Larry Partain

Megavoltage cone-beam computed tomography using a high-efficiency image receptor

PURPOSE To develop an image receptor capable of forming high-quality megavoltage CT images using modest radiation doses. METHODS AND MATERIALS A flat-panel imaging system consisting of a conventional flat-panel sensor attached to a thick CsI scintillator has been fabricated. The scintillator consists of individual CsI crystals 8 mm thick by 0.38 mm x 0.38-mm pitch. Five sides of each crystal are coated with a reflecting powder/epoxy mixture, and the sixth side is in contact with the flat-panel sensor. A timing interface coordinates acquisition by the imaging system and pulsing of the linear accelerator. With this interface, as little as one accelerator pulse (0.023 cGy at the isocenter) can be used to form projection images. Different CT phantoms irradiated by a 6-MV X-ray beam have been imaged to evaluate the performance of the imaging system. The phantoms have been mounted on a rotating stage and rotated while 360 projection images are acquired in 48 s. These projections have been reconstructed using the Feldkamp cone-beam CT reconstruction algorithm. RESULTS AND DISCUSSION Using an irradiation of 16 cGy (360 projections x 0.046 cGy/projection), the contrast resolution is approximately 1% for large objects. High-contrast structures as small as 1.2 mm are clearly visible. The reconstructed CT values are linear (R(2) = 0.98) for electron densities between 0.001 and 2.16 g/cm(3), and the reconstruction time for a 512 x 512 x 512 data set is 6 min. Images of an anthropomorphic phantom show that soft-tissue structures such as the heart, lung, kidneys, and liver are visible in the reconstructed images (16 cGy, 5-mm-thick slices). CONCLUSIONS The acquisition of megavoltage CT images with soft-tissue contrast is possible with irradiations as small as 16 cGy.

Mercuric iodide medical imagers for low exposure radiography and fluoroscopy

Photoconductive polycrystalline mercuric iodide deposited on flat panel thin film transistor (TFT) arrays is being developed for direct digital X-ray detectors that can perform both radiographic and fluoroscopic medical imaging. The mercuric iodide is either vacuum deposited by Physical Vapor Deposition (PVD) or coated onto the array by a wet Particle-In-Binder (PIB) process. The PVD deposition technology has been scaled up to the 20 cm x 25 cm size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns is used for these imagers. Arrays of 10 cm x 10 cm size have been used to evaluate performance of mercuric iodide imagers. Radiographic and fluoroscopic images of diagnostic quality at up to 15 pulses per second were demonstrated. As we previously reported, the resolution is limited to the TFT array Nyquist frequency of ~3.9 lp/mm (127 micron pixel pitch). Detective Quantum Efficiency (DQE) has been measured as a function of spatial frequency for these imagers. The DQE is lower than the theoretically calculated value due to some additional noise sources of the electronics and the array. We will retest the DQE after eliminating these noise sources. Reliability and stress testing was also began for polycrystalline mercuric iodide PVD and PIB detectors. These are simplified detectors based upon a stripe electrode or circular electrode structure. The detectors were stressed under various voltage bias, temperature and time conditions. The effects of the stress tests on the detector dark current and sensitivity were determined.

Mercuric iodide and lead iodide x-ray detectors for radiographic and fluoroscopic medical imaging

Mercuric iodide (HgI2) and lead iodide (PbI2) have been under development for several years as direct converter layers in digital x-ray imaging. Previous reports have covered the basic electrical and physical characteristics of these and several other materials. We earlier reported on 5cm x 5cm and 10cm x 10cm size imagers, direct digital radiography X-ray detectors, based on photoconductive polycrystalline mercuric iodide deposited on a flat panel thin film transistor (TFT) array, as having great potential for use in medical imaging, NDT, and security applications. This paper, presents results and comparison of both lead iodide and mercuric iodide imagers scaled up to 20cm x 25cm sizes. Both the mercuric iodide and lead iodide direct conversion layers are vacuum deposited onto TFT array by Physical Vapor Deposition (PVD). This process has been successfully scaled up to 20cm x 25cm -- the size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns was used for this imager. In addition to increasing detector size, more sophisticated, non-TFT based small area detectors were developed in order to improve analysis methods of the mercuric and lead iodide photoconductors. These small area detectors were evaluated in radiographic mode, continuous fluoroscopic mode and pulsed fluoroscopic mode. Mercuric iodide coating thickness ranging between 140 microns and 300 microns and lead iodide coating thickness ranging between 100 microns and 180 microns were tested using beams with energies between 40 kVp and 100 kVp, utilizing exposure ranges typical for both fluoroscopic and radiographic imaging. Diagnostic quality radiographic and fluoroscopic images have been generated at up to 15 frames per second. Mercuric iodide image lag appears adequate for fluoroscopic imaging. The longer image lag characteristics of lead iodide make it only suitable for radiographic imaging. For both material the MTF is determined primarily by the aperture and pitch of the TFT array (Nyquist frequency of ~3.93 mm-1 (127 micron pixel pitch).

Dark current and DQE improvements of mercuric iodide medical imagers

A new TFT array has been developed specifically for mercuric iodide (HgI2) deposition. This new TFT array combined with a modified HgI2 Physical Vapor Deposition (PVD) process provides less than 10 pA/mm2 dark current at room temperature (22 °C) measured at 1 V/&mgr;m electrical field. This photoconductor (direct) imager was run at 10 fr/s framerate and gave a measured sensitivity of 19 μC/(R*cm2) using a RQA5 radiation quality x-ray beam (70kVp x-ray with 21 mm Al filtering). This sensitivity value is higher than the sensitivity reported by our group for any previous HgI2 imagers. MTF, NPS and DQE values were also evaluated on this 13 cm x 13 cm size imager with 127 μm pixel pitch. The MTF value is higher than 40% at the Nyquist frequency (3.9 lp/mm). This is much better than the MTF of a 600 μm CsI scintillator/photodiode (indirect) imager, which is only 16% (Varian internal data) and it is similar to the MTF value of the a-Se (another photoconductor) imagers. The first frame image lag is less than 8% when the imager was run at a 10 fr/s framerate. The low dark current and some noise reduction in the detector electronics, made it possible for the DQE to be measured down to low fluoroscopic dose levels (< 4 μR/fr). The DQE(0) value is over 50% at a dose of 35 μR/fr and still about 40% at 3.76 μR/fr. The 270 μm thick PVD HgI2 layer only absorbs less than 75% of the ~51 keV mean energy X-ray photons (70 kVp RQA5 filtered beam). This means that if the thickness of the HgI2 layer is increased to 500 μm (increasing the absorption up to over 90%) the DQE(0) should then increase to about 60- 65% (assuming everything else remains unchanged). This value is close to the 65 - 70 % DQE(0), measured for the indirect (CsI) imagers at higher doses. Such a high DQE value makes this material competitive both for fluoroscopic and for radiographic applications.

Improved properties of PbI2 x-ray imagers with tighter process control and using positive bias voltage

Vapor deposited lead iodide films show a wide range of physical attributes dependant upon fabrication conditions. High density is most readily achieved with films less than 100 μm. Thicker films, with lessening density, often show lower response (gain) as charge collection becomes less efficient. Lack of consistency in density throughout a deposition invariably leads to non-uniform electronic properties, which is challenging to both model and predict. To overcome this, tighter control of deposition parameters is required during the slow growth process (<10 μm/hour). Lead iodide films are characterized in forms of planar devices deposited onto conductive glass and active pixel arrays deposited onto a-Si TFT arrays1. Electronic properties (e.g. leakage current, gain) show little variation that can be traced to substrate choice. Films generally provide less than 100 pA/mm2 leakage current as they show saturation in gain (at approximate fields of 1 V/μm). We recently modified our readout electronics to accept positive bias. Using positive bias on the top electrode provides better charge collection for the lower mobility electrons and (despite process variability) better quality films can provide sensitivities greater than 6 μC/R*cm2, with only partial x-ray absorption, and show less than 20 pA/mm2 dark current.

50 μm pixel size a-Se mammography imager with high DQE and increased temperature resistance

The imager presented in this paper has a special blocking structure that ensures very low dark current of less than 1 pA/mm2 even with a 20 V/μm electric field. Hence the electric field can be increased from the generally applied 10 V/μm to 20V/μm, this reduces the energy required to produce an electron hole (e-h) pair from 60 eV to about 36 eV at the given (19.3 keV mean) mammo energy. Furthermore, with special doping and manufacturing processes this a-Se layer is very stable in the 0-70 C° temperature range as demonstrated by Ogusu et al. [1]. A new 5 cm × 5 cm size TFT array was developed with 50 μm pixel size, specifically for testing the resolution of photoconductor based imagers. The first new imager of this type had a 200 μm thick a-Se layer evaporated onto the array. Its MTF, NPS, and DQE values were evaluated using 28kVp Mo anode x-ray source with a 0.03mm thick Mo and an additional 2 mm thick Al filters. The MTF value is about 40% and 50% in x-and y directions at the Nyquist frequency of 10 lp/mm. The low frequency DQE at 20 V/μm electrical field is ~70% at 151 μGy dose and drops only about 10% when going down to 23 μGy. This new array also has excellent lag properties. The measured first frame image lag at 20 V/μm is less than 1%. Such low lag provides opportunities to use this material not only for mammography but also for breast tomosynthesis applications. Breast phantom images demonstrate that even the smallest 0.13 mm calcifications are clearly visible with this high-resolution imager.

Characterization of mercuric iodide photoconductor for radiographic and fluoroscopic medical imagers

Photoconductive polycrystalline mercuric iodide deposited on flat panel thin film transistor (TFT) arrays is one of the best candidates for direct digital X-ray detectors for radiographic and fluoroscopic medical imaging. The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This deposition technology has been scaled up to the 20cmX25cm size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns is used for these imagers. In addition to successful imager scale up, non-TFT based detectors were developed in order to improve analysis methods of the mercuric iodide photoconductor itself. These substrates consist of an array of palladium or ITO stripes on a glass substrate. Following deposit of the photoconductor, striped bias electrodes are deposited on top of the photoconductor at a 90 degree orientation to the bottom electrodes. These substrates provide more information than was previously available on the dark current and signal uniformity of the mercuric iodide photoconductor without the use of expensive TFT arrays. Mercuric iodide photoconductor thicknesses between 110 microns and 300 microns were tested with beam energy between 40 kVp and 120 kVp utilizing exposure ranges typical for both fluoroscopic and radiographic imaging. Diagnostic quality radiographic and fluoroscopic images at up to 15 pulses per second were demonstrated. Resolution tests on resolution target phantoms were performed and performance close to the theoretical sinc function up to the Nyquist frequency of ~3.9 lp/mm is shown (127 micron pixel pitch).

Medical imaging with mercuric iodide direct digital radiography flat-panel x-ray detectors

Photoconductive polycrystalline mercuric iodide coated on amorphous silicon flat panel thin film transistor (TFT) arrays is the best candidate for direct digital X-ray detectors for radiographic and fluoroscopic applications in medical imaging. The mercuric iodide is vacuum deposited by Physical Vapor Deposition (PVD). This coating technology is capable of being scaled up to sizes required in common medical imaging applications. Coatings were deposited on 2”×2” and 4”×4” TFT arrays for imaging performance evaluation and also on conductive-coated glass substrates for measurements of X-ray sensitivity and dark current. TFT arrays used included pixel pitch dimensions of 100, 127 and 139 microns. Coating thickness between 150 microns and 250 microns were tested with beam energy between 25 kVP and 100kVP utilizing exposure ranges typical for both fluoroscopic, and radiographic imaging. X-ray sensitivities measured for the mercuric iodide samples and coated TFT detectors were superior to any published results for competitive materials (up to 7100 ke/mR/pixel for 100 micron pixels). It is believed that this higher sensitivity can result in fluoroscopic imaging signal levels high enough to overshadow electronic noise. Diagnostic quality of radiographic and fluoroscopic images of up to 15 pulses per second were demonstrated. Image lag characteristics appear adequate for fluoroscopic rates. Resolution tests on resolution target phantoms showed that resolution is limited to the TFT array Nyquist frequency including detectors with pixel size of 139 microns resolution ~3.6 lp/mm) and 127 microns (resolution~3.9 lp/mm). The ability to operate at low voltages (~0.5 volt/micron) gives adequate dark currents for most applications and allows low voltage electronics designs.

Initial evaluation of a newly developed high resolution CT imager for dedicated breast CT

A new, high resolution 40x30cm2 area CsI-TFT based CT imager having 127μm pixel pitch was developed for fully-3D breast CT imaging as part of a SPECT-CT system. The imager has two narrow edges suited for pendant breast CT imaging close to the chest wall. The scintillator thickness of 600 microns provides >90% absorption for the 36keV mean x-ray energy of the cone beam source. The 2D MTF is ˜7.5% at the 3.9 lp/mm Nyquist frequency. The imager has excellent linearity over the full dynamic range. The imager is mounted on the CT device and initial tomographic imaging of geometric and breast phantoms demonstrate the reliable and robust imaging capabilities of this device for breast CT.

A forward bias method for lag correction of an a-Si flat panel detector.

PURPOSE Digital a-Si flat panel (FP) x-ray detectors can exhibit detector lag, or residual signal, of several percent that can cause ghosting in projection images or severe shading artifacts, known as the radar artifact, in cone-beam computed tomography (CBCT) reconstructions. A major contributor to detector lag is believed to be defect states, or traps, in the a-Si layer of the FP. Software methods to characterize and correct for the detector lag exist, but they may make assumptions such as system linearity and time invariance, which may not be true. The purpose of this work is to investigate a new hardware based method to reduce lag in an a-Si FP and to evaluate its effectiveness at removing shading artifacts in CBCT reconstructions. The feasibility of a novel, partially hardware based solution is also examined. METHODS The proposed hardware solution for lag reduction requires only a minor change to the FP. For pulsed irradiation, the proposed method inserts a new operation step between the readout and data collection stages. During this new stage the photodiode is operated in a forward bias mode, which fills the defect states with charge. A Varian 4030CB panel was modified to allow for operation in the forward bias mode. The contrast of residual lag ghosts was measured for lag frames 2 and 100 after irradiation ceased for standard and forward bias modes. Detector step response, lag, SNR, modulation transfer function (MTF), and detective quantum efficiency (DQE) measurements were made with standard and forward bias firmware. CBCT data of pelvic and head phantoms were also collected. RESULTS Overall, the 2nd and 100th detector lag frame residual signals were reduced 70%-88% using the new method. SNR, MTF, and DQE measurements show a small decrease in collected signal and a small increase in noise. The forward bias hardware successfully reduced the radar artifact in the CBCT reconstruction of the pelvic and head phantoms by 48%-81%. CONCLUSIONS Overall, the forward bias method has been found to greatly reduce detector lag ghosts in projection data and the radar artifact in CBCT reconstructions. The method is limited to improvements of the a-Si photodiode response only. A future hybrid mode may overcome any limitations of this method.