Megavoltage CT Scans Research Articles

TH-C-J-6B-04: Performance Characterization of Megavoltage CT Imaging On a Helical Tomotherapy Unit

Purpose: This study characterizes the image performance of the megavoltage computed tomography (MVCT) imaging from the Hi‐ART II tomotherapy treatment unit using measures of noise, uniformity, contrast, linearity, and spatial resolution as a function of absorbed dose.Method and Materials: MVCT image performance was characterized using a AAPM CT Performance Phantom. The following tests were performed as a function of collimator pitch and image pixel size: imagenoise,spatial uniformity, objective (modulation transfer function) and subjective (hole patterns) spatial resolution,contrast linearity, and contrast‐detail resolution. The multiple scan average dose (MSAD) was determined in a 20 cm diameter cylindrical phantom as a function of collimator pitch, and the beam energy was estimated from percent depth dose measurements. Results: The uniformity and spatial resolution of MVCT images generated by the HI‐ART II are comparable to that of diagnostic CTimages. The MVCT scan contrast is linear with respect to electron density of target material. MVCT images do not have the same performance characteristics as state‐of‐the‐art diagnostic images based on objective measures of noise and low‐contrast resolution, but large object low contrast resolution is sufficient to for identification of many important soft tissue structures. The MSAD ranges from 0.25 cGy to 1.1 cGy, and the effective beam energy is 3.5 MV. Conclusion: MVCT images from the Hi‐ART II are adequate for verifying a patient's position at the time of radiation therapy. These images also provide sufficient contrast to delineate many soft‐tissue structures. Hence, these images are adequate for reconstruction of the dose actually delivered to anatomical structures during radiotherapy. The dose delivered by the MVCT is low enough that images can be acquired on a daily basis.

TH-C-T-6C-05: Evaluation of TomoTherapy's MVCT Number Integrity

Purpose: Megavoltage CT (MVCT) images of patients are acquired daily on a helical tomotherapy unit (TomoTherapy, Inc., Madison, WI). While these images are primarily used for patient alignment, they can also be used to recalculate the treatment plans in the daily anatomy. This, however, requires a reliable CT number to electron density calibration curve. We tested the integrity of the MVCT numbers by determining the variation of this calibration with time, spatial arrangement of the calibration phantom, and MVCT acquisition parameters. Method and Materials: To evaluate the stability of the calibration with time a CT to electron density phantom was scanned repeatedly over time intervals up to 9 months. To test the sensitivity to the spatial phantom arrangement the outer and inner ring test plugs were exchanged. The phantom was scanned with different pitch ratios to test for any sensitivity to this parameter. MVCT scans encompassed the length of the phantom plus a region where the test plugs are extended into air. These in-air slices were compared with the in-phantom slices. The two calibration curves that enveloped all observed variations were applied to six clinical MVCT images for recalculations to test for dosimetric uncertainties. Results: The largest variation of the calibration curve was observed between in-air and in-phantom scans. For the two extreme test plugs the scans the MVCT numbers differed by up to 82 MVCT numbers. Using these two calibration curves, the largest difference in any of the dosimetric endpoints among the clinical images was 3.1 % . More typically the dosimetric endpoints varied by less than 2 %. Conclusion: The stability of the electron density calibration curve is comparable to that of kVCT scanners and allows the establishment of a reliable MVCT to electron density calibration. Conflict of Interest: Three of the co-authors are employees of TomoTherapy, Inc.

Megavoltage computed tomography for tomotherapeutic verification

A helical tomotherapy system is being developed at the University of Wisconsin—Madison as a means of improving Intensity Modulated Radiotherapy (IMRT) delivery. The goal of this thesis was to develop megavoltage computed tomography (MVCT) capabilities to image the patient in the treatment position, before, during, or after IMRT treatment. This MVCT system uses a 738‐channel xenon detector and is similar to helical kilovoltage CT (kVCT) except that it utilizes the treatment linac as the x‐ray source. Due to the high (4 MV) photon energy, MVCT contrast is inferior to kVCT, but should be adequate to verify patient positioning for radiotherapy. Techniques were developed to preprocess transmission data and to correct for artifacts. Image quality was compared for several different reconstruction algorithms. MVCT scans on a tomotherapy research system demonstrated relative contrasts below 1% for 25 mm objects using a 7 cGy dose to the center of the phantom. High‐contrast resolutions are 1.25 mm for air cavities. Through calibration of this system with known materials, the reconstructed values have been demonstrated to be linear with electron density, which benefits subsequent dose calculations. MVCT using transmission data collected during IMRT treatment delivery was also investigated. By overcoming the problems of incomplete data availability and difficulty normalizing modulated beams, treatment data was incorporated into the reconstruction process. This has demonstrated improved image contrast while reducing the additional imaging dose, and also presents the opportunity to collect all of the data necessary for image reconstruction during the treatment delivery.

Real-time beam monitoring in dynamic conformation therapy

Purpose : Although portal imaging is a promising method of verification during static multiport irradiation, it cannot be applied directly to dynamic irradiation such as rotational conformation with multileaf collimator movement. A real-time beam monitoring system based on megavoltage computed tomography scanning has been developed to establish a verification method for the rotational conformation technique. Methods and Materials : Exit beam through the patient is extracted by the same detector unit as used for megavoltage scanning during the actual treatment. Beam edge is defined as the 50% level of the maximum dose of the detector array. Megavoltage computed tomography is done after patient setup and just prior to the actual irradiation. Detected beam pathways are overlaid on this image approximately every 1 s. Therapists can monitor correlation between the target and actual beam pathways on a real-time computer display. Results : The accuracy of field edge detection has been proven to be less than 2 mm from various measurements. Real-time monitoring is more useful in rotational conformation than in static multiport irradiation due to dynamic movement of the collimator. Field errors were identified in two of 54 sessions using this method. Conclusions : Although several limitations remain to be solved, the method presented is a useful tool for treatment verification of high accuracy radiation therapy, particularly rotational conformation irradiation.

Technological features and clinical feasibility of megavoltage CT scanning

Megavoltage CT scanning using 4-MV and 6-MV radiotherapy beams has been developed and applied to verify errors in patient positioning. A detect or system composed of 120 pairs of cadmium tungstate scintillators with photodiodes is mounted to the treatment unit at a distance of 160 cm from the beam source. Image reconstruction is performed with a standard filtered back-projection algorithm. Scanning time and reconstruction time for a slice is approximately 35 s and 60 s respectively. Although spatial resolution is as large as 4 mm, it has sufficient image quality to be applied for treatment planning and verification. The delivered dose with 4 MV and 6 MV is about 1.4 cGy and 28. cGY respectively. When a megavoltage CT image is taken in treatment position, the positioning errors are easily detected by comparing it with diagnostic CT sections for treatment planning. Several clinical examples are presented.

A quantitative patient set-up and verification system using megavoltage CT scanning

A quantitative patient set-up and verification system using megavoltage CT scanning