Technical Note: Combination of multiple EPID imager layers improves image quality and tracking performance of low contrast-to-noise objects

PURPOSE

We hypothesized that combining multiple amorphous silicon flat panel layers increases photon detection efficiency in an electronic portal imaging device (EPID), improving image quality and tracking accuracy of low-contrast targets during radiotherapy.

METHODS

The prototype imager evaluated in this study contained four individually programmable layers each with a copper converter layer, Gd₂ O₂ S scintillator, and active-matrix flat panel imager (AMFPI). The imager was placed on a Varian TrueBeam linac and a Las Vegas phantom programmed with sinusoidal motion (peak-to-peak amplitude = 20 mm, period = 3.5 s) was imaged at a frame rate of 10 Hz with one to four layers activated. Number of visible circles and CNR of least visible circle (depth = 0.5 mm, diameter = 7 mm) was computed to assess the image quality of single and multiple layers. A previously validated tracking algorithm was employed for auto-tracking. Tracking error was defined as the difference between the programmed and tracked positions of the circle. Pearson correlation coefficient (R) of CNR and tracking errors was computed.

RESULTS

Motion-induced blurring significantly reduced circle visibility. During four cycles of phantom motion, the number of visible circles varied from 11-23, 13-24, 15-25, and 16-26 for one-, two-, three-, and four-layer imagers, respectively. Compared with using only a single layer, combining two, three, and four layers increased the median CNR by factors of 1.19, 1.42, and 1.71, respectively and reduced the average tracking error from 3.32 mm to 1.67 mm to 1.47 mm, and 0.74 mm, respectively. Significant correlations (P~10^-9 ) were found between the tracking error and CNR.

CONCLUSION

Combination of four conventional EPID layers significantly improves the EPID image quality and tracking accuracy for a poorly visible object which is moving with a frequency and amplitude similar to respiratory motion.

Super-resolution imaging in a multiple layer EPID

A new portal imager consisting of four vertically stacked conventional electronic portal imaging device (EPID) layers has been constructed in pursuit of improved detective quantum efficiency (DQE). We hypothesize that super-resolution (SR) imaging can also be achieved in such a system by shifting each layer laterally by half a pixel relative to the layer above. Super-resolution imaging will improve resolution and contrast-to-noise ratio (CNR) in megavoltage (MV) planar and cone beam computed tomography (MV-CBCT) applications. Simulations are carried out to test this hypothesis with digital phantoms. To assess planar resolution, 2 mm long iron rods with 0.3 × 0.3 mm² square cross-section are arranged in a grid pattern at the center of a 1 cm thick solid water. For measuring CNR in MV-CBCT, a 20 cm diameter digital phantom with 8 inserts of different electron densities is used. For measuring resolution in MV-CBCT, a digital phantom featuring a bar pattern similar to the Gammex™ phantom is used. A 6 MV beam is attenuated through each phantom and detected by each of the four detector layers. Fill factor of the detector is explicitly considered. Projections are blurred with an estimated point spread function (PSF) before super-resolution reconstruction. When projections from multiple shifted layers are used in SR reconstruction, even a simple shift-add fusion can significantly improve the resolution in reconstructed images. In the reconstructed planar image, the grid pattern becomes visually clearer. In MV-CBCT, combining projections from multiple layers results in increased CNR and resolution. The inclusion of two, three and four layers increases CNR by 40%, 70% and 99%, respectively. Shifting adjacent layers by half a pixel almost doubles resolution. In comparison, using four perfectly aligned layers does not improve resolution relative to a single layer.

A novel EPID design for enhanced contrast and detective quantum efficiency

Beams-eye-view imaging applications such as real-time soft-tissue motion estimation are hindered by the inherently low image contrast of electronic portal imaging devices (EPID) currently available for clinical use. We introduce and characterize a novel EPID design that provides substantially increased detective quantum efficiency (DQE), contrast-to-noise ratio (CNR) and sensitivity without degradation in spatial resolution. The prototype design features a stack of four conventional EPID layers combined with low noise integrated readout electronics. Each layer consists of a copper plate, a scintillator ([Formula: see text]) and a photodiode/TFT-switch (aSi:H). We characterize the prototype's signal response to a 6 MV photon beam in terms of modulation transfer function (MTF), DQE and CNR. The presampled MTF is estimated using a slanted slit technique, the DQE is calculated from measured normalized noise power spectra (nNPS) and the MTF and CNR is estimated using a Las Vegas contrast phantom. The prototype has been designed and built to be interchangeable with the current clinical EPID on the Varian TrueBeam platform (AS-1200) in terms of size and data output specifications. Performance evaluation is conducted in absolute values as well as in relative terms using the Varian AS-1200 EPID as a reference detector. A fivefold increase of DQE(0) to about 6.7% was observed by using the four-layered design versus the AS-1200 reference detector. No substantial differences are observed between each layer's individual MTF and the one for all four layers operating combined indicating that defocusing due to beam divergence is negligible. Also, using four layers instead of one increases the signal to noise ratio by a factor of 1.7.

A piecewise-focused high DQE detector for MV imaging

PURPOSE

Electronic portal imagers (EPIDs) with high detective quantum efficiencies (DQEs) are sought to facilitate the use of the megavoltage (MV) radiotherapy treatment beam for image guidance. Potential advantages include high quality (treatment) beam's eye view imaging, and improved cone-beam computed tomography (CBCT) generating images with more accurate electron density maps with immunity to metal artifacts. One approach to increasing detector sensitivity is to couple a thick pixelated scintillator array to an active matrix flat panel imager (AMFPI) incorporating amorphous silicon thin film electronics. Cadmium tungstate (CWO) has many desirable scintillation properties including good light output, a high index of refraction, high optical transparency, and reasonable cost. However, due to the 0 1 0 cleave plane inherent in its crystalline structure, the difficulty of cutting and polishing CWO has, in part, limited its study relative to other scintillators such as cesium iodide and bismuth germanate (BGO). The goal of this work was to build and test a focused large-area pixelated "strip" CWO detector.

METHODS

A 361 × 52 mm scintillator assembly that contained a total of 28 072 pixels was constructed. The assembly comprised seven subarrays, each 15 mm thick. Six of the subarrays were fabricated from CWO with a pixel pitch of 0.784 mm, while one array was constructed from BGO for comparison. Focusing was achieved by coupling the arrays to the Varian AS1000 AMFPI through a piecewise linear arc-shaped fiber optic plate. Simulation and experimental studies of modulation transfer function (MTF) and DQE were undertaken using a 6 MV beam, and comparisons were made between the performance of the pixelated strip assembly and the most common EPID configuration comprising a 1 mm-thick copper build-up plate attached to a 133 mg/cm(2) gadolinium oxysulfide scintillator screen (Cu-GOS). Projection radiographs and CBCT images of phantoms were acquired. The work also introduces the use of a lightweight edge phantom to generate MTF measurements at MV energies and shows its functional equivalence to the more cumbersome slit-based method.

RESULTS

Measured and simulated DQE(0)'s of the pixelated CWO detector were 22% and 26%, respectively. The average measured and simulated ratios of CWO DQE(f) to Cu-GOS DQE(f) across the frequency range of 0.0-0.62 mm(-1) were 23 and 29, respectively. 2D and 3D imaging studies confirmed the large dose efficiency improvement and that focus was maintained across the field of view. In the CWO CBCT images, the measured spatial resolution was 7 lp/cm. The contrast-to-noise ratio was dramatically improved reflecting a 22 × sensitivity increase relative to Cu-GOS. The CWO scintillator material showed significantly higher stability and light yield than the BGO material.

CONCLUSIONS

An efficient piecewise-focused pixelated strip scintillator for MV imaging is described that offers more than a 20-fold dose efficiency improvement over Cu-GOS.

SU-E-I-11: A Novel High Resolution CT Imager for a Breast SPECT-CT System

PURPOSE

To evaluate the intrinsic characteristics of a new high resolution, large area, small bezel, digital imager developed for a dedicated SPECT-CT system for fully-3D mammotomographic imaging applications.

METHODS

A new flat panel digital imager was developed having 127μm pixels (identical to those in the Paxscan 2520) with an active 40×30cm area and 3200×2304 total pixels, which is well suited for pendant breast imaging close to the chest wall. The readout and driver ASIC TAB bonding pads were arranged on only two sides of the imager plate, to eliminate dead edge spacing. A special housing was developed to accommodate this imaging plate providing very narrow 8mm edges (bezels) on two orthogonal sides The TFT array imager was coated with 600μm thick micro-columnar CsI, which provides >90% absorption in the breast CT (60-80kVp) x-ray energy range. It provides better than 70% absorption for RQA5 (70kVp 21mm Al filter) x- ray radiation. The readout ASICs are connected to 14 bit A/D converters, and special readout ASICs with gain switching feature, which provide an additional 4bit virtual (2-3 effective) dynamic range.

RESULTS

The MTF is ∼7.5% at the 3.9 lp/mm Nyquist frequency. The NPS curves continuously decrease with increasing spatial frequencies, characteristic of scintillator based imagers. The excellent DQE (0) measured at RQA5 is ∼75% in the 0.1-1mR dose range and ∼10% at the Nyquist frequency. The imager has excellent linearity over the full dynamic range, and high contrast images are readily acquired.

CONCLUSIONS

The new 4030 flat panel imager is a high resolution, state of the art detector, ideal for breast imaging and other applications. This detector is included in the SPECT-CT device in development.

[Emergency treatment of corrosive gastrointestinal injuries]

Authors discuss their experience of management for caustic gastrointestinal injury. Importance of urgent endoscopy in establishing the extend of injury is emphasized. In eight out of 12 surgical intervention an esophagogastrectomy were performed. According to their experience in case of serious caustic burn the only hope to save the patients life is the early radical operation.

The pharmacokinetics, urinary and biliary excretion of pipecuronium bromide

The pharmacodynamics and -kinetics of pipecuronium were studied in 12 patients, six of whom received 100 micrograms kg-1 for laryngectomy (Group L), and six who underwent choledochotomy after insertion of the T-drain and were given 50 micrograms kg-1 (Group C). Onset time and clinical duration were 2.3 and 109 min and 2.8 and 39 min in Groups L and C, respectively. All patients could be sufficiently reversed with neostigmine. Terminal half-lives were 101.5 min (Group L) and 179 min (Group C) in a three-exponent decay; the distribution volumes at steady state 0.339 l kg-1 (Group L) and 0.506 l kg-1 (Group C); the plasma clearance 3.4 ml kg-1 min-1 (Group L) and 2.5 ml kg-1 min-1 (Group C). Within 24 h, 38.6% and 37% were excreted unchanged in the urine and 4.4% and 1% as 3-desacetyl pipecuronium in Groups L and C, respectively. Within 24 h only 2% was excreted into the bile in Group C. Distribution volume and terminal half-life in Group C were positively correlated with pre-operative serum aminotransferase levels (P less than 0.005).

Emergency esophagogastrectomy for treatment of hydrochloric acid injury

Eight patients with severe hydrochloric acid injury of the esophagus and stomach had early esophagogastrectomy. Four patients survived and 6 to 8 weeks later underwent successful substernal colon replacement. The value of early diagnosis and subsequent aggressive surgical management of patients who have ingested hydrochloric acid is stressed.