Adaptive SPECT: personalizing medical imaging

We develop modern techniques for image quality evaluation and optimization of imaging systems, and use them to control adaptive SPECT systems. Our results should contribute to the development of more personalized and efficient medical imaging

Efficient optimization based on local shift-invariance for adaptive SPECT systems

Adaptive SPECT systems automatically change some of their settings to maximize the image quality for a given subject and purpose. This approach has a lot of potential, and could lead to drastic improvements in performance. In particular, it would be very useful in high resolution pinhole SPECT, where the low sensitivity requires higher radiation doses or longer imaging times compared to other systems. In order to have adaptation in real-time, we need a fast method for optimizing the adaptive settings according to a given figure of merit. This is still a big challenge. Based on previous work, we address in this paper the issue of fast evaluation of image quality and optimization, for a class of adaptive SPECT systems. We evaluate the image quality in a voxel of interest, reconstructed using post- filtered MLEM, with the contrast-to-noise ratio (CNR). The CNR is computed analytically, using an approximation based on the Fisher information matrix and assuming local shift-invariance on the Fisher information matrices per adaptation parameter. We maximize the CNR with a gradient based optimization approach. We then test this method in the optimization of the angular sampling of a single-head SPECT system which rotates around a phantom. In this case, the method proved to be very efficient, and at the same time showed good agreement with previous results in literature and with the outcome from reconstructions

Evaluation of fisher information matrix approximation-based methods for fast assessment of image quality in pinhole SPECT

The accurate determination of the local impulse response and the covariance in voxels from penalized maximum likelihood reconstructed images requires performing reconstructions from many noise realizations of the projection data. As this is usually a very time-consuming process, efficient analytical approximations based on the Fisher information matrix (FIM) have been extensively used in PET and SPECT to estimate these quantities. For 3D imaging, however, additional approximations need to be made to the FIM in order to speed up the calculations. The most common approach is to use the local shift-invariant (LSI) approximation of the FIM, but this assumes specific conditions which are not always necessarily valid. In this paper we take a single-pinhole SPECT system and compare the accuracy of the LSI approximation against two other methods that have been more recently put forward: the non-uniform object-space pixelation (NUOP) and the subsampled FIM. These methods do not assume such restrictive conditions while still increasing the speed of the calculations considerably. Our results indicate that in pinhole SPECT the NUOP and subsampled FIM approaches could be more reliable than the LSI approximation, especially when a high accuracy is required

Parallel-hole collimator concept for stationary SPECT imaging

Parallel-hole SPECT collimators have traditionally been manufactured by stacking sheets of lead foil or by casting. These techniques significantly restrict our options in terms of collimator geometry. However, recent developments in metal additive manufacturing are making novel collimator designs possible, giving rise to new opportunities in SPECT imaging. In this paper we propose an innovative type of collimator for stationary SPECT, using parallel-holes whose collimation direction depends on their axial position. Its main advantage compared to current stationary SPECT systems (which are based on pinholes) is that, using only axial bed translations, we can achieve complete angular sampling of an increased portion of the transaxial area of the collimator bore. This allows the system to be much more compact than current stationary SPECT systems that image objects of the same size. We describe three possible designs, for full-body, brain and small-animal imaging, respectively, and test their feasibility using simulations. The system modeling method is validated against realistic Monte Carlo simulations, and then used in the evaluation of the systems’ performances and reconstructions. The simulations show that the system is able to reconstruct objects occupying the predicted field of view (75% of the transaxial area of the bore) without sampling artifacts. In particular, we perform reconstructions from noisy projection data obtained for an activity and scanning time similar to standard protocols for the three applications, and the resulting images indicate the possibility of using the proposed systems in practice

Characterization of a SPECT pinhole collimator for optimal detector usage (the lofthole)

In single-photon emission computed tomography (SPECT), multi-pinhole collimation is often employed nowadays. Most multi-pinhole collimators avoid overlap (multiplexing) of the projections on the detector. This can be done by using additional shielding or by spacing the pinholes far enough apart. Using additional shielding has the drawback that it increases weight, design complexity and cost. Spacing the pinholes far enough apart results in suboptimal detector usage, the valuable detector area is not entirely used. This is due to the circular projections of pinholes on the detector; these ellipses can not be tiled with high detector coverage. To overcome this we designed a new pinhole geometry, the lofthole, that has a rectangular projection on the detector. The lofthole has a circular aperture and a rectangular entrance/exit opening. Sensitivity formulae have been derived for pinholes and loftholes. These formulae take the penumbra effect into account; the proposed formulae do not take penetration into account. The derived formulae are valid for geometries where the field-of-view and the sensitivity of the aperture are solely limited by the exit window. A flood map measurement was performed to compare the rectangular projection of a lofthole with the circular projection of a pinhole. Finally, measurements were done to compare the amount of penetration of pinholes with the amount of penetration of a lofthole. A square lofthole collimator has less penetration than a knife-edge pinhole collimator that irradiates the same rectangular detector area with full coverage. A multilofthole collimator allows high detector coverage without using additional shielding. An additional advantage is the lower amount of penetration

Evaluation of the local shift-invariance approximation in pinhole SPECT

The local shift-invariance approximation of the Fisher information matrix is commonly used in 3D PET and SPECT to speed up the analytical estimation of the contrast recovery coefficient and variance at voxels of interest in a reconstructed image. However, the approximation should only be used under specific conditions, which are usually assumed to be valid. In this work we investigate the two main assumptions behind this approach in a single-pinhole SPECT system, by analyzing Fisher Information Matrix column images. Our results show that the first assumption (localness) is less applicable when low angular sampling is used, and consequently also for highly non-uniform sampling strategies. With the second assumption (local shift-invariance), on the other hand, there seem to be issues in any angular sampling strategy. This indicates that the local shift-invariance approach might not be applicable in many other situations, even for very simple systems and phantoms, particularly in pinhole SPECT. Our findings are mainly intended to show that the local shift-invariance approximation should be used with care. One should always make sure that the assumptions made, as well as any additional approximations, are valid for a particular imaging system, phantom and protocol

Design and simulation of a stationary SPECT imaging system based on axially varying tilted parallel-hole collimation

Stationary SPECT systems present many advantages compared to non-stationary systems: more stability (less calibration necessary), no need for large and expensive rotation / positioning mechanisms... They are normally based on multi-pinhole collimation, which gives complete angular sampling but only for a small portion of the transaxial field-of-view, thereby limiting them to imaging small objects relative to the bore. We propose a new type of collimator for stationary SPECT imaging that uses tilted parallel-holes whose viewing direction changes according to their axial position. This collimator allows complete angular sampling of a larger percentage of the bore, using only longitudinal bed movement. We have simulated a small animal system with a hexagonal detector configuration that fits inside a cube of 100mm side. The collimator fits inside this detector system, and is designed such that we can image a 65mm diameter rat with a target resolution at the center of 2mm. Our results show that we are indeed able to reconstruct objects of 65mm diameter without sampling artifacts with the simulated system, using only longitudinal bed translations. The (mean) volume sensitivity and resolution obtained were 170 cps/MBq and 1.4mm, respectively, for a field-of-view of 65 mm diameter and 46 mm length (75% of the bore's volume). We envision this novel collimator concept as having many applications, from small-animal to human imaging. It is particularly useful for systems with spatial restrictions, such as SPECT/MR inserts, due to its very efficient use of space and no need for a rotation mechanism. It could also be used to perform a full-body scout scan, prior to a targeted region-of-interest scan with a pinhole collimator. Furthermore, such a collimator can be inserted and used in existing scanners, since it only requires axial bed movement