Presenter: Daniel Letourneau Presenter's Affiliation: William Beaumont Hospital Type of Session: Scientific
Cone-beam CT (CBCT) imaging is an emerging technology which uses a flat panel detector in conjuction with a kilovoltage or megavoltage linear accelerator to produce CT images. The ability to produce CT images from these accelators, such as simulators and treatment machines, has the potential to greatly enhance physicians' ability to visualize target and normal structures during simulation and provide a much higher level of on-treatment imaging than is currently available. The current study analyzes patient and phantom images to determine irradiation parameters that provide optimal image quality without excessive patient dose from radiation source.
Materials and Methods
A kilovoltage system consisting of a 120 kVp x-ray source and an opposing 41x41cm flat panel imager were utilized to generate the cone-beam images.
CBCT images were obtained on a stereotactic radiosurgery patient using the above system.
Convential CT images using a commercially available CT machine were concurrently obtained on a phantom.
The irradiation parameters, including imaging dose, beam filtration, number of projections, and use of a scatter rejection grid were varied.
Comparisons of these various images were made with regard to zero-frequency signal-to-noise ratio (SNR0), and contrast detail was measured by determining an observers' ability to visualize holes of varying diameters in acrylic disks of varying diameters within the phantom.
These images were compared to helical CT scans of the phantom in 3 mm slices.
CBCT images delivering a skin dose of 1 cGy to the patient were obtained that provided high quality images.
The SNR0 was 4 times higher for the helical CT taking 3 mm slices compared to the CBCT taking 1 mm slices; however the SNR0 of the CBCT improved 50% by increasing the dose delivered by a factor of 2.
Increasing the number of projections of the CBCT increased the air-to-bone contrast (420 projections resulted in contrast approaching that of helical CT)
The use of a 10:1 scatter rejection grid reduced the cupping artifact.
Using a 2.5 cGy entrance dose on a head scan, the CBCT could visualize a 2 mm hole in a 0.4 mm disk, whereas the helical CT could only visualize a 4 mm disk.
Likewise, using a 2.5 cGy entrance dose on a body scan, the CBCT visualized a 5 mm hole in a 0.8 mm disk, while this could not be seen on a helical CT.
CBCT is feasible for image-guided radiosurgery using bony structures.
Adjustments in the irradiation parameters can produce high quality images even in thicker body regions with acceptable patient doses.
Clinical CBCT imaging studies are needed.
The emerging technology of CBCT has a very high potential of impacting patient care. The addition of CT quality imaging to kilovoltage machines and megavoltage treatment machines would allow physicians to image patient anatomy in greater detail than ever before. This current study demonstrates that proper variation in irradiation parameters can produce images that are nearly equivalent to, and in some cases superior to helical CT images while delivering acceptable doses of radiation to the patient. The doses delivered to the patient in this study are comparable to the doses received by the patient from weekly port films. There are currently several clinical studies of CBCT that are ongoing which may demostrate the clinical utility of this technology.
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