AAPM Task Group Report 238: 3D C-arms with volumetric imaging capability

Abstract

This report reviews the image acquisition and reconstruction characteristics of C-arm Cone Beam Computed Tomography (C-arm CBCT) systems and provides guidance on quality control of C-arm systems with this volumetric imaging capability. The concepts of 3D image reconstruction, geometric calibration, image quality, and dosimetry covered in this report are also pertinent to CBCT for Image-Guided Radiation Therapy (IGRT). However, IGRT systems introduce a number of additional considerations, such as geometric alignment of the imaging at treatment isocenter, which are beyond the scope of the charge to the task group and the report. Section 1 provides an introduction to C-arm CBCT systems and reviews a variety of clinical applications. Section 2 briefly presents nomenclature specific or unique to these systems. A short review of C-arm fluoroscopy quality control (QC) in relation to 3D C-arm imaging is given in Section 3. Section 4 discusses system calibration, including geometric calibration and uniformity calibration. A review of the unique approaches and challenges to 3D reconstruction of data sets acquired by C-arm CBCT systems is give in Section 5. Sections 6 and 7 go in greater depth to address the performance assessment of C-arm CBCT units. First, Section 6 describes testing approaches and phantoms that may be used to evaluate image quality (spatial resolution and image noise and artifacts) and identifies several factors that affect image quality. Section 7 describes both free-in-air and in-phantom approaches to evaluating radiation dose indices. The methodologies described for assessing image quality and radiation dose may be used for annual constancy assessment and comparisons among different systems to help medical physicists determine when a system is not operating as expected. Baseline measurements taken either at installation or after a full preventative maintenance service call can also provide valuable data to help determine whether the performance of the system is acceptable. Collecting image quality and radiation dose data on existing phantoms used for CT image quality and radiation dose assessment, or on newly developed phantoms, will inform the development of performance criteria and standards. Phantom images are also useful for identifying and evaluating artifacts. In particular, comparing baseline data with those from current phantom images can reveal the need for system calibration before image artifacts are detected in clinical practice. Examples of artifacts are provided in Sections 4, 5, and 6.

FIGURE 1

Geometry of C-Arm systems. Target object is conventionally centered at the world origin ($X$, $Y$, $Z$) with the central ray passing through the origin and intersecting at the detector center pixel or piercing point ($u_0$, $v_0$). During CBCT acquisition, the source and detector positions are rotated and translated around the world origin.

FIGURE 2

While utmost care is taken during construction of CBCT-enabled equipment to ensure level detector orientation, small perturbations can exist and result in detector rotation around the piercing point. Existing perturbations can manifest as roll (rotation around $Z_d$ axis defined in Figure 1 and illustrated at left), pitch (rotation around $X_d$ axis defined in Figure 1 and illustrated at center) and yaw (rotation around $t$ $Y_{\text{d}}$-axis defined in Figure 1 and illustrated at right).

FIGURE 3

Calibration phantom examples. (a) Projection image of an ellipsoid-based phantom. As in the helical phantom, marker size and pitch variations are used to create a unique projection image. This phantom layout is used by Philips Healthcare. (b) Volumetric reconstruction of a helical calibration phantom with constant marker size. Helical pitch, marker size, and marker-to-marker distance are used to create a non-ambiguous phantom. This phantom layout is used by Siemens Healthineers and Canon Medical Systems.

FIGURE 4

Calibration procedure setup as suggested by different vendors. (a) The cylindrical calibration phantom with spheres embedded in a helical pattern is attached to the table and placed at isocenter (Siemens Healthineers). (b) The cylindrical calibration phantom, with embedded ellipsoid and sphere combination, is placed at isocenter on the patient table (Philips Healthcare).

FIGURE 5

Reconstructed images of the Gammex ACR phantom before and after application of the correct geometric calibration for the system. Note the correction or removal of a tuning-fork artifact, restoration of beads, and reduction of the shading artifact.

FIGURE 6

A marked “arc” artifact indicates need to update offset and gain calibrations.

FIGURE 7

Reconstructions without and with accurate geometric and detector corrections applied.

FIGURE 8

Measured intensity data before and after log normalization.

FIGURE 9

Attenuation data and a typical weighting image of the FDK algorithm.

FIGURE 10

Attenuation data before and after convolution with a Ram-Lak kernel.

FIGURE 11

Axial slice of the volume $f\left(\mathit{r}\right)$ after backprojection.

FIGURE 12

Example assessment of C-arm CBCT spatial resolution from the line pairs in the Phantom Lab CATPHAN. (a) Subjective evaluation in terms of a line-pair test pattern, shown here for various settings of voxel size (avox). (b) More quantitative evaluation in terms of the MTF, shown here for various settings of smoothing kernel.

FIGURE 13

Fourier domain depiction of spatial-frequency sampling in CBCT with a circular source-detector orbit. A range (“cone”) of spatial frequencies is unsampled (the null cone),with the extent of the cone increasing with increasing distance above or below the central axial plane.

FIGURE 14

C-arm CBCT Noise. A region of interest within a phantom containing a uniform circular insert of enhanced attenuation relative to a uniform background may be useful for analyzing noise and CNR. Shown are example NPS figures for a C-arm CBCT image showing the 3D NPS as an isosurface in the 3D Fourier domain and central axial, sagittal, and coronal slices illustrating distinct noise-power characteristics in different directions.

FIGURE 15

Example of C-arm CBCT image artifacts. (a) Ring artifacts arising from imperfect detector offset, gain, and defect calibration. (b) Geometric calibration artifacts evident as streaks and distortion in high-contrast objects, such as a metal wire. (c) Lateral truncation artifacts associated with both the object (a chest phantom) and support table exceeding the detector FOV. (d) Artifact associated with an incomplete source-detector orbit less than the required 180^o per voxel. (e) Cupping and streak artifacts associated with $x$-ray scatter. (f) Shading or streak artifact arising from beam hardening. (g) Artifact arising from image lag. (h) Illustration of cone-beam artifacts in a coronal C-arm CBCT image of a stack of disks (strongly degraded at increased distance from the central axial plane) interspersed by plastic spheres. (i) Artifacts associated with patient motion; in this case, a phantom approximating respiratory motion during the C-arm CBCT scan.

FIGURE 16

Measuring dose free-in-air (${\text{D}}_{\text{air}}$). Place the thimble chamber free-in-air at the isocenter, verifying that it is centered in the FOV in both (a) anteroposterior and (b) lateral projections as seen in the display monitor. (c) The setup for the floor-mounted plane of a bi-plane system shows the table in the FOV.

FIGURE 17

Measuring $D_{\text{air}}$ in imaging mode. A screenshot of detailed information of the 3D protocol selected.

FIGURE 18

Setup of single dosimetry phantom positioned at isocenter for rotational acquisition protocol on the (a) table with the (b) ion chamber positioned at isocenter in posterior-anterior view on a C-arm CBCT system. Note the photograph in (a) shows the ion chamber inserted into a peripheral hole of the phantom.

FIGURE 19

Use of multiple phantoms to capture the entire contribution of scatter to the central plane dose. (a) The setup of three contiguous 16-cm CTDI phantoms to capture the entire contribution of dose from scatter for a ~15 cm beam width. (b) Picture of the PA view through the three-phantom setup with the ion chamber at isocenter. (c) Differences in measured dose from the same CBCT acquisition protocol acquired with three contiguous CTDI phantoms (left bar) and one phantom (right bar). Values are provided in the bar chart for measurements at each of the dose bars as labeled as well as the planar average dose defined in Equation (23). The data supports the use of even a single 16-cm CTDI phantom to perform $\bar{f}\left(0\right)$ measurements for beam widths on the order of ~15 cm.

FIGURE 20

The dose profile measured at different positions along the longitudinal extent of the central bore of three, 16-cm CTDI phantoms placed in series from a C-arm CBCT acquisition. Distributions such as this may be used to calculate the integral dose and $A_{\text{eq}}$ as described in Equation (28).

FIGURE 21

The dose profile measured at different dose bore and longitudinal positions along the extent of the ICRU/AAPM phantom for a C-arm CBCT acquisition. Distributions such as this may be used to calculate the integral dose and $A_{\text{eq}}$ as described in Equation (28).