Collimators

In this article the present knowledge of the collimators used for single photon imaging in Nuclear Medicine is recapped. In the introduction, the classic formulas to describe the performances of multihole collimator are presented with an overview of the basic concepts. In the subsequent sections, the tools available for the characterization of multihole collimators are described. The characterization is divided into aspects related to the geometrical response and aspects related to effects due to interaction of photons with the collimator. Nowadays a rather complete theoretical description of the collimator geometrical response is available. In section Collimator characterization: theory a general formulation that is valid for a wide range of collimators is outlined. In section Collimator characterization: numerical methods the kind of knowledge that can be derived today from numerical computing methods about the penetration and scatter effects is presented. The problem of performances optimization in collimator design is described in section Collimator design. Difficulties and some new perspectives on avoiding both penetration and hole pattern artifacts while designing high-energy collimators are mentioned.

Geometrical response of multihole collimators

A complete theory of camera multihole collimators is presented. The geometrical system response is determined in closed form in frequency space. This closed form accounts for the known efficiency and resolution formulae for parallel beam, fan beam, cone beam and astigmatic collimators as well as for the most frequent hole array patterns and hole shapes. The point spread function in the space domain for a certain collimator and source position can be calculated via a discrete fast Fourier transform. Beside the complete theoretical definition of the response of multihole collimators, this theory allows the definition of accurate models of the geometrical response for SPECT reconstruction and it is suitable for designing new collimators.

World Wide Web interface for advanced SPECT reconstruction algorithms implemented on a remote massively parallel computer

Data from Single Photon Emission Computed Tomography (SPECT) studies are blurred by inevitable physical phenomena occurring during data acquisition. These errors may be compensated by means of reconstruction algorithms which take into account accurate physical models of the data acquisition procedure. Unfortunately, this approach involves high memory requirements as well as a high computational burden which cannot be afforded by the computer systems of SPECT acquisition devices. In this work the possibility of accessing High Performance Computing and Networking (HPCN) resources through a World Wide Web interface for the advanced reconstruction of SPECT data in a clinical environment was investigated. An iterative algorithm with an accurate model of the variable system response was ported on the Multiple Instruction Multiple Data (MIMD) parallel architecture of a Cray T3D massively parallel computer. The system was accessible even from low cost PC-based workstations through standard TCP/IP networking. A speedup factor of 148 was predicted by the benchmarks run on the Cray T3D. A complete brain study of 30 (64 x 64) slices was reconstructed from a set of 90 (64 x 64) projections with ten iterations of the conjugate gradients algorithm in 9 s which corresponds to an actual speed-up factor of 135. The technique was extended to a more accurate 3D modeling of the system response for a true 3D reconstruction of SPECT data; the reconstruction time of the same data set with this more accurate model was 5 min. This work demonstrates the possibility of exploiting remote HPCN resources from hospital sites by means of low cost workstations using standard communication protocols and an user-friendly WWW interface without particular problems for routine use.

High-performance computing and networking as tools for accurate emission computed tomography reconstruction

It is well known that the quantitative potential of emission computed tomography (ECT) relies on the ability to compensate for resolution, attenuation and scatter effects. Reconstruction algorithms which are able to take these effects into account are highly demanding in terms of computing resources. The reported work aimed to investigate the use of a parallel high-performance computing platform for ECT reconstruction taking into account an accurate model of the acquisition of single-photon emission tomographic (SPET) data. An iterative algorithm with an accurate model of the variable system response was ported on the MIMD (Multiple Instruction Multiple Data) parallel architecture of a 64-node Cray T3D massively parallel computer. The system was organized to make it easily accessible even from low-cost PC-based workstations through standard TCP/IP networking. A complete brain study of 30 (64x64) slices could be reconstructed from a set of 90 (64x64) projections with ten iterations of the conjugate gradients algorithm in 9 s, corresponding to an actual speed-up factor of 135. This work demonstrated the possibility of exploiting remote high-performance computing and networking resources from hospital sites by means of low-cost workstations using standard communication protocols without particular problems for routine use. The achievable speed-up factors allow the assessment of the clinical benefit of advanced reconstruction techniques which require a heavy computational burden for the compensation effects such as variable spatial resolution, scatter and attenuation. The possibility of using the same software on the same hardware platform with data acquired in different laboratories with various kinds of SPET instrumentation is appealing for software quality control and for the evaluation of the clinical impact of the reconstruction methods.

Least squares algorithm for region-of-interest evaluation in emission tomography

An accurate model of the nonstationary geometrical response of a camera-collimator system is discussed. The algorithm is compared to three other algorithms that are specialized for region-of-interest evaluation, as well as to the conventional method for summing the reconstructed quantity over the regions of interest. For noise-free data and for regions of accurate shape, least-squares estimates were unbiased within roundoff errors. For noisy data, estimates were still unbiased but precision worsened for regions smaller than resolution: simulating typical statistics of brain perfusion studies performed with a collimated camera, the estimated standard deviation for a 1-cm-square region was 10% with an ultra-high-resolution collimator and 7% with a low-energy all-purpose collimator. Conventional region-of-interest estimates show comparable precision but are heavily biased if filtered backprojection is used for image reconstruction. Using the conjugate-gradient iterative algorithm and the model of nonstationary geometrical response, bias of estimates decreased on increasing the number of iterations, but precision worsened, thus achieving an estimated standard deviation of more than 25% for the same 1-cm region.

An analysis of the arterial input curve for technetium-99m-HMPAO: quantification of rCBF using single-photon emission computed tomography

Arterial radioactivity content after the intravenous administration of HMPAO in seven human subjects was analyzed. Arterial sampling of 99mTc-HMPAO was performed on each subject over a 25-min period postinjection. The lipophilic fraction of the tracer present in the blood was rapidly extracted with octanol. An analysis of the time course of the extracted and nonextracted octanol fractions was performed in order to calculate the arterial input of the tracer available for brain extraction. HMPAO net regional brain clearances were then calculated and compared with rCBF values obtained in the same patients using 99mTc-microspheres injected into the left ventricle of the heart. HMPAO brain clearances were 0.41 +/- 0.01 and 0.27 +/- 0.01 ml/min/g for grey and white matter, respectively. Linear regression analysis was performed and the following result was obtained: clearance (HMPAO) = 0.07 + 0.43 . rCBF with a high significance (p less than 0.001). This equation can be used for the transformation of HMPAO clearances into rCBF values. Our study demonstrates that by using HMPAO and SPECT it is possible to obtain a quantitative estimate of rCBF in humans.

Evaluation of technetium 99m cyclobutylpropylene amine oxime as a potential brain perfusion imaging agent for SPET

Technetium 99m d,l-cyclobutylpropylene amine oxime (99mTc-CBPAO) has been developed as a brain-imaging agent for single photon emission tomography (SPET). 99mTc-CBPAO can be prepared using a simple labelling procedure suitable for routine clinical use. It has a high in vitro stability, as has been demonstrated by high-pressure liquid chromatography (HPLC) analysis. This shows that 3 h after labelling, less than 5% of the primary lipophilic complex which is capable of crossing the blood-brain barrier (BBB) converts to a secondary hydrophilic complex. Brain uptake (% dose/g wet tissue) of 99mTc-CBPAO, determined at 5 and 30 min after injection in two groups of six adult male Sprague-Dawley rats, was found to be 0.74 +/- 0.06 and 0.73 +/- 0.13 (mean +/- SD), respectively. These values are not significantly different from those obtained repeating the experiment with 99mTc-labelled hexamethylpropylene amine oxime (99mTc-HMPAO) (0.72 +/- 0.15 at 5 min and 0.88 +/- 0.24 at 30 min after injection). Since the rat brain uptake of 99mTc-CBPAO remained unchanged for a period of time suitable for tomographic study, the comparison of the two tracers was extended to two groups of ten patients. The latter were affected by neurological and psychiatric disorders and were studied with SPET. Human brain uptake (% dose/cc cortical grey matter) of 99mTc-CBPAO and 99mTc-HMPAO were 3.04 +/- 0.57 and 4.22 +/- 0.46 (mean x 10(-3) +/- SD x 10(-3), respectively, with a 32% significant difference. In two other groups of five patients, the first transit time-activity curves of the two tracers were compared.(ABSTRACT TRUNCATED AT 250 WORDS)

A brain phantom for studying contrast recovery in emission computerized tomography

A brain phantom is described that is characterized by a high anatomical definition and by the possibility of varying the internal contrast with the use of a single radioactive solution. The experimental work was done with a single-photon emission computerized tomographic (SPET) rotating camera. The phantom was used to study the contrast recovery of both the filtered back-projection and an iterative reconstruction algorithm. Moreover, it was also used to find a cross-calibration factor between activity concentrations in the SPET slices and an external reference.

Compensation of spatial system response in SPECT with conjugate gradient reconstruction technique

A procedure for the determination of the system matrix in single photon emission tomography (SPECT) is described which uses the conjugate gradient reconstruction technique in order to take into account the variable system resolution of a camera equipped with parallel-hole collimators. The procedure involves the acquisition of the system line spread functions (LSF) in the region occupied by the object to be studied. Those data are used to generate a set of weighting factors based on the assumption that the LSFs of the collimated camera are of Gaussian shape with the full width at half maximum (FWHM) linearly dependent on the source depth in the span of image space. The factors are stored on a disc file for subsequent use in the reconstruction process. Afterwards the reconstruction is performed using the conjugate gradient method with the system matrix modified by the incorporation of these precalculated factors in order to take into account the variable geometrical system response. The set of weighting factors is regenerated whenever the acquisition conditions are changed (collimator, radius of rotation). In the case of an ultra high resolution (UHR) collimator 2000 weighting factors need to be calculated. The modification of the system matrix for the geometrical response allows the number of iterations to increase, considerably improving image definition without the appearance of noise artifacts. Moreover, phantom studies show that the number of iterations is less critical because of improved stability in the convergence to the solution. For brain studies of patients 10-15 iterations are usually performed. Studies with a single line source give a value between 7 and 8 mm for the FWHM of the point spread function (PSF) when the conjugate gradient method with modified system matrix is used on data acquired with a UHR collimator, whereas without the modification of the system matrix the result is 9 mm FWHM, if filtered backprojection (FBP) is used with the same filter as in the clinical studies the result is 15 mm FWHM. The results of this work show that proper definition of the system matrix using conjugate gradients influences the quality of the reconstruction remarkably. Nevertheless, further work has to be done in order to assess to what extent the system matrix is ill-conditioned and, eventually, to define a suitable regularization technique.

Renal sequential scintigraphy: unilateral clearance rate determination based on external measurements only

A method of estimating the 131I-hippuran clearance of each kidney during renal sequential scintigraphy is presented. The clearance rates were obtained from the elaboration of the renal histograms and of a blood curve recorded on the precordium, directly compared with the injected dose. A function simulating the progressive deformation of the radioactive bolus as kidney perfusion is impaired was introduced into the computation. The method was tested on 23 patients who underwent simultaneous determination of the clearance rate with the Sapirstein method, the results being corrected to 1.73 m2 of body surface area. The correlation coefficient r between the obtained data (the sum of the clearance rates of the two sides) and the results of the Sapirstein method was 0.89.