Image registration and analysis for quantitative myocardial perfusion: application to dynamic circular cardiac CT

Large area detector computed tomography systems with fast rotating gantries enable volumetric dynamic cardiac perfusion studies. Prospectively, ECG-triggered acquisitions limit the data acquisition to a predefined cardiac phase and thereby reduce x-ray dose and limit motion artefacts. Even in the case of highly accurate prospective triggering and stable heart rate, spatial misalignment of the cardiac volumes acquired and reconstructed per cardiac cycle may occur due to small motion pattern variations from cycle to cycle. These misalignments reduce the accuracy of the quantitative analysis of myocardial perfusion parameters on a per voxel basis. An image-based solution to this problem is elastic 3D image registration of dynamic volume sequences with variable contrast, as it is introduced in this contribution. After circular cone-beam CT reconstruction of cardiac volumes covering large areas of the myocardial tissue, the complete series is aligned with respect to a chosen reference volume. The results of the registration process and the perfusion analysis with and without registration are evaluated quantitatively in this paper. The spatial alignment leads to improved quantification of myocardial perfusion for three different pig data sets.

[Automatic phase point determination of minimal motion reconstruction intervals with motion maps in ECG-gated CT diagnostics of coronary sclerosis]

PURPOSE

Cardio-CT motion maps for automated cardiac phase point determination were evaluated for image quality and reliability of coronary calcium scores.

MATERIALS AND METHODS

24 patients underwent ECG-gated non-enhanced cardiac CT for calcium scoring. From raw data the motion map software reconstructed low-resolution images in 2 % steps of the RR interval and automatically generated cardiac motion maps for determination of minimal motion phase points. Diagnostic images were reconstructed in 10% steps of the RR interval (RR data) and according to the motion maps (MM data). For every data set, the Agatston score was calculated. Image quality was evaluated by two independent observers. Image quality was correlated with the Agatston score.

RESULTS

The Agatston score calculated over the RR interval showed a mean variation of 127 with 41% of patients assigned to more than one risk group. If the motion map RR intervals were calculated, only 16% patients were assigned to different risk categories with a mean variation of 55. Regarding the image quality, the inter-rater variance was moderate. The best image quality was achieved with the 30 - 40% and 70 - 80% RR interval. Over the complete RR interval motion map reconstructions produced a good image quality.

CONCLUSION

Calculation of the Agatston score requires selection of the proper reconstruction interval to guarantee the assignment of patients into the appropriate risk category. By using motion maps for phase point determination, the amount of necessary reconstruction can be minimized and the assignment to different risk groups is also reduced.

Experimental 16-Row CT Evaluation of In-Stent Restenosis using New Stationary and Moving Cardiac Stent Phantoms: Experimental Examination

PURPOSE

The aim of this study was to evaluate in-stent restenosiss using a newly developed stationary and moving cardiac stent phantom with three built-in artificial stenoses and a 16-row MDCT.

MATERIALS AND METHODS

A newly developed coronary stent phantom with three artificial stenoses--low (approx. 30 %), medium (approx. 50 %) and high (approx. 70 %)--was attached to a moving heart phantom and used to evaluate the ability of 16-row MDCT to visualize in-stent restenosis. High resolution scans (16 x 0.75 mm, 250 mm FOV) were made to identify the baseline for image quality. The non-moving phantom was scanned (16 x 0.75 mm, routine cardiac scan protocol) first without and then with implementation of an ECG signal at various simulated heart rates (HR 40 to 120 bpm) and pitches (0.15 to 0.3). The moving cardiac phantom was scanned at the same simulated heart rates but at a pitch of 0.15. Images were reconstructed at every 10 % of the RR interval using a multi-cycle real cone-beam reconstruction algorithm. Multi-planar reformations (MPR) were made for the image evaluation. The image quality was assessed using a three-point scale, and stent patency and stenoses detection were evaluated using a four-point scale. To evaluate the image quality and to grade the stent stenoses, the median values were calculated while considering the reconstruction interval.

RESULTS

The image quality for the static phantom was adequate in 97 % of the measurements. In this phantom, every stenosis was detected independent of the pitch and heart rate used. The dynamic stent phantom yielded the best results at 0 %, 40 %, and 50 % of the RR interval at a pitch of 0.15. The low stenosis was visible at a simulated heart rate of up to 80 bpm. Patency can be detected at heart rates greater than 80 bpm.

CONCLUSION

The newly developed moving stent phantom allowed a nearly in-vivo condition for detecting re-stenoses within a stent. In this phantom study the use of a 16-row MDCT allowed the detection of re-stenosis within a coronary stent at a heart rate of up to 80 bpm. This phantom can then be used for future studies, e. g. with a 64-row MDCT.

Evaluation of spatial and temporal resolution for ECG-gated 16-row multidetector CT using a dynamic cardiac phantom

Measurements of spatial and temporal resolution for ECG-gated scanning of a stationary and moving heart phantom with a 16-row MDCT were performed. A resolution phantom with cylindrical holes from 0.4 to 3.0 mm diameter was mounted to a cardiac phantom, which simulates the motion of a beating heart. Data acquisition was performed with 16x0.75 mm at various heart rates (HR, 60-120 bpm), pitches (0.15-0.30) and scanner rotation times (RT, 0.42 and 0.50 s). Raw data were reconstructed using a multi-cycle real cone-beam reconstruction algorithm at multiple phases of the RR interval. Multi-planar reformations (MPR) were generated and analyzed. Temporal resolution and cardiac cycles used for image reconstruction were calculated. In 97.2% (243/250) of data obtained with the stationary phantom, the complete row of holes with 0.6 mm was visible. These results were independent of heart rate, pitch, scanner rotation time and phase point of reconstruction. For the dynamic phantom, spatial resolution was determined during phases of minimal motion (116/250). In 40.5% (47/116), the resolution was 0.6 mm and in 37.1% (43/116) 0.7 mm. Temporal resolution varied between 63 and 205 ms, using 1.5-4.37 cardiac cycles for image reconstruction.

A reconstruction algorithm for coherent scatter computed tomography based on filtered back-projection

Coherent scatter computed tomography (CSCT) is a reconstructive x-ray imaging technique that yields the spatially resolved coherent-scatter form factor of the investigated object. Reconstruction from coherently scattered x-rays is commonly done using algebraic reconstruction techniques (ART). In this paper, we propose an alternative approach based on filtered back-projection. For the first time, a three-dimensional (3D) filtered back-projection technique using curved 3D back-projection lines is applied to two-dimensional coherent scatter projection data. The proposed algorithm is tested with simulated projection data as well as with projection data acquired with a demonstrator setup similar to a multi-line CT scanner geometry. While yielding comparable image quality as ART reconstruction, the modified 3D filtered back-projection algorithm is about two orders of magnitude faster. In contrast to iterative reconstruction schemes, it has the advantage that subfield-of-view reconstruction becomes feasible. This allows a selective reconstruction of the coherent-scatter form factor for a region of interest. The proposed modified 3D filtered back-projection algorithm is a powerful reconstruction technique to be implemented in a CSCT scanning system. This method gives coherent scatter CT the potential of becoming a competitive modality for medical imaging or nondestructive testing.

Persistent light-induced absorption in oxidized iron-doped barium titanate crystals

Illumination of oxidized iron-doped BaTiO(3) crystals with visible or ultraviolet light yields absorption changes of as much as 1300 m(-1) that are stable at room temperature. These photochromic effects cannot be erased by light, but heating the crystal to a moderate temperature (100 degrees C) can switch it back to its initial state. These effects, in particular the optical irreversibility, might be of interest, e.g., for optical control of charge-transport properties and for persistent data storage.