When performing Quality Control (QC) on a CT system, it is important to have a phantom that fits the scanner’s capabilities. The test objects should usefully probe the systems limits, without being exceeded, and more curiously, without all being “too good” for this system. A phantom that is over-spec’d for a system is not only a waste of money; it can altogether fail to provide necessary QC information.
This problem can impact conventional diagnostic scanners, and is even more acute for some different CT implementations, such as radiation therapy cone-beam CT (CBCT) and Megavoltage CT (MVCT). QC phantoms developed for one system may not provide much analytical value on another system. Another issue is that phantoms that were originally designed for manual or visual evaluation might not be optimally suited for automated analysis.
Thus, many clinics might be hindered in their ability to achieve a basic goal: To simplify testing equipment with a single phantom that can usefully test a variety of scanner types, whether in one department, or spanning multiple departments.
The new IQphan™ from Sun Nuclear (Figure 1) addresses this issue.
Figure 1: IQphan, shown partially-transparent for illustration purposes only.
The test range specifications were re-evaluated to create a versatile phantom with useful analysis ranges for a diverse array of CT systems, in both the diagnostic and radiation therapy spaces.
One key example regards low contrast detectability (LCD). This test is designed to evaluate how well a CT system can detect faint objects. It is common for CT phantoms to have test objects that span a range of sizes and contrasts, typically maxing out around a 1% contrast at 15 mm. This is generally appropriate for typical diagnostic CT scanners. However, CBCT and MVCT systems, as well as systems running low-dose applications, may not see even the largest object at that contrast.
The IQphan phantom rectifies this by adding objects with both higher contrasts, up to 2%, and larger sizes, up to 25 mm. Appropriately, these additional targets are more visible, especially on RT imaging systems, and also reflect the size and contrast of common visualization targets.
This is demonstrated in the CBCT image of IQphan taken on a Varian TrueBeam system (Figure 2). The section highlighted in green is the quadrant of 2% contrast objects, as well as a 1% contrast object in a 25 mm size. Numerous objects are visible in this green region, meaning that this phantom could usefully evaluate system performance. Conversely, contrast objects maxing out at 1% and 15 mm are not visible, making them unsuitable this imaging task. As such, if those objects in the red section were the most attenuating ones in a phantom, then that phantom would not suffice for evaluating low contrast detectability of the system.
In conclusion, an image quality phantom needs to have specs that encompass the characteristics of the imaging systems being tested. As illustrated by the example of low contrast detectability, the new Sun Nuclear IQphan was designed with a broad range of CT scanner needs in mind, to create a versatile solution that can improve and standardize testing.
Figure 2: Low Contrast Detectability of a CBCT scan on IQphan. The higher contrast and larger diameter objects are critical for achieving useful measurements.
This article was originally published in the EFOMP Spring 2023 newsletter. See it here