Characterization of image quality and image-guidance performance of a preclinical microirradiator

Abstract

Purpose: To assess image quality and image-guidance capabilities of a cone-beam CT based small-animal image-guided irradiation unit (micro-IGRT).

Methods: A micro-IGRT system has been developed in collaboration with the authors' laboratory as a means to study the radiobiological effects of conformal radiation dose distributions in small animals. The system, the X-Rad 225Cx, consists of a 225 kVp x-ray tube and a flat-panel amorphous silicon detector mounted on a rotational C-arm gantry and is capable of both fluoroscopic x-ray and cone-beam CT imaging, as well as image-guided placement of the radiation beams. Image quality (voxel noise, modulation transfer, CT number accuracy, and geometric accuracy characteristics) was assessed using water cylinder and micro-CT test phantoms. Image guidance was tested by analyzing the dose delivered to radiochromic films fixed to BB's through the end-to-end process of imaging, targeting the center of the BB, and irradiation of the film/BB in order to compare the offset between the center of the field and the center of the BB. Image quality and geometric studies were repeated over a 5-7 month period to assess stability.

Results: CT numbers reported were found to be linear (R2 0.998) and the noise for images of homogeneous water phantom was 30 HU at imaging doses of approximately 1 cGy (to water). The presampled MTF at 50% and 10% reached 0.64 and 1.35 mm(-1), respectively. Targeting accuracy by means of film irradiations was shown to have a mean displacement error of [deltax, deltay, deltaz] = [-0.12, -0.05, -0.02] mm, with standard deviations of [0.02, 0.20, 0.17] mm. The system has proven to be stable over time, with both the image quality and image-guidance performance being reproducible for the duration of the studies.

Conclusions: The micro-IGRT unit provides soft-tissue imaging of small-animal anatomy at acceptable imaging doses (< or =1 cGy). The geometric accuracy and targeting systems permit dose placement with submillimeter accuracy and precision. The system has proven itself to be stable over 2 yr of routine laboratory use (>1800 irradiations) and provides a platform for the exploration of targeted radiation effects in small-animal models.

Figure 1

Pictures of the micro-IGRT (X-Rad 225Cx) unit. (a) The exterior of the self-shielded cabinet. (b) Interior, showing the C-arm setup with collimator and 3D linear translation stage. (c) Geometry of the system, with d_SAD=30.7 cm, d_SDD=64.5 cm, and d_SCD=23 cm. The primary beam is collimated to cover the entire detector surface (D_width×D_length=20.4×20.4 cm²), giving a FOV_z=FOV_xy=9.7 cm.

Figure 2

Image analysis performed using two phantoms. (a) The micro-CT image quality phantom (left) and 2.5 cm diameter cylindrical water phantom. (b) The phantoms were set up so cylindrical axis lined up with gantry rotation axis. (c) Sagittal view of the image quality phantom, with white lines added to show the different sections to be analyzed. Figures (d)–(i) are (proceeding up through sagittal view) images of the (d) linearity, (e) geometric accuracy, (f) uniformity, (g) slanted-edge, (h) resolution, and (i) CT number sections of the quality phantom, respectively. (j) is a section of the water phantom, with ROIs for uniformity and noise calculations added in gray.

Figure 3

Stability and accuracy studies probe the relative movement of the detector and collimated beam with respect to isocenter. (a) Mechanical nonidealities in the gantry can be measured using the BB. This measurement is used to digitally compensate for flex during the reconstruction process. This map is monitored for stability over time through routine QA. (b) With the collimator in place, a Winston–Lutz type of test is used to measure the movement of the collimated beam relative to a nominal isocenter. These deviations are recorded and a compensatory displacement of the stage in x-y-z is applied as a function of gantry angle during irradiation. Two types of tests were performed to confirm targeting performance: (c) Image-guided Winston–Lutz tests and (d) “star-shot” irradiations to evaluate compensation during rotational delivery.

Figure 4

Plot of the signal intensities vs iodine concentrations (Insert: Image of linearity section, with ROIs overlaid in gray). The plot shows the average of all of the linearity curves for the 5 month period over which measurements were taken. The boxes represent the standard deviation in the average and the error bars represent the min/max attenuation coefficients numbers. The system has individual linear fit of R²≥0.998 in all cases. Slopes for each fit are as given in legend.

Figure 5

Quantitative and qualitative analyses of the geometric accuracy of the system. (a) and (b) show the geometric accuracy plate, with the distances between beads (found from the image) overlaid in gray. (c) and (d) show the resolution coils, where the wire coils have spacing (clockwise from leftmost coil) of 0.5, 0.3, 0.2, and 0.15 mm. Note that the micro-IGRT can resolve the (c) 0.300 mm coil, while the GE Ultra micro-CT can resolve the (d) 0.200 mm coil.

Figure 6

Plots of the (averaged) presampled MTF curves from micro-IGRT (solid line), with the micro-CT (+) and clinical CT (*) for comparison purposes. Note that the 10% MTF level for the micro-IGRT was 1.35 mm⁻¹, lying between the accuracy of the clinical (0.85 mm⁻¹) and micro-CT (2.34 mm⁻¹), as one would expect, given the respective resolutions. Insert: Image of slanted-edge section, with ROI region shown in gray.

Figure 7

Evaluation of dynamic mechanical flex compensation system. In (a), (b), and (c), the plane of radiochromic film is parallel to the incident beams. (a), (b), and (c) were analyzed by taking horizontal and vertical dose profiles (gray lines, plotted as percentage dose, with horizontal profile shown to the right of each image; vertical profiles are similar). [(a) and (b)] Star-shot irradiations, with stage corrections (a) off and (b) on. (c) 360° arc, with stage corrections on, again showing sharp central region and dose profile having sharp drop-offs.

Figure 8

Typical dosimetric test of targeting using a BB and fixed vertical beam. The center of BB differed from center of irradiation field by an average of [Δx,Δy,Δz]=[−0.12,−0.05,−0.02].

Figure 9

Micro-IGRT images of a C3H∕HeJ male mouse, 8 weeks old, taken while the mouse was under inhaled anesthetic. Volumes (a), (b), and (c) were taken at 40 kVp and 0.5 mA and volumes (d), (e), and (f) at 40 kVp and 2.5 mA for qualitative comparison. Image-guided setup images are typically taken using the lower dose [(a), (b), and (c)] settings, where the purpose of the scan is for targeting while keeping the scan dose as low as possible. The improved image quality in a higher dose image [(d), (e), and (f)] is typical of an image that might be used for diagnostics or evaluation. (Note: No contrast used; window∕level adjusted for display.)