A novel technique for the measurement of CBF and CBV with robot-arm-mounted flat panel CT in a large-animal model.

BACKGROUND AND PURPOSE: Endovascular therapy is an emerging treatment option in patients with acute ischemic stroke and especially in cases presenting late after symptom onset. Information about remaining viable tissue as measured with perfusion imaging is crucial for proper patient selection. The aim of this study was to investigate whether perfusion imaging with C-arm CT in the angiography suite is feasible and provides measurements comparable with ones made by CTP. MATERIALS AND METHODS: The MCA was occluded surgically in 6 sheep. Perfusion studies were performed before surgery, immediately after, and at 3 hours after MCA occlusion by using a robotic flat panel detector C-arm angiographic system. For comparison, conventional CTP was performed at the same time points. Two different protocols with the C-arm CT were tested. Images were analyzed by 2 readers with regard to the presence and size of perfusion abnormalities. RESULTS: With C-arm CT, perfusion abnormalities were detected with a high sensitivity and specificity when vessel occlusion was confirmed by criterion standard DSA. No difference was found between lesions sizes measured with the 2 C-arm CT protocols and CTP. Growth of the CBV lesions with time was captured with C-arm CT and CTP. CONCLUSIONS: In this small study, it was feasible to qualitatively measure CBV and CBF by using a flat panel detector angiographic system.

Volume-of-interest imaging of the inner ear in a human temporal bone specimen using a robot- driven C-arm flat panel detector CT system.

VOI imaging can provide higher image quality at a reduced dose for a subregion. In this study with a robot-driven C-arm FDCT system, the goals were proof of feasibility for inner ear imaging, higher flexibility during data acquisition, and easier processing during reconstruction. First a low-dose OV scan was acquired allowing an orientation and enabling the selection of the VOI. The C-arm was then moved by the robotic system without a need for patient movement and the VOI was scanned with adapted parameters. Uncompromised artifact-free image quality was achieved by the 2-scan approach and the dose was reduced by 80%-90% in comparison with conventional MSCT and FPCT scans.

[Digital volume tomography (DVT) and multislice spiral CT (MSCT): an objective examination of dose and image quality]. / Digitale Volumentomografie (DVT) und Mehrschicht-Spiral-CT (MSCT): eine objektive Untersuchung von Dosis und Bildqualität.

PURPOSE: In the last five years digital volume tomographs (DVT) have found their way into the diagnostic imaging of the facial skull. In this study both the image quality and dose of DVT and multislice spiral CT (MSCT) in this field of application were investigated using established physical methods for CT. MATERIALS AND METHODS: Measurements on DVT scanners of various manufacturers and on a modern MSCT scanner were performed. The investigation was based on equivalent dose levels for both modalities (CT dose index, CTDI). For this purpose, the dose was measured with an ionization chamber in a cylindrical PMMA phantom. For the evaluation of image quality, the spatial resolution, contrast and noise were investigated with phantoms established for CT. RESULTS: MSCT exhibited spatial resolution values of 1.0 to 1.6 lp/mm, while DVT provided resolution between 0.6 and 1.0 lp/mm only. Thus, MSCT offered similar or better resolution at an equivalent dose. For soft tissue resolution, DVT showed significant image artifacts. MSCT yielded higher homogeneity and no significant artifacts, and the contrast steps of the phantom were more verifiable. The different DVT devices, from image intensifiers to modern flat-detector (FD) devices, showed significant differences in favor of the FD devices. CONCLUSION: For medium and high contrast applications (teeth/bones), DVT scanners can be an alternative to MSCT at comparable radiation exposure. However, MSCT offers advantages in terms of constantly good and controlled image quality with significantly more flexible scan parameters at a constant or lower dose and should therefore be given preference.

Development, implementation and evaluation of a dedicated metal artefact reduction method for interventional flat-detector CT.

The purpose of this study was to develop, implement and evaluate a dedicated metal artefact reduction (MAR) method for flat-detector CT (FDCT). The algorithm uses the multidimensional raw data space to calculate surrogate attenuation values for the original metal traces in the raw data domain. The metal traces are detected automatically by a three-dimensional, threshold-based segmentation algorithm in an initial reconstructed image volume, based on twofold histogram information for calculating appropriate metal thresholds. These thresholds are combined with constrained morphological operations in the projection domain. A subsequent reconstruction of the modified raw data yields an artefact-reduced image volume that is further processed by a combining procedure that reinserts the missing metal information. For image quality assessment, measurements on semi-anthropomorphic phantoms containing metallic inserts were evaluated in terms of CT value accuracy, image noise and spatial resolution before and after correction. Measurements of the same phantoms without prostheses were used as ground truth for comparison. Cadaver measurements were performed on complex and realistic cases and to determine the influences of our correction method on the tissue surrounding the prostheses. The results showed a significant reduction of metal-induced streak artefacts (CT value differences were reduced to below 22 HU and image noise reduction of up to 200%). The cadaver measurements showed excellent results for imaging areas close to the implant and exceptional artefact suppression in these areas. Furthermore, measurements in the knee and spine regions confirmed the superiority of our method to standard one-dimensional, linear interpolation.

The influence of the heel effect in cone-beam computed tomography: artifacts in standard and novel geometries and their correction.

For decades, the heel effect has been known to cause an angular dependence of the emitted spectrum of an x-ray tube. In radiography, artifacts were observed and attributed to the heel effect. However, no problems due to the heel effect were discerned in multi-slice computed tomography (MSCT) so far. With flat-detector CT (FDCT), involving larger cone angles and different system geometries, the heel effect might cause new artifacts. These artifacts were analyzed in this paper for system geometries different from the ones widely used nowadays. Simulations and measurements were performed. Simulations included symmetric as well as asymmetric detector layouts and different x-ray tube orientations with respect to the detector plane. The measurements were performed on a micro-CT system in an asymmetric detector layout. Furthermore, an analytical correction scheme is proposed to overcome heel effect artifacts. It was shown that the type of artifact greatly depends on the orientation of the x-ray tube and also on the type of detector alignment (i.e. symmetric or different types of asymmetric alignment). Certain combinations exhibited almost no significant artifact while others greatly influenced the quality of the reconstructed images. The proposed correction scheme showed good results that were further improved when also applying a scatter correction. When designing CT systems, care should be taken when placing the tube and the detector. Orientation of the x-ray tube like in most MSCT systems seems advisable in asymmetric detector layouts. However, a different type of tube orientation can be overcome with suitable correction schemes.

Metal artifact reduction for clipping and coiling in interventional C-arm CT.

BACKGROUND AND PURPOSE: Metallic implants induce massive artifacts in CT images which deteriorate image quality and often superimpose structures of interest. The purpose of this study was to apply and evaluate a dedicated MAR method for neuroradiologic intracranial clips and detachable platinum coiling events. We here report the first clinical results for MAR in FDCT. MATERIALS AND METHODS: FDCT volume scans of several patients treated with endovascular coiling or intracranial clipping were corrected by using a dedicated FDCT MAR correction algorithm combined with an edge-preserving attenuation-normalization method in the projection space. Corrected and uncorrected images were compared by 2 experienced radiologists and evaluated for several image-quality features. RESULTS: After application of our algorithm, implant delineation and visibility were highly improved. CT values compared with values in metal artifact-unaffected areas showed good agreement (average correction of 1300 HU). Image noise was reduced overall by 27%. Intracranial hemorrhage in the direct surroundings of the implanted coil or clip material was displayed without worrisome metal artifacts, and our algorithm even allowed diagnosis in areas where extensive information losses were seen. The high spatial resolution provided by FDCT imaging was well preserved. CONCLUSIONS: Our MAR method provided metal artifact-reduced images in every studied case. It reduced image noise and corrected CT values to levels comparable with images measured without metallic implants. An overall improvement of brain tissue modeling and implant visibility was achieved. MAR in neuroradiologic FDCT imaging is a promising step forward for better image quality and diagnosis in the presence of metallic implants.

[Basic principles of flat detector computed tomography (FD-CT)]. / Grundlagen der Flachdetektor-CT (FD-CT).

Flat detectors (FDs) have been developed for use in radiography and fluoroscopy to replace standard X-ray film, film-screen combinations and image intensifiers (II). In comparison to X-ray film and II, FD technology offers higher dynamic range, dose reduction, fast digital readout and the possibility for dynamic acquisitions of image series, yet keeping to a compact design. It appeared logical to employ FD designs also for computed tomography (CT) imaging. FDCT has meanwhile become widely accepted for interventional and intra-operative imaging using C-arm systems. Additionally, the introduction of FD technology was a milestone for soft-tissue CT imaging in the interventional suite which was not possible with II systems in the past. This review focuses on technical and performance issues of FD technology and its wide range of applications for CT imaging. FDCT is not aimed at challenging standard clinical CT as regards to the typical diagnostic examinations, but it has already proven unique for a number of dedicated CT applications offering distinct practical advantages, above all the availability of immediate CT imaging during an intervention.

Reduction of motion artefacts in non-gated dual-energy radiography.

The objective of this work was to reduce motion artefacts in non-gated dual-energy subtraction radiography whilst preserving the contrast-to-noise ratio (CNR) in regions with low motion. Dual-energy radiography provides material-selective information (soft-tissue and bone images) that may be used for improved detection of calcifications in lung nodules. The weighted logarithmic dual-energy subtraction of thoracic images performed without electrocardiogram gating results in motion-induced artefacts. The low-energy image was acquired at the usual dose setting at 60 kV. To obtain the high-energy (120 kV) information, a series of consecutive images at a time interval of 30 ms were made. The series integral dose was equivalent to the dose of a single conventional high-energy image. A motion-free merging technique was introduced that combines standard images yielding low image noise with phase-selective images yielding motion artefact-free image regions which are used for dual-energy subtraction. Evaluations of the method were performed with simulations and measurements using a C-arm system (Axiom Artis; Siemens AG, Germany) equipped with a flat detector of 40 x 30 cm(2). The merging approach conserved standard image noise levels and the CNR in areas without cardiac motion, whereas image noise in pericardial lung regions and in the heart was increased compared with standard images. Motion artefacts in the heart and in the lung areas close to the heart are significantly reduced in the material-selective images when compared with a standard non-gated subtraction.

Simultaneous misalignment correction for approximate circular cone-beam computed tomography.

Currently, CT scanning is often performed using flat detectors which are mounted on C-arm units or dedicated gantries as in radiation therapy or micro CT. For perspective cone-beam backprojection of the Feldkamp type (FDK) the geometry of an approximately circular scan trajectory has to be available for reconstruction. If the system or the scan geometry is afflicted with geometrical instabilities, referred to as misalignment, a non-perfect approximate circular scan is the case. Reconstructing a misaligned scan without knowledge of the true trajectory results in severe artefacts in the CT images. Unlike current methods which use a pre-scan calibration of the geometry for defined scan protocols and calibration phantoms, we propose a real-time iterative restoration of reconstruction geometry by means of entropy minimization. Entropy minimization is performed combining a simplex algorithm for multi-parameter optimization and iterative graphics card (GPU)-based FDK-reconstructions. Images reconstructed with the misaligned geometry were used as an input for the entropy minimization algorithm. A simplex algorithm changes the geometrical parameters of the source and detector with respect to the reduction of entropy. In order to reduce the size of the high-dimensional space required for minimization, the trajectory was described by only eight fix points. A virtual trajectory is generated for each iteration using a least-mean-squares algorithm to calculate an approximately circular path including these points. Entropy was minimal for the ideal dataset, whereas strong misalignment resulted in a higher entropy value. For the datasets used in this study, the simplex algorithm required 64-200 iterations to achieve an entropy value equivalent to the ideal dataset, depending on the grade of misalignment using random initialization conditions. The use of the GPU reduced the time per iteration as compared to a quad core CPU-based backprojection by a factor of 10 resulting in a total of 15-20 ms per iteration, and thus providing an online geometry restoration after a total computation time of approximately 1-3 s, depending on the number of iterations. The proposed method provides accurate geometry restoration for approximately circular scans and eliminates the need for an elaborate off-line calibration for each scan. If a priori information about the trajectory is used to initialize the simplex algorithm, it is expected that the entropy minimization will converge significantly faster.

Neuroradiologic applications with routine C-arm flat panel detector CT: evaluation of patient dose measurements.

BACKGROUND AND PURPOSE: Since the introduction of flat panel detector-equipped C-arms, the use of flat panel detector CT (FPCT) in the neuroradiologic angiography suite has become more frequent. This examination implicates its own specific radiation exposure. We used the CT dose index (CTDI) concept and adapted it to the special FPCT geometry to provide a consistent comparison with multisection head CT (cCT). MATERIALS AND METHODS: Exposure data obtained for routine scanning during a period of 1 year were used to assess a specific dose of a total of 217 rotational scans performed in 105 patients. One hundred seventy-two scans were 3D digital subtraction angiography (DSA) scans. There were 45 scans that were performed to achieve high-quality, soft-tissue resolution. Dose measurements in cylindrical polymethylmethacrylate (PMMA) phantoms were used to determine the CTDI value and to compare it with the reference values for cCT. In addition, the dose-area product (DAP) was registered and correlated with the CTDI and corresponding dose-length product (DLP) values. Exposure data and dose values were compared with cCT. RESULTS: Mean-weighted CTDI value of 3D-DSA was approximately 9 mGy per scan. High-quality, soft-tissue resolution FPCT scans, comparable with cCT, revealed a mean dose value of 75 mGy (reference value for cCT, CTDI(w) approximately 60 mGy). CONCLUSION: The high-speed scans used for 3D-DSA revealed a significantly lower CTDI(w) and DLP compared with clinical CT. The high-quality FPCT protocol resulted in a higher dose and should therefore be limited to acute cases, when patient transfer to a CT scanner is considered to be a disadvantage for patient management.