Simultaneous fluoroscopic and nuclear imaging: impact of collimator choice on nuclear image quality

Nuclear Cardiac Imaging in the Interventional Suite

PURPOSE OF REVIEW

This review presents the current state of imaging approaches that enable real-time molecular imaging in the interventional suite and discusses the potential future use of integrated nuclear imaging and fluoroscopy for intraprocedural guidance in the evaluation and treatment of both cardiovascular and oncological diseases.

RECENT FINDINGS

Although there are no commercially available real-time hybrid nuclear imaging devices that are approved for use in the interventional suite, prototype open gantry hybrid nuclear imaging and x-ray c-arm imaging systems and theranostic catheter for location radiotracer detection are currently undergoing development and testing by multiple groups. The integration of physiological and molecular targeted nuclear imaging for real-time delivery of targeted theranostics in the interventional laboratory may enable more personalized care for a wide variety of cardiovascular procedures and improve patient outcomes.

A Dual-layer Detector for Simultaneous Fluoroscopic and Nuclear Imaging

Purpose To develop and evaluate a dual-layer detector capable of acquiring intrinsically registered real-time fluoroscopic and nuclear images in the interventional radiology suite. Materials and Methods The dual-layer detector consists of an x-ray flat panel detector placed in front of a γ camera with cone beam collimator focused at the x-ray focal spot. This design relies on the x-ray detector absorbing the majority of the x-rays while it is more transparent to the higher energy γ photons. A prototype was built and dynamic phantom images were acquired. In addition, spatial resolution and system sensitivity (evaluated as counts detected within the energy window per second per megabecquerel) were measured with the prototype. Monte Carlo simulations for an improved system with varying flat panel compositions were performed to assess potential spatial resolution and system sensitivity. Results Experiments with the dual-layer detector prototype showed that spatial resolution of the nuclear images was unaffected by the addition of the flat panel (full width at half maximum, 13.6 mm at 15 cm from the collimator surface). However, addition of the flat panel lowered system sensitivity by 45%-60% because of the nonoptimized transmission of the flat panel. Simulations showed that an attenuation of 27%-35% of the γ rays in the flat panel could be achieved by decreasing the crystal thickness and housing attenuation of the flat panel. Conclusion A dual-layer detector was capable of acquiring real-time intrinsically registered hybrid images, which could aid interventional procedures involving radionuclides. Published under a CC BY-NC-ND 4.0 license. Online supplemental material is available for this article.

Fast technetium-99m liver SPECT for evaluation of the pretreatment procedure for radioembolization dosimetry

PURPOSE

The efficiency of radioembolization procedures could be greatly enhanced if results of the 99m Tc-MAA pretreatment procedure were immediately available in the interventional suite, enabling 1-day procedures as a result of direct estimation of the hepatic radiation dose and lung shunt fraction. This would, however, require a relatively fast, but still quantitative, SPECT procedure, which might be achieved with acquisition protocols using nonuniform durations of the projection images.

METHODS

SPECT liver images of the 150-MBq 99m Tc-MAA pretreatment procedure were simulated for eight different lesion locations and two different lesion sizes using the digital XCAT phantom for both single- and dual-head scanning geometries with respective total acquisition times of 1, 2, 5, 10, and 30 min. Three nonuniform projection-time acquisition protocols ("half-circle SPECT (HCS)," "nonuniform SPECT (NUS) I," and "NUS II") for fast quantitative SPECT of the liver were designed and compared with the standard uniform projection-time protocol. Images were evaluated in terms of contrast-to-noise ratio (CNR), activity recovery coefficient (ARC), tumor/non-tumor (T/N) activity concentration ratio, and lung shunt fraction (LSF) estimation. In addition, image quality was verified with a physical phantom experiment, reconstructed with both clinical and Monte Carlo-based reconstruction software.

RESULTS

Simulations showed no substantial change in image quality and dosimetry by usage of a nonuniform projection-time acquisition protocol. Upon shortening acquisition times, CNR dropped, but ARC, T/N ratio, and LSF estimates were stable across all simulated acquisition times. Results of the physical phantom were in agreement with those of the simulations.

CONCLUSION

Both uniform and nonuniform projection-time acquisition liver SPECT protocols yield accurate dosimetric metrics for radioembolization treatment planning in the interventional suite within 10 min, without compromising image quality. Consequently, fast quantitative SPECT of the liver in the interventional suite is feasible.

The physics of radioembolization

Radioembolization is an established treatment for chemoresistant and unresectable liver cancers. Currently, treatment planning is often based on semi-empirical methods, which yield acceptable toxicity profiles and have enabled the large-scale application in a palliative setting. However, recently, five large randomized controlled trials using resin microspheres failed to demonstrate a significant improvement in either progression-free survival or overall survival in both hepatocellular carcinoma and metastatic colorectal cancer. One reason for this might be that the activity prescription methods used in these studies are suboptimal for many patients.In this review, the current dosimetric methods and their caveats are evaluated. Furthermore, the current state-of-the-art of image-guided dosimetry and advanced radiobiological modeling is reviewed from a physics' perspective. The current literature is explored for the observation of robust dose-response relationships followed by an overview of recent advancements in quantitative image reconstruction in relation to image-guided dosimetry.This review is concluded with a discussion on areas where further research is necessary in order to arrive at a personalized treatment method that provides optimal tumor control and is clinically feasible.

A Monte Carlo study on the performance evaluation of a parallel hole collimator for a HiReSPECT: A dedicated small-animal SPECT

Collimator geometry has an important contribution on the image quality in SPECT imaging. The purpose of this study was to investigate the effect of parallel hole collimator hole-size on the functional parameters (including the spatial resolution and sensitivity) and the image quality of a HiReSPECT imaging system using SIMIND Monte Carlo program. To find a proper trade-off between the sensitivity and spatial resolution, the collimator with hole diameter ranges of 0.3-1.5 mm (in steps of 0.3 mm) were used with a fixed septal and hole thickness values (0.2 mm and 34 mm, respectively). Lead, Gold, and Tungsten as the LEHR collimator material were also investigated. The results on a ^99mTc point source scanning with the experimental and also simulated systems were matched to validate the simulated imaging system. The results on the simulation showed that decreasing the collimator hole size, especially in the Gold collimator, improved the spatial resolution to 18% and 3.2% compared to the Lead and the Tungsten, respectively. Meanwhile, the Lead collimator provided a good sensitivity in about of 7% and 8% better than that of Tungsten and Gold, respectively. Overall, the spatial resolution and sensitivity showed small differences among the three types of collimator materials assayed within the defined energy. By increasing the hole size, the Gold collimator produced lower scatter and penetration fractions than Tungsten and Lead collimator. The minimum detectable size of hot rods in micro-Jaszczak phantom on the iterative maximum-likelihood expectation maximization (MLEM) reconstructed images, were determined in the sectors of 1.6, 1.8, 2.0, 2.4 and 2.6 mm for scanning with the collimators in hole sizes of 0.3, 0.6, 0.9, 1.2 and 1.5 mm at a 5 cm distance from the phantom. The Gold collimator with hole size of 0.3 mm provided a better image quality with the HiReSPECT imaging.

Estimation of lung shunt fraction from simultaneous fluoroscopic and nuclear images

Radioembolisation with yttrium-90 (⁹⁰Y) is increasingly used as a treatment of unresectable liver malignancies. For safety, a scout dose of technetium-99m macroaggregated albumin (^99mTc-MAA) is used prior to the delivery of the therapeutic activity to mimic the deposition of ⁹⁰Y. One-day procedures are currently limited by the lack of nuclear images in the intervention room. To cope with this limitation, an interventional simultaneous fluoroscopic and nuclear imaging device is currently being developed. The purpose of this simulation study was to evaluate the accuracy of estimating the lung shunt fraction (LSF) of the scout dose in the intervention room with this device and compare it against current clinical methods.

METHODS

A male and female XCAT phantom, both with two respiratory profiles, were used to simulate various LSFs resulting from a scout dose of 150 MBq ^99mTc-MAA. Hybrid images were Monte Carlo simulated for breath-hold (5 s) and dynamic breathing (10 frames of 0.5 s) acquisitions. Nuclear images were corrected for attenuation with the fluoroscopic image and for organ overlap effects using a pre-treatment CT-scan. For comparison purposes, planar scintigraphy and mobile gamma camera images (both 300 s acquisition time) were simulated. Estimated LSFs were evaluated for all methods and compared to the phantom ground truth.

RESULTS

In the clinically relevant range of 10-20% LSF, hybrid imaging overestimated LSF with approximately 2 percentage points (pp) and 3 pp for the normal and irregular breathing phantoms, respectively. After organ overlap correction, LSF was estimated with a more constant error. Errors in planar scintigraphy and mobile gamma camera imaging were more dependent on LSF, body shape and breathing profile.

CONCLUSION

LSF can be estimated with a constant minor error with a hybrid imaging device. Estimated LSF is highly dependent on true LSF, body shape and breathing pattern when estimated with current clinical methods. The hybrid imaging device is capable of accurately estimating LSF within a few seconds in an interventional setting.