Low-dose megavoltage cone-beam CT imaging using thick, segmented scintillators

El‐Mohri, Youcef; Antonuk, Larry E.; Zhao, Qihua; Choroszucha, Richard; Jiang, Hao; Liu, Langechuan

doi:10.1088/0031-9155/56/6/001

Cited by 30 publications

(58 citation statements)

References 47 publications

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“…The crystals are arranged in the form of a twodimensional matrix of optically isolated elements-with each element consisting of a scintillating crystal surrounded by optically opaque septal wall material. Such segmented scintillators have been examined in numerous empirical 6,[8][9][10][11] and theoretical [12][13][14][15][16] studies of various designs with crystal thicknesses ranging up to 4 and 6 cm, respectively. The use of segmented scintillators in early prototypes has, thus far, been shown to significantly increase DQE values up to levels of ∼25%.…”

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“…This, in turn, makes it possible to acquire the large number of images required to perform megavoltage cone-beam computed tomography (MV CBCT) at a clinically practical dose-comparable to that required to capture a single projection image with a conventional MV AMFPI. 10,11 Compared to kV CBCT images obtained in the treatment room, MV CBCT images exhibit significantly reduced streak artifacts for patients with metal implants. [17][18][19] In addition, CT-numbers obtained from MV CBCT can be more readily used for dose calculation in treatment planning.…”

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“…BGO was chosen due to the generally promising performance of previous prototype segmented scintillators incorporating the material, resulting from properties such as high electron density and refractive index, as well as a high degree of optical transparency. 6,11,26 The present investigation involves simulation of reconstructed images of a contrast phantom using a CBCT geometry at clinically realistic radiation doses-accounting for both radiation and optical transport effects. Given the very large number of x-ray histories required for the study, a purely event-by-event Monte Carlo modeling approach would have been computationally prohibitive even had the scintillation yield been reduced to a level consistent with avoiding deleterious effect on accuracy.…”

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“…CNR is a widely accepted metric that is commonly employed for characterization of the performance of volumetric CBCT imaging systems employing AMFPIs. 11,12,[28][29][30][31][32][33][34] MTF is a standard metric commonly employed for characterization of 2D imaging system spatial resolution performance. Improvements to system MTF resulting from adjustments to scintillator design are expected to lead to 3D spatial resolution improvement, the degree of which depends on the multitude of factors that affect the latter, as described in Sec.…”

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

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“…These high‐Z image artifacts are greatly reduced in MV X‐ray imaging. Past efforts of using the therapeutic MV beams for imaging have proven its feasibility especially in the area of portal imaging while the implementation of megavoltage cone‐beam imaging (MVCB) has also been studied by several different groups 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 and is commercially available (MVision, Siemens Medical Solutions USA, Malvern, PA).…”

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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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