ACTG 398 data) and correlated with plasma HIV RNA better than average 4-day recall. An adherence index formulated using weights of PC1 showed substantially greater variability in the range of adherence scores in comparison to average 4-day adherence recall alone. PC1 compared favorably with 2 indices derived from medication event monitoring system data as well. CONCLUSIONS: Findings indicate that a superior assessment of antiretroviral adherence may be obtained with the ACTG Adherence Questionnaire by using the method employed in this analysis.     

T1 measurement using a short acquisition period for quantitative cardiac applications

Myocardial signal intensity curves for myocardial perfusion studies may be made quantitative by the use of T1 measurements made after the first-pass of contrast agent. A short data acquisition method for T1 mapping is presented in which all data for each T1 map are acquired in a short breath hold, and the slice geometry and timing in the cardiac cycle exactly match that of the dynamic first-pass perfusion sequence. This allows accurate image registration of the T1 map with the first-pass series of images. The T1 method is based on varying the preparation-pulse delay time of a saturation recovery sequence, and in this implementation employs an ECG-triggered, single-shot, spoiled gradient echo technique with SENSE reconstruction. The method allows T1 estimates of three slices to be made in fifteen heartbeats. For a range of samples with T1 values equivalent to those found in the myocardium during the first-pass of contrast agent, T1 estimates were accurate to within 6%, and the variation between slices was 2% or less.     

Optimizing bioimpedance measurement configuration for dual-gated nuclear medicine imaging: a sensitivity study

Motion artefacts due to respiration and cardiac contractions may deteriorate the quality of nuclear medicine imaging leading to incorrect diagnosis and inadequate treatment. Motion artefacts can be minimized by simultaneous respiratory and cardiac gating, dual-gating. Currently, only cardiac gating is often performed. In this study, an optimized bioimpedance measurement configuration was determined for simultaneous respiratory and cardiac gating signal acquisition. The optimized configuration was located on anterolateral upper thorax based on sensitivity simulations utilizing a simplified thorax model. The validity of the optimized configuration was studied with six healthy volunteers. In the peak-to-peak and frequency content analyses the optimized configuration showed consistently higher peak-to-peak values and frequency content than other studied measurement configurations. This study indicates that the bioimpedance method has potential for the dual-gating in nuclear medicine imaging. The method would minimize the need of additional equipment, is easy for the technologists to use and comfortable for the patients.