Megavoltage planar and cone‐beam imaging with low‐ targets: Dependence of image quality improvement on beam energy and patient separation

Robar, James L.; Connell, T; Huang, Wei-Hong; Kelly, R

doi:10.1118/1.3183499

Cited by 28 publications

(47 citation statements)

References 33 publications

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“…The thinner thickness of the target also decreases the amount of self‐absorption and any further attenuation of these low‐energy photons can be eliminated by removing the flattening filter. This beamline configuration subsequently improves the image contrast as demonstrated by several previous studies 10 , 12 , 13 , 14 , 15 …”

Section: Introductionsupporting

confidence: 62%

Low‐dose 2.5 MV cone‐beam computed tomography with thick CsI flat‐panel imager

Tang

Moussot

Morf

et al. 2016

J Applied Clin Med Phys

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Most of the treatment units, both new and old models, are equipped with a megavoltage portal imager but its use for volumetric imaging is limited. This is mainly due to the poor image quality produced by the high‐energy treatment beam (>6 MV). A linac at our center is equipped with a prototype 2.5 MV imaging beam. This study evaluates the feasibility of low‐dose megavoltage cone‐beam imaging with the 2.5 MV beam and a thick cesium iodide detector, which is a high‐efficiency imager. Basic imaging properties such as spatial resolution and modulation transfer function were assessed for the 2.5 MV prototype imaging system. For image quality and imaging dose, a series of megavoltage cone‐beam scans were acquired for the head, thorax, and pelvis of an anthropomorphic phantom and were compared to kilovoltage cone‐beam and 6X megavoltage cone‐beam images. To demonstrate the advantage of MV imaging, a phantom with metallic inserts was scanned and the image quality was compared to CT and kilovoltage cone‐beam scans. With a lower energy beam and higher detector efficiency, the 2.5 MV imaging system generally yields better image quality than does the 6 MV imaging system with the conventional MV imager. In particular, with the anthropomorphic phantom studies, the contrast to noise of bone to tissue is generally improved in the 2.5 MV images compared to 6 MV. With an image quality sufficient for bony alignment, the imaging dose for 2.5 MV cone‐beam images is 2.4−3.4 MU compared to 26 MU in 6 MV cone‐beam scans for the head, thorax, and pelvis regions of the phantom. Unlike kilovoltage cone‐beam, the 2.5 MV imaging system does not suffer from high‐Z image artifacts. This can be very useful for treatment planning in cases where high‐Z prostheses are present.PACS number(s): 87.57.Q‐

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Section: Introductionsupporting

confidence: 62%

Low‐dose 2.5 MV cone‐beam computed tomography with thick CsI flat‐panel imager

Tang

Moussot

Morf

et al. 2016

J Applied Clin Med Phys

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“…For example, it would be anticipated that optimization of such detectors for lower megavoltage energies realized through use of low-Z target materials [43][44][45][46] would favor designs with thinner crystals. Alternatively, the methodology could be applied toward the design of detectors required for other imaging geometries, such as fan-beam configurations of the type used in Tomotherapy treatment machines.…”

Section: Discussionmentioning

confidence: 99%

Optimization of the design of thick, segmented scintillators for megavoltage cone-beam CT using a novel, hybrid modeling technique

et al. 2014

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Purpose: Active matrix flat-panel imagers (AMFPIs) incorporating thick, segmented scintillators have demonstrated order-of-magnitude improvements in detective quantum efficiency (DQE) at radiotherapy energies compared to systems based on conventional phosphor screens. Such improved DQE values facilitate megavoltage cone-beam CT (MV CBCT) imaging at clinically practical doses. However, the MV CBCT performance of such AMFPIs is highly dependent on the design parameters of the scintillators. In this paper, optimization of the design of segmented scintillators was explored using a hybrid modeling technique which encompasses both radiation and optical effects. Methods: Imaging performance in terms of the contrast-to-noise ratio (CNR) and spatial resolution of various hypothetical scintillator designs was examined through a hybrid technique involving Monte Carlo simulation of radiation transport in combination with simulation of optical gain distributions and optical point spread functions. The optical simulations employed optical parameters extracted from a best fit to measurement results reported in a previous investigation of a 1.13 cm thick, 1016 μm pitch prototype BGO segmented scintillator. All hypothetical designs employed BGO material with a thickness and element-to-element pitch ranging from 0.5 to 6 cm and from 0.508 to 1.524 mm, respectively. In the CNR study, for each design, full tomographic scans of a contrast phantom incorporating various soft-tissue inserts were simulated at a total dose of 4 cGy. Results: Theoretical values for contrast, noise, and CNR were found to be in close agreement with empirical results from the BGO prototype, strongly supporting the validity of the modeling technique. CNR and spatial resolution for the various scintillator designs demonstrate complex behavior as scintillator thickness and element pitch are varied-with a clear trade-off between these two imaging metrics up to a thickness of ∼3 cm. Based on these results, an optimization map indicating the regions of design that provide a balance between these metrics was obtained. The map shows that, for a given set of optical parameters, scintillator thickness and pixel pitch can be judiciously chosen to maximize performance without resorting to thicker, more costly scintillators. Conclusions: Modeling radiation and optical effects in thick, segmented scintillators through use of a hybrid technique can provide a practical way to gain insight as to how to optimize the performance of such devices in radiotherapy imaging. Assisted by such modeling, the development of practical designs should greatly facilitate low-dose, soft tissue visualization employing MV CBCT imaging in external beam radiotherapy.

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“…Such BEV kV imaging has been investigated using different approaches: through the integration of an additional kV source in the treatment head, [25][26][27] and through modification of the treatment beam line so as to increase the low energy component of the beam. [28][29][30][31][32] The latter approach has been explored through the use of a low-Z target, [28][29][30] a modified linear accelerator waveguide, 31 or reduction of the electron beam energy. 32 Compared with the current kV CBCT imaging which is performed with the aforementioned rotational offset of the kV source relative to the MV source, BEV kV imaging would eliminate the geometric uncertainties associated with that offset and the need for additional quality assurance effort to ensure the coincidence of the isocenters of the kV and MV radiation fields, while preserving the superior contrast of images compared to that obtained using the MV treatment beam.…”

Section: Introductionmentioning

confidence: 99%

Theoretical investigation of the design and performance of a dual energy (kV and MV) radiotherapy imager

et al. 2015

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Purpose: In modern radiotherapy treatment rooms, megavoltage (MV) portal imaging and kilovoltage (kV) cone-beam CT (CBCT) imaging are performed using various active matrix flat-panel imager (AMFPI) designs. To expand the clinical utility of MV and kV imaging, MV AMFPIs incorporating thick, segmented scintillators and, separately, kV imaging using a beam's eye view geometry have been investigated by a number of groups. Motivated by these previous studies, it is of interest to explore to what extent it is possible to preserve the benefits of kV and MV imaging using a single AMFPI design, given the considerably different x ray energy spectra used for kV and MV imaging. In this paper, considerations for the design of such a dual energy imager are explored through examination of the performance of a variety of hypothetical AMFPIs based on x ray converters employing segmented scintillators. Methods: Contrast, noise, and contrast-to-noise ratio performances were characterized through simulation modeling of CBCT imaging, while modulation transfer function, Swank factor, and signal performance were characterized through simulation modeling of planar imaging. The simulations were based on a previously reported hybrid modeling technique (accounting for both radiation and optical effects), augmented through modeling of electronic additive noise. All designs employed BGO scintillator material with thicknesses ranging from 0.25 to 4 cm and element-to-element pitches ranging from 0.508 to 1.016 mm. A series of studies were performed under both kV and MV imaging conditions to determine the most advantageous imager configuration (involving front or rear x ray illumination and use of a mirror or black reflector), converter design (pitch and thickness), and operating mode (pitch-binning combination). Results: Under the assumptions of the present study, the most advantageous imager design was found to employ rear illumination of the converter in combination with a black reflector, incorporate a BGO converter with a 0.508 mm pitch and a 2 cm thickness, and operate at full resolution for kV imaging and 2 × 2 binning mode for MV imaging. Such a dual energy imager design should provide soft tissue visualization at low, clinically practical doses under MV conditions, while helping to preserve the high spatial resolution and high contrast offered by kV imaging. Conclusions: The authors' theoretical investigation suggests that a dual energy imager capable of largely preserving the desirable characteristics of both kV and MV imaging is feasible. Such an imager, when coupled to a dual energy radiation source, could facilitate simplification of current treatment room imaging systems (as well as their associated quality assurance), and facilitate more precise integration of kV and MV imaging information by virtue of reduced geometric uncertainties. C 2015 American Association of Physicists in Medicine. [http://dx

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Megavoltage planar and cone‐beam imaging with low‐ targets: Dependence of image quality improvement on beam energy and patient separation

Abstract: The use of low-Z external targets in the linear accelerator improves megavoltage planar and CBCT image quality significantly. CNR may be increased by a factor of 4 or greater. Improvement of the spatial resolution is also apparent.

Cited by 28 publications

References 33 publications

Low‐dose 2.5 MV cone‐beam computed tomography with thick CsI flat‐panel imager

Low‐dose 2.5 MV cone‐beam computed tomography with thick CsI flat‐panel imager

Optimization of the design of thick, segmented scintillators for megavoltage cone-beam CT using a novel, hybrid modeling technique

Theoretical investigation of the design and performance of a dual energy (kV and MV) radiotherapy imager

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