Dual-contrast photon-counting detector computed tomography enables quantitative assessment of articular cartilage

Dual-contrast photon-counting detector computed tomography enables quantitative assessment of articular cartilage

 

Satu I. Inkinen^1,2 and Petri Paakkari^3,4

 

¹Research Unit of Medical Imaging, Physics and Technology, University of Oulu, Oulu, Finland

²Helsinki University Hospital, Diagnostic Center, Helsinki, Finland

³Department of Applied Physics, University of Eastern Finlan0064, Kuopio, Finland

⁴Diagnostic Imaging Center, Kuopio University Hospital, Kuopio, Finland

 

 

Highlights

-       Photon-counting detector computed tomography (PCD-CT) enables multi-energy imaging in a single scan acquisition

-       Multi-energy CT imaging can be used for quantitative assessment of contrast agent concentrations in tissues

-       Dual-contrast enhanced imaging of articular cartilage can be used for the assessment/evaluation of tissue composition and structural integrity

 

 

Contrast-enhanced computed tomography imaging of articular cartilage

 

Articular cartilage tissue composition and structural integrity can be investigated using contrast-enhanced computed tomography (CECT) [1], [2]. Especially, the early detection of post-traumatic osteoarthritic changes, such as collagen fibrillation, increase in tissue water content, and proteoglycan loss, are of interest in CECT, as imaging of the early changes are very challenging. Contrast agents (e.g., iodine and gadolinium) attenuate X-rays, and when contrast agents diffuse in articular cartilage, the tissue contrast is improved [3]. Also, the material properties of the contrast agents can be modified to cationic, anionic, or non-ionic which affects the diffusivity in cartilage. For example, a cationic contrast agent is sensitive to proteoglycan content at diffusion equilibrium [4]and it can be applied for differentiation between reparative, degenerative, and healthy articular cartilage [5].

 

In addition, it is possible to administer several different contrast agents targeting a more comprehensive assessment of tissue composition [6]–[8]. The quantification of several contrast agents requires spectral information which can be conventionally acquired using dual-energy CT (DECT) imaging performed, e.g., using kVp switching during acquisition or dual acquisition with different kVp and filter settings.

 

 

 

Photon-counting detector computed tomography (PCD-CT)

 

PCD-CT enables spectral imaging in a single acquisition since photon-counting detectors (PCDs) count the individual incoming photons and assign them to energy bins based on photon energy [9], [10]. PCDs can have several bin counters (usually varies between two to six bins), and thus, more energy bin thresholds can be set to detect different photon energies. PCDs are usually Cadmium-Telluride (CdTe) or Cadmium-Zinc-Telluride (CZT) semiconductor detectors that operate through direct conversion. Therefore, PCDs have better detection efficiency enabling radiation dose reductions compared to conventional energy integrating detectors (EIDs) which use indirect conversion (scintillation). Furthermore, PCD has a pixelated anode which enables smaller pixels (e.g., 50x50 ¬µm²) than in EIDs, and thus PCD-CT can provide ultra-high-resolution images which are extremely useful for musculoskeletal applications [11], [12]. First FDA clearance for a clinical PCD-CT device was gained at the end of September 2021 [13].

 

 

PCD-CT and CECT imaging of articular cartilage

 

There exist only a few studies on contrast-enhanced PCD-CT imaging of articular cartilage [14], [15]. In our recent study, a dual-contrast agent method using a cationic iodinated CA4+ and non-ionic gadolinium-based gadoteridol was applied in human osteochondral tissue imaging (N=53) with PCD-CT [15]. The PCD-CT scanning was performed in diffusion equilibrium. The cationic iodinated CA4+ targets the proteoglycan distribution and the non-ionic gadolinium-based gadoteridol reflects the tissue water content. The obtained concentration partitions were compared against biomechanical and optical density measurements, and Mankin scoring. The PCD had two-bin counters enabling spectral imaging, and the material decomposition utilized a calibration-based approach to estimate iodine and gadolinium concentrations (Figure 1) [7], [15].

Figure 1. Workflow for assessing the iodine and gadolinium partitions (i.e., measured contrast agent concentration normalized to initial contrast agent bath concentration) using a reconstruction domain material decomposition. First, the raw projection data is collected in two energy bins (Total energy [30-120] keV and High energy [60-120] keV) by the PCD (data not shown). Subsequently, projection data is corrected using the signal-to-thickness calibration method [16], and then low and high bin projection data is reconstructed. After reconstruction, the material decomposition is performed by assessing least-the squares solution for each voxel utilizing the material matrix (M) determined based on attenuation vs. concentration slope of calibration solutions of different concentrations of iodinated CA4+ and gadoteridol.

 

 

To summarize, PCD-CT enabled simultaneous quantification of the two contrast agent concentrations inside articular cartilage (Figure 1). The iodine CA4+ concentrations correlated strongly with the proteoglycan content (ρ = 0.836, p < 0.001, Spearman’s correlation), moderately with Mankin score (ρ = -0.307, p < 0.05) and equilibrium modulus (ρ = 0.328, p < 0.05) [15].
 
In conclusion, PCD-CT enables spectral imaging with improved detection efficiency and higher resolution compared to conventional CT. With this improved soft-tissue contrast and resolution, combined with the comprehensive assessment of proteoglycan and water content via dual-contrast imaging, PCD-CT holds promise for the detection of post-traumatic osteoarthritis in the future.
 
 
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