A performance comparison of flat‐panel imager‐based MV and kV cone‐beam CT

Abstract

The use of cone-beam computed tomography (CBCT) has been proposed for guiding the delivery of radiation therapy, and investigators have examined the use of both kilovoltage (kV) and megavoltage (MV) x-ray beams in the development of such CBCT systems. In this paper, the inherent contrast and signal-to-noise ratio (SNR) performance for a variety of existing and hypothetical detectors for CBCT are investigated analytically as a function of imaging dose and object size. Theoretical predictions are compared to the results of experimental investigations employing largearea flat-panel imagers (FPIs) at kV and MV energies. Measurements were performed on two different FPI-based CBCT systems: a bench-top prototype incorporating an FPI and kV x-ray source (100 kVp x rays), and a system incorporating an FPI mounted on the gantry of a medical linear accelerator (6 MV x rays). The SNR in volume reconstructions was measured as a function of dose and found to agree reasonably with theoretical predictions. These results confirm the theoretically predicted advantages of employing kV energy x rays in imaging soft-tissue structures found in the human body. While MV CBCT may provide a valuable means of correcting 3D setup errors and may offer an advantage in terms of simplicity of mechanical integration with a linear accelerator (e.g., implementation in place of a portal imager), kV CBCT offers significant performance advantages in terms of image contrast and SNR per unit dose for visualization of soft-tissue structures. The relatively poor SNR performance at MV energies is primarily a result of the low x-ray quantum efficiencies (approximately a few percent or less) that are currently achieved with FPIs at high energies. Furthermore, kV CBCT with an FPI offers the potential of combined volumetric and radiographic/fluoroscopic imaging using the same device.

Flat-detector computed tomography (FD-CT)

“…Modern C-arm CT systems presently find rapid acceptance since they offer improved image quality, versatility and dedicated applications for planning, guiding, monitoring and assessing interventional procedures [5][6][7][8][9][10][11][12]. These are above all their use in interventional procedures and in intra-operative imaging.…”

A software tool of digital tomosynthesis application for patient positioning in radiotherapy

“…The radiation dose for a CBCT scan is about 2‐4 cGy and the cumulative dose for a treatment consisting of 30 fractions is about 60‐120 cGy. For a 4D CBCT scan, three or four thousand CB projections are necessary, and the total dose would be more than 10 cGy per scan 3 , 4 , 5 . Therefore, the use of CBCT was precautious in clinical practice.…”

Monte Carlo investigations of megavoltage cone‐beam CT using thick, segmented scintillating detectors for soft tissue visualization

“…For example, Groh et al have reported the use of 32 cGy to delineate ~4% density differences. 8 Also, Morin et al have reported using 14.4 cGy to obtain pelvic images with visible soft tissues (e.g., prostate, rectum, fat, and muscle) 22 , and 9 MU to obtain pelvic images having sufficient soft tissue information to guide 3D positioning 23 . If the x-ray detector quantum efficiency could be greatly improved, it is reasonable to expect that soft tissues could be visualized at considerably lower, clinically acceptable doses using megavoltage cone-beam (or fan-beam) CT. For this reason, high efficiency x-ray detectors have been widely investigated.…”

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17] One such technique that has been clinically implemented involves acquiring cone-beam computed tomography (CBCT) images with a kilovoltage (kV) x-ray source and a diagnostic active matrix flat-panel imager (AMFPI), both orthogonally mounted to the treatment gantry. [5][6][7][8][9][10][11] This system can produce useful CT images at clinically acceptable doses (i.e., a few cGy, which is approximately equal to the dose for one portal image). [8][9][10] Another CT imaging technique that is also under investigation for soft tissue visualization involves the use of the megavoltage (MV) therapy beam and an electronic portal imaging device (EPID).…”

“…[8][9][10] Another CT imaging technique that is also under investigation for soft tissue visualization involves the use of the megavoltage (MV) therapy beam and an electronic portal imaging device (EPID). [5][6][7][8]11,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Although the intrinsic contrast of human anatomical structures is lower at MV energies compared to kV energies, it has been demonstrated that soft tissues can be visualized on MV CBCT images obtained using conventional, as well as investigational, imagers. 8,24,30,33 In addition, MV CBCT can offer some advantages over the kV technique.…”