James Scuffham

Pixellated Cd(Zn)Te high-energy X-ray instrument

We have developed a pixellated high energy X-ray detector instrument to be used in a variety of imaging applications. The instrument consists of either a Cadmium Zinc Telluride or Cadmium Telluride (Cd(Zn)Te) detector bump-bonded to a large area ASIC and packaged with a high performance data acquisition system. The 80 by 80 pixels each of 250 μm by 250 μm give better than 1 keV FWHM energy resolution at 59.5 keV and 1.5 keV FWHM at 141 keV, at the same time providing a high speed imaging performance. This system uses a relatively simple wire-bonded interconnection scheme but this is being upgraded to allow multiple modules to be used with very small dead space. The readout system and the novel interconnect technology is described and how the system is performing in several target applications.

K-edge subtraction imaging using a pixellated energy-resolving detector

This paper presents preliminary work aimed at assessing the feasibility of K-edge subtraction imaging using the spectroscopic information provided by a pixellated energy-resolving Cadmium Zinc Telluride detector, having an active area of 20×20 pixels 250 μm in size. Images of a test object containing different amounts of Iodine-based contrast agent were formed above and below the K-edge of Iodine (33.2 keV) by integrating, pixel by pixel, different windows of the spectrum. The results show that the optimum integration window for details 1-2 mm in diameter is between 2 keV and 5 keV. Concentrations of down to 50 μg Iodine/ml were detected in a 1-mm diameter tube with an entrance dose of 100 μGy.

A CdTe detector for hyperspectral SPECT imaging

A Cadmium Telluride (CdTe) detector has been developed for multiple-radioisotope SPECT imaging. The 2 × 2 cm detector has 80 × 80 pixels on a 250 μm pitch and a three-side buttable design so that it can be tiled into larger arrays. The detector is termed hyperspectral as it measures the energy of every photon that interacts in the CdTe to give fully spectroscopic information from 5–200 keV in each pixel. The detector has been tested for applications in multiple-radioisotope SPECT imaging using a 1 mm diameter pinhole configuration and standard phantom test objects containing Tc-99m, I-123 and Ga-67. The detector has an average pixel energy resolution (FWHM) of 0.75% at the I-123 photopeak of 159 keV. We demonstrate the systems capability of resolving spatial features of 2 mm, although the spatial resolution of the detector is limited only by the pixel size and pinhole magnification factor. These characteristics are superior to alternative detectors currently in use in clinical SPECT systems. When imaging multiple radioisotopes simultaneously, we show that there is very little cross-talk between adjacent photopeaks, leading to superior image contrast. The detector is also capable of resolving fluorescence x-rays from the radioactive source, which could be used to improve image count statistics or derive information about the attenuation properties of the source. The performance presented here, and the ability to tile the detector modules to create a clinically useful field of view, makes this technology a strong candidate to be used in future solid-state SPECT cameras.

A 10 cm × 10 cm CdTe Spectroscopic Imaging Detector based on the HEXITEC ASIC

The 250 μ m pitch 80x80 pixel HEXITEC detector systems have shown that spectroscopic imaging with an energy resolution of <1 keV FWHM per pixel can be readily achieved in the range of 5–200 keV with Al-pixel CdTe biased to −500 V. This level of spectroscopic imaging has a variety of applications but the ability to produce large area detectors remains a barrier to the adoption of this technology. The limited size of ASICs and defect free CdTe wafers dictates that building large area monolithic detectors is not presently a viable option. A 3-side buttable detector module has been developed to cover large areas with arrays of smaller detectors. The detector modules are 20.35 × 20.45 mm with CdTe bump bonded to the HEXITEC ASIC with coverage up to the edge of the module on three sides. The fourth side has a space of 3 mm to allow I/O wire bonds to be made between the ASIC and the edge of a PCB that routes the signals to a connector underneath the active area of the module. The detector modules have been assembled in rows of five modules with a dead space of 170 μ m between each module. Five rows of modules have been assembled in a staggered height array where the wire bonds of one row of modules are covered by the active detector area of a neighboring row. A data acquisition system has been developed to digitise, store and output the 24 Gbit/s data that is generated by the array. The maximum bias magnitude that could be applied to the CdTe detectors from the common voltage source was limited by the worst performing detector module. In this array of detectors a bias of −400 V was used and the detector modules had 93 % of pixels with better than 1.2 keV FWHM at 59.5 keV. An example of K-edge enhanced imaging for mammography was demonstrated. Subtracting images from the events directly above and below the K-edge of the Iodine contrast agent was able to extract the Iodine information from the image of a breast phantom and improve the contrast of the images. This is just one example where the energy spectrum per pixel can be used to develop new and improve existing X-ray imaging techniques.

The importance of scatter correction for the assessment of lung shunting prior to yttrium-90 radioembolization therapy.

BackgroundTreatment of inoperable hepatocellular carcinoma or secondary metastases by radioembolization using yttrium-90 (90Y) microspheres is a promising method for the treatment of unresectable liver metastases. A pretreatment scintigraphy planar scan using 99mTc-labelled macroaggregated albumin (99mTc-MAA) injected directly into the hepatic artery is carried out to assess the degree of portal shunting of blood between the liver and the lungs. The quantitative results of this scan are used to modulate the activity of therapeutic 90Y microspheres injected into the patient to limit the radiation dose received by the lungs. The presence of scattered events in the MAA lung shunt scan leads to an overestimation of the true shunting ratio, which in turn leads to the administered therapeutic activity being lowered unnecessarily to comply with the protocols of radiation protection. Overall, this may impact the efficacy of treatment. Materials and methodsThis study analyses the impact of a window-based analytical scatter-correction method on lung shunt analysis using an anthropomorphic torso phantom, and retrospectively analysed three patient case studies. ResultsOur results of scatter in the phantom show a marked decrease in the lung shunt percentage. Clinical analysis of patient data shows that the lung shunt percentage can be overestimated by up to 50% in clinical cases, and depending on the lung shunt percentage, the efficacy of treatment by therapeutic dose reduction may be compromised. ConclusionOur results indicate that scatter correction should be used on 90Y pretreatment 99mTc-MAA scans in order to more accurately assess the lung shunting percentage before therapy.

Algorithms for spectral calibration of energy-resolving small-pixel detectors

Small pixel Cd(Zn)Te detectors often suffer from inter-pixel variations in gain, resulting in shifts in the individual energy spectra. These gain variations are mainly caused by inclusions and defects within the crystal structure, which affect the charge transport within the material causing a decrease in the signal pulse height. In imaging applications, spectra are commonly integrated over a particular peak of interest. This means that the individual pixels must be accurately calibrated to ensure that the same portion of the spectrum is integrated in every pixel. The development of large-area detectors with fine pixel pitch necessitates automated algorithms for this spectral calibration, due to the very large number of pixels. Algorithms for automatic spectral calibration require accurate determination of characteristic x-ray or photopeak positions on a pixelwise basis. In this study, we compare two peak searching spectral calibration algorithms for a small-pixel CdTe detector in gamma spectroscopic imaging. The first algorithm uses rigid search ranges to identify peaks in each pixel spectrum, based on the average peak positions across all pixels. The second algorithm scales the search ranges on the basis of the position of the highest-energy peak relative to the average across all pixels. In test spectra acquired with Tc-99m, we found that the rigid search algorithm failed to correctly identify the target calibraton peaks in up to 4% of pixels. In contrast, the scaled search algorithm failed in only 0.16% of pixels. Failures in the scaled search algorithm were attributed to the presence of noise events above the main photopeak, and possible non-linearities in the spectral response in a small number of pixels. We conclude that a peak searching algorithm based on scaling known peak spacings is simple to implement and performs well for the spectral calibration of pixellated radiation detectors.

Three dosimetry models of lipoma arborescens treated by 90Y synovectomy.

PURPOSE Lipoma arborescens (LA) is a benign intra-articular lipomatous proliferation of the synovial membrane. This extremely rare condition has previously been treated by intra-articular (90)Y radiosynoviorthesis but dosimetry literature on this form of radionuclide therapy is nonexistent. The authors detail methodology for successful treatment of LA and provide for the first time estimates of radiation dosimetry. The authors also analyze the biodistribution of the radiopharmaceutical over the course of the patients treatment through sequential imaging. METHODS A patient with bilateral LA underwent intracavity injection of (90)Y citrate colloid to the right and left knee joint spaces (181 and 198 MBq, respectively). SPECT/CT datasets were acquired over 9 days to quantify the biodistribution and kinetics of the radiopharmaceutical. Radiation dosimetry was performed using the MIRD schema (through OLINDA software), a custom voxel-based method, and a direct Monte Carlo calculation (OEDIPE). RESULTS Follow-up MRI showed marked reduction in LA size in both knees. Mean absorbed doses to the LA were 21.2 ± 0.8 and 42.9 ± 2.3 Gy using OLINDA, 8.1 ± 0.3 and 16.7 ± 0.5 Gy using voxel based methodology, and 8.2 ± 0.3 and 15.7 ± 0.5 Gy for OEDIPE in the right and left LA, respectively. Distribution of the radiopharmaceutical within the joint space alters over the imaging period, with less than 1% of the remaining activity having moved posteriorly in the knee cavity. No uptake was detected outside of the joint space after assessment with whole-body scintigraphy. CONCLUSIONS An activity of approximately 185 MBq successfully relieved clinical symptoms of LA. There was good correlation between direct Monte Carlo and voxel based techniques, but OLINDA was shown to overestimate the absorbed dose to the tumor. Accurate dosimetry may help select an activity more tailored to the specific size and location of the LA.

Lipoma Arborescens Successfully Treated With 90Y Synovectomy

Lipoma arborescens (LA) in the knee is a benign intra-articular lipomatous proliferation of the synovial membrane, and data on treatment of LA by radionuclide therapy is sparse. We present a rare case of bilateral LA in the knees successfully treated with injection of Y-citrate colloid. We assessed the biodistribution of the radiopharmaceutical through the use of SPECT/CT imaging. Our images show slight redistribution of the radiocolloid in the knee joint, whereas most of the radioactivity remains localized around the LA. MRI scans confirmed the efficacy of this treatment, with a significant reduction in LA volume after 6 months.

Optimization of K-edge subtraction imaging using a pixellated spectroscopic detector

Conventional K-edge subtraction imaging is based around the acquisition of two separate images at energies respectively below and above the K-edge of a contrast agent. This implies increased patient dose with respect to a conventional procedure and potentially incorrect image registration due to patient motion. We present results obtained with a pixellated spectroscopic CdTe detector. A spectroscopic detector allows simultaneous acquisition of the two images by integrating appropriate bands from the transmitted X-ray spectrum, thus removing the above limitations; the photon counting capability of the detector limits the noise to statistical noise, thus minimizing the dose for a given signal-to-noise ratio. Furthermore, an appropriate choice of the integration bands allows optimization of image quality, resulting from a trade-off between background removal (maximum with a narrow band) and low statistical noise (achieved with a broad band). Results obtained with a custom test-object, simulating breast structures, and with an iodine-based contrast agent, are presented for two different image subtraction algorithms: logarithmic subtraction and dual-energy linear combination. Whilst being conceptually simpler, logarithmic subtraction is strongly dependent upon the position and width of the band selected, while linear combination allows better background removal even with a broad energy band, and therefore better image quality.

Scatter free imaging for the improvement of breast cancer detection in mammography

In mammography, the reduction of scattered x-rays is vital due to the low contrast or small dimension of the details that are searched for. The typical method of doing so in current conventional mammography is the anti-scatter grid. The disadvantage of this method is the absorption of a proportion of the primary beam and therefore an increase in dose is required to compensate for the loss of counts. An alternative method is proposed, using quasi-monochromatic beams and a pixellated spectroscopic detector. As Compton-scattered x-rays lose energy in the scattering process, they are detected at a lower energy in the spectrum. Therefore the spectrum can be windowed around the monochromatic energy peak, removing the scattered x-rays from the image. The work presented here shows contrast improvement of up to 50% and contrast to noise ratio improvements of around 20% for scatter free imaging in comparison to full spectrum imaging. Contrast improvements of around 45% were found when comparing scatter free images to conventional polychromatic imaging for both the low contrast test object and the Rachel anthropomorphic breast phantom.