Clinical advantages of ultra-high resolution
imaging using photon-counting CT
siemens-healthineers.com/naeotom-alpha
Introduction
Background
Ultra-High Resolution (UHR) imaging is crucial in radiology because it allows for the precise visualization of anatomical structures. The fine details captured in imaging can significantly impact diagnostic accuracy and treatment planning.
Spatial resolution, as defined by Radiopedia, refers to the ability of an imaging modality to differentiate two adjacent structures as distinct from one another. It is expressed in line pairs per cm (lp/cm). Improved spatial resolution enhances the definition and visibility of detail, enabling radiologists to make more confident diagnoses.
In this whitepaper, we explore the direct bearing of Quantum HD spatial resolution on clinical decision making with specific examples in thoracic, body, neurological, muscoloskeletal, pediatric, and cardiovascular imaging.
The technology
The trend in CT detector technology is moving towards smaller pixels to achieve better spatial resolution. Smaller pixels provide clearer visualization of small structures and textures, which could imply more diagnostic confidence. However, conventional Energy Integrating Detector CTs (EID-CTs) face challenges with radiation dose efficiency as pixel sizes decrease. Smaller pixels in EID-CTs suffer from geometric inefficiency, leading to reduced dose efficiency.
More specifically, these conventional EID-CT detectors employ a two-step process: first, the X-ray photon interacts with a scintillator to create visible light, which is then converted by a photodiode into an electric pulse to form the image. The primary challenge arises in the scintillator, where the created light can spread in any direction, including into neighboring detector elements.
Opaque barriers between detector elements, called septa, are required to prevent this optical crosstalk, but these septa also act as photon sinks, wasting any photon that lands on them. As the detector element size decreases, the space dedicated to septa increases, exacerbating the inefficiency. Alternatively, a comb may be placed between the patient and the detector to create the effect of smaller detector elements, thus blocking photons which have already passed through the patient. Again, resulting in a dose penalty.
Quantum HD is the UHR mode in NAEOTOM Alpha class photon-counting CT (PCCT) systems. It overcomes these challenges by removing the scintillation step in favor of a direct conversion of photons into an electric signal and thus eliminating the need for septa. The consequence is smaller pixels which allow for greater detail by avoiding the averaging of features under a larger pixel area. More detail means the system can tolerate more noise, allowing for lower doses while maintaining contrast-to-noise ratio (CNR) as expressed in a better Modulation Transfer Function (MTF) see Table 1.
The Vectron X-ray tube, capable of small focal spot of 0.4 × 0.5 mm at high power, is also key to achieving these gains in image quality.
Specifics
The pixel dimensions of NAEOTOM Alpha in Quantum HD mode are 0.151 mm × 0.176 mm at the iso-center, capable of achieving a resolution of 0.11 mm in-plane. The resolution is 44.3 lp/cm at 2% of the MTF. The Quantum HD option is available for many of the protocols on NAEOTOM Alpha class systems and is indicated by a scan mode that includes “HighResUltra” in the name.
There are some limitations in invoking Quantum HD. These are not constraints due to physical phenomena but rather restrictions of implementation (i.e., the massive data transmission and processing capacities which are required). Collimation is currently limited to 120 detector rows of 0.2 mm slice thickness, and a non-spectral “T3D” reconstruction is required for slice thickness of 0.2 mm. While a high pitch of 3.2 is available for standard resolution scans, the lower bound on pitch is 0.2 and may vary depending on protocol. Spectral information may be available in Quantum HD mode but is limited to 0.4 mm standard resolution. Additionally, due to the requirement for a small focal spot, there are upper bounds on tube power.
“We have never been so excited to not understand what we are seeing.”Prof. Anders Persson, Linkoping University Linkoping, Sweden
| High-contrast resolution with Quantum HD |
|---|
x-y plane | |
50% MTF | 39.0 lp/cm |
10% MTF | 42.9 lp/cm |
2% MTF | 44.3 lp/cm |
Technique | 120 × 0.2, 120 kV, 0.2-mm slices, Br98 |
Table 1: Modulation transfer function values of Quantum HD mode