Ashish Panse

Assessment of a three-dimensional line-of-response probability density function system matrix for PET

To achieve optimal PET image reconstruction through better system modeling, we developed a system matrix that is based on the probability density function for each line of response (LOR-PDF). The LOR-PDFs are grouped by LOR-to-detector incident angles to form a highly compact system matrix. The system matrix was implemented in the MOLAR list mode reconstruction algorithm for a small animal PET scanner. The impact of LOR-PDF on reconstructed image quality was assessed qualitatively as well as quantitatively in terms of contrast recovery coefficient (CRC) and coefficient of variance (COV), and its performance was compared with a fixed Gaussian (iso-Gaussian) line spread function. The LOR-PDFs of three coincidence signal emitting sources, (1) ideal positron emitter that emits perfect back-to-back γ rays (γγ) in air; (2) fluorine-18 (¹⁸F) nuclide in water; and (3) oxygen-15 (¹⁵O) nuclide in water, were derived, and assessed with simulated and experimental phantom data. The derived LOR-PDFs showed anisotropic and asymmetric characteristics dependent on LOR-detector angle, coincidence emitting source, and the medium, consistent with common PET physical principles. The comparison of the iso-Gaussian function and LOR-PDF showed that: (1) without positron range and acollinearity effects, the LOR-PDF achieved better or similar trade-offs of contrast recovery and noise for objects of 4 mm radius or larger, and this advantage extended to smaller objects (e.g. 2 mm radius sphere, 0.6 mm radius hot-rods) at higher iteration numbers; and (2) with positron range and acollinearity effects, the iso-Gaussian achieved similar or better resolution recovery depending on the significance of positron range effect. We conclude that the 3D LOR-PDF approach is an effective method to generate an accurate and compact system matrix. However, when used directly in expectation-maximization based list-mode iterative reconstruction algorithms such as MOLAR, its superiority is not clear. For this application, using an iso-Gaussian function in MOLAR is a simple but effective technique for PET reconstruction.

Evaluation of the Micro-Angiographic Fluoroscope as a High-Resolution, Single-Photon Counting and Energy-Integrating Imager for Transmission and Emission Imaging

X-ray and radionuclide imaging are widely popular medical imaging modalities. The requirements for each imaging modality are different, and different detectors are normally used. For this paper, we demonstrate our new detector capable of both fluoroscopy and angiography, to be used as an imager for both single-photon and integral-energy imaging and applied to the dual modalities of radionuclide imaging and X-ray imaging. This newly developed micro-angiographic fluoroscope (MAF) has 1024 × 1024 pixels of 35 μm effective size and is capable of real-time imaging at 30 fps. The large variable gain of its light image intensifier (LII) provides quantum-limited operation with essentially no additive instrumentation noise. We demonstrate that the MAF can be operated in single-photon counting (SPC) mode for X-ray imaging with substantially better resolution than in energy integration (EI) mode. We may use high LII gain with very low exposure (less than 1 X-ray photon/pixel) per frame for SPC mode (with X-ray and radionuclide) and higher exposure with lower gain for EI mode (transmission imaging with X-rays). For a demonstration of the operation in both EI and SPC mode, a heavily K-edge filtered X-ray beam (average energy of 31 keV) was used to provide a nearly monochromatic spectrum. The MTF measured using a standard slit method showed a dramatic improvement for the SPC mode over the EI mode at all spatial frequencies. Images of a line-pair phantom also showed improved spatial resolution in SPC mode compared to EI mode. In SPC mode, images of human distal and middle phalanges showed the trabecular structures of the bone with far better contrast and detail. We also show MAF operation in SPC mode for radionuclide imaging using a custom-built phantom filled with I-125. A 1-mm-diameter parallel hole, medium-energy gamma camera collimator was placed between the phantom, and the MAF and was moved multiple times at equal intervals in random directions to eliminate the pattern corresponding to the collimator septa. Data was acquired at 20 fps, and multiple signal-thresholded frames were summed in SPC mode to provide an integrated frame. The sharpness of the emission image is limited by the collimator resolution and could be improved by optimized collimator design. We demonstrate that the same MAF is capable of operating in both SPC and EI modes and can be used in both X-ray transmission imaging and radionuclide emission imaging.

Focal spot measurements using a digital flat panel detector

Focal spot size is one of the crucial factors that affect the image quality of any x-ray imaging system. It is, therefore, important to measure the focal spot size accurately. In the past, pinhole and slit measurements of x-ray focal spots were obtained using direct exposure film. At present, digital detectors are replacing film in medical imaging so that, although focal spot measurements can be made quickly with such detectors, one must be careful to account for the generally poorer spatial resolution of the detector and the limited usable magnification. For this study, the focal spots of a diagnostic x-ray tube were measured with a 10-μm pinhole using a 194-μm pixel flat panel detector (FPD). The twodimensional MTF, measured with the Noise Response (NR) Method was used for the correction for the detector blurring. The resulting focal spot sizes based on the FWTM (Full Width at Tenth Maxima) were compared with those obtained with a very high resolution detector with 8-μm pixels. This study demonstrates the possible effect of detector blurring on the focal spot size measurements with digital detectors with poor resolution and the improvement obtained by deconvolution. Additionally, using the NR method for measuring the two-dimensional MTF, any non-isotropies in detector resolution can be accurately corrected for, enabling routine measurement of non-isotropic x-ray focal spots. This work presents a simple, accurate and quick quality assurance procedure for measurements of both digital detector properties and x-ray focal spot size and distribution in modern x-ray imaging systems.

Development of a small animal SPECT and CT dual function imager with a microcolumnar CsI(Tl) and CCD based detector

We report on the development of a small animal SPECT/CT dual function imager with a single detector that consists of a 300 μm thick micro-columnar structured CsI(TI) phosphor, a light image intensifier (LII), and a CCD image sensor. The detector was originally designed for x-ray micro-angiographic fluoroscopy (MAF) imaging but used here for both x-ray and gamma-ray detection in the dual function imager. The large variable gain of the LII enables the detector to meet the very different SPECT and x-ray CT sensitivity requirements. The prototype imager allowed SPECT and CT imaging on a common image-subject movement platform. A detachable 5-pinhole tungsten collimator enabled switching between the two modes. Several technology modules including projection preprocessing, imager geometrical calibration, and iterative reconstruction, were developed to generate SPECT and CT images. A 10-mm diameter rod phantom and a 25-gram mouse were scanned with the imager in both SPECT and CT modes. The images showed the expected system resolution and sensitivity performance.

The micro-angiographic fluoroscope (MAF) in High Definition (HD) mode for improved contrast-to-noise ratio and resolution in fluoroscopy and roadmapping

During image guided interventional procedures, superior resolution and image quality is critically important. Operating the MAF in the new High Definition (HD) fluoroscopy mode provides high resolution and increased contrast-to-noise ratio. The MAF has a CCD camera and a 300 micron cesium iodide x-ray convertor phosphor coupled to a light image intensifier (LII) through a fiber-optic taper. The MAF captures 1024 × 1024 pixels with an effective pixel size of 35 microns, and is capable of real-time imaging at 30 fps. The HD mode uses the advantages of higher exposure along with a small focal spot effectively improving the contrast-to-noise ratio (CNR) and the spatial resolution. The Control Acquisition Processing and Image Display System (CAPIDS) software for the MAF controls the LII gain. The interventionalist can select either fluoroscopic or angiographic modes using the two standard foot pedals. When improved image quality is needed and the angiography foot-pedal is used for HD mode, the x-ray machine will operate at a preset higher exposure rate using a small focal spot, while the CAPIDS will automatically adjust the LII gain to achieve proper image brightness. HD mode fluoroscopy and roadmapping are thus achieved conveniently during the interventional procedure. For CNR and resolution evaluation we used a bar phantom with images taken in HD mode with both the MAF and a Flat Panel Detector (FPD). It was seen that the FPD could not resolve more than 2.8 lp/mm whereas the MAF could resolve more than 5 lp/mm. The CNR of the MAF was better than that of the FPD by 60% at lower frequencies and by 600% at the Nyquist frequency of the FPD. The HD mode has become the preferred mode during animal model interventions because it enables detailed features of endovascular devices such as stent struts to be visualized clearly for the first time. Clinical testing of the MAF in HD mode is imminent.

Dose reduction technique using a combination of a region of interest (ROI) material x-ray attenuator and spatially different temporal filtering for fluoroscopic interventions

We demonstrate a novel approach for achieving patient dose savings during image-guided neurovascular interventions, involving a combination of a material x-ray region of interest (ROI) attenuator and a spatially different ROI temporal filtering technique. The part of the image under the attenuator is reduced in dose but noisy and less bright due to fewer x-ray quanta reaching the detector, as compared to the non-attenuating (or less attenuating) region. First the brightness is equalized throughout the image by post processing and then a temporal filter with higher weights is applied to the high attenuating region to reduce the noise, at the cost of increased lag; however, in the regions where less attenuation is present, a lower temporal weight is needed and is applied to preserve temporal resolution. A simulation of the technique is first presented on an actual image sequence obtained from an endovascular image guided interventional (EIGI) procedure. Then the actual implementation of the technique with a physical ROI attenuator is presented. Quantitative analysis including noise analysis and integral dose calculations are presented to validate the proposed technique.

SU-E-I-192: Improved High-Resolution Imaging through an Aneurysm Coil Mass Using the MAF Compared with a Flat Panel Detector

Purpose: To compare the ability of a standard flat panel detector(FPD) with that of the new Microangiographic Fluoroscope (MAF) to visualize individual coils used in minimally invasive (endovascular) treatment of cerebro‐vascular aneurysms where high‐resolution image guidance may be critical to the accuracy and hence outcome of the intervention. Methods: A human neurovascular aneurysm elastomer phantom was filled with standard platinum coils (GDC‐10 Coil SR) and placed on an anthropomorphic head phantom to provide a realistic challenge to the two detectors, both of which were mounted on a Toshiba Infinix C‐arm gantry. The standard FPD (Varian PaxScan‐2020, 194μm pixel‐width, 600μm thick CsI) was compared with a custom MAF (35μm pixel‐width and 300μm thick CsI‐HR type). Using the same x‐ray technique parameters for the same projection view, the images from each detector were digitally contrast matched to enable comparisons of important features such as individual coils and unfilled spaces within the coil mass both for overall qualitative detail and for quantitative measures of corresponding profiles across the features. Results: The FPDimages gave an overall impression of being blurred compared to the MAF images where more individual coils and a clearer demarcation of the spaces between the coils were apparent. The ratio of the average individual coil wire diameter taken from four separate profiles for the FPD to that for the MAF was 1.22, indicating a substantial improvement in detail visualization for the MAF. Conclusions: Standard FPDs have difficulty separating coil wires and visualizing through dense aneurysm coil masses whereas the MAF with its large adjustable dynamic gain and high spatial resolution provides improved visualization and should be able to provide superior image guidance during delicate neurovascular interventions. Support‐NIH Grants R01‐EB008425, R01‐EB002873.

High resolution emission and transmission imaging using the same detector

We demonstrate the capability of one detector, the Micro-Angiographic Fluoroscope (MAF) detector, to image for two types of applications: nuclear medicine imaging and radiography. The MAF has 1024 × 1024 pixels with an effective pixel size of 35 microns and is capable of real-time imaging at 30 fps. It has a CCD camera coupled by a fiber-optic taper to a light image intensifier (LII) viewing a 300-micron thick CsI phosphor. The large variable gain of the LII provides quantum-limited operation with little additive instrumentation noise and enables operation in both energy-integrating (El) and sensitive low-exposure single photon counting (SPC) modes. We used the El mode to take a radiograph, and the SPC mode to image a custom phantom filled with 1 mCi of 1–125. The phantom is made of hot rods with diameters ranging from 0.9 mm to 2.3 mm. A 1 mm diameter parallel hole, medium energy gamma camera collimator was placed between the phantom and the MAF and was moved multiple times at equal intervals in random directions to eliminate the grid pattern corresponding to the collimator septa. Data was acquired at 20 fps. Two algorithms to localize the events were used: 1) simple threshold and 2) a weighted centroid method. Although all the hot rods could be clearly identified, the image generated with the simple threshold method shows more blurring than that with the weighted centroid method. With the diffuse cluster of pixels from each single detection event localized to a single pixel, the weighted centroid method shows improved spatial resolution. A radiograph of the phantom was taken with the same MAF in El mode without the collimator. It shows clear structural details of the rods. Compared to the radiograph, the sharpness of the emission image is limited by the collimator resolution and could be improved by optimized collimator design. This study demonstrated that the same MAF detector can be used in both radioisotope and x-ray imaging, combining the benefits of each.

Dose Reduction Technique Using a Combination of a Region of Interest (ROI) Material X-Ray Attenuator and Spatially Different Temporal Filtering for Fluoroscopic Interventions.

We demonstrate a novel approach for achieving patient dose savings during image-guided neurovascular interventions, involving a combination of a material x-ray region of interest (ROI) attenuator and a spatially different ROI temporal filtering technique. The part of the image under the attenuator is reduced in dose but noisy and less bright due to fewer x-ray quanta reaching the detector, as compared to the non-attenuating (or less attenuating) region. First the brightness is equalized throughout the image by post processing and then a temporal filter with higher weights is applied to the high attenuating region to reduce the noise, at the cost of increased lag; however, in the regions where less attenuation is present, a lower temporal weight is needed and is applied to preserve temporal resolution. A simulation of the technique is first presented on an actual image sequence obtained from an endovascular image guided interventional (EIGI) procedure. Then the actual implementation of the technique with a physical ROI attenuator is presented. Quantitative analysis including noise analysis and integral dose calculations are presented to validate the proposed technique.

Dose reduction by moving a region of interest (ROI) beam attenuator to follow a moving object of interest.

Region-of-interest (ROI) fluoroscopy takes advantage of the fact that most neurovascular interventional activity is performed in only a small portion of an x-ray imaging field of view (FOV). The ROI beam filter is an attenuating material that reduces patient dose in the area peripheral to the object of interest. This project explores a method of moving the beam-attenuator aperture with the object of interest such that it always remains in the ROI. In this study, the ROI attenuator, which reduces the dose by 80% in the peripheral region, is mounted on a linear stage placed near the xray tube. Fluoroscopy is performed using the Microangiographic Fluoroscope (MAF) which is a high-resolution, CCD-based x-ray detector. A stainless-steel stent is selected as the object of interest, and is moved across the FOV and localized using an object-detection algorithm available in the IMAQ Vision package of LabVIEW. The ROI is moved to follow the stent motion. The pixel intensities are equalized in both FOV regions and an adaptive temporal filter dependent on the motion of the object of interest is implemented inside the ROI. With a temporal filter weight of 5% for the current image in the peripheral region, the SNR measured is 47.8. The weights inside the ROI vary between 10% and 33% with a measured SNR of 57.9 and 35.3 when the object is stationary and moving, respectively. This method allows patient dose reduction as well as maintenance of superior image quality in the ROI while tracking the object.