Three-dimensional computed tomographic reconstruction using a C-arm mounted XRII: correction of image intensifier distortion

Validation of marker-based tracking with a biplanar fluoroscopy system optimized for the foot and ankle

Biplanar fluoroscopy is a powerful, maturing technique for providing clinicians and biomechanists with in vivo kinematic data of the human skeleton during a variety of tasks. Marker-based tracking with biplane systems has applications in both the in vivo and in vitro realms and serves as the established means of validating model-based tracking algorithms. We have developed a custom biplane system for dynamic imaging of the entire foot and ankle complex during gait as well as a custom software suite to perform the required data preprocessing and marker-based tracking. We demonstrate our ability to repeatably model the biplane imaging chains and then accurately and precisely reconstruct the positions of markers in the foot during static and dynamic motion trials. Finally, we simulate the effects of marker localization errors in reconstructing the poses of the calcaneus, navicular, and proximal phalanx during gait in order to contextualize the extent to which marker-based tracking may be considered ground truth compared to future model-based tracking algorithms.

Quantitative assessments of image intensifier distortion induced by weak (Sub-Gauss) magnetic fields during fluoroscopically-guided procedures

BACKGROUND

Fluoroscopically-guided procedures at our hospital have been aborted due to sigmoidal distortion (S-distortion) when an image intensifier (II) system is used in a surgical environment distant from any apparent sources of strong magnetic fields, such as a nearby magnetic resonance imaging (MRI) scanner. Clearly, current clinical practice fails to account for the impact of ambient weak magnetic fields and/or other contributing factors on S-distortion induction.

PURPOSE

This study attempts to quantitatively assess the threshold level of magnetic field, along with other potential factors, that can induce intolerable S-distortion during image-intensified fluoroscopically-guided procedures. We will also discover the origins of such level of magnetic field in typical surgical facilities and provide our practical mitigation strategies accordingly.

METHODS

Ten surgical facilities and their accessory equipment (e.g., surgical tables) were screened using an AC/DC gaussmeter for the distribution and magnitude of magnetic field (magnetic flux density). A 'hot spot' of magnetic field was identified to further investigate the induction of S-distortion by scanning a titanium rod phantom using a GE OEC 9900 Elite II system placed at increasing distance from the 'hot spot' corresponding to decreasing magnetic field experienced by the II. The measurements were compared to that on a 'cold spot', and a GE flat panel detector (FPD) fluoroscopy was used as the negative control. Rod phantoms made of various magnetic susceptible materials (titanium, steel, aluminium, and copper) were scanned to explore the potential effects of implant material on S-distortion. An upper extremity anthropomorphic phantom was imaged on various surgical tables to mimic clinical sceneries. The GE II model and Siemens ARCADIS Orbic II model were compared to evaluate if S-distortion induction varied among different II models. Two metrics, angle of rotation (θ) and deviation/length ratio, were used to quantify the degree of S-distortion. Three designs of external magnetic shielding were evaluated for mitigating S-distortion.

RESULTS

We identified static magnetic fields up to 2500 µT and 70 µT on the floor and at 1-meter height, respectively, in random locations of surgical facilities. A large variation of magnetic field (64 ± 20 µT) was detected on the surface of surgical tables, with background magnetic fields of ∼35 µT. Quantitative assessments demonstrated that even weak magnetic fields at sub-Gauss level (<100 µT) could induce noticeable distortion artifacts, deemed unacceptable (θ > 4°). S-distortion was independent of the implant material being imaged but dependent on the II model - the threshold magnetic fields (4° distortion induction) were as low as 47 µT and 94 µT for the GE and Siemens II models. Mitigation possibilities of S-distortion include relocating the II to an area with subthreshold magnetic fields and shielding the II utilizing cylindrical mu-metal shields with an extension for alleviating the effect of openings.

CONCLUSIONS

This work demonstrates that ambient sub-Gauss magnetic fields originating from any possible sources in a surgical environment have to be carefully considered when performing an image-intensified fluoroscopically-guided procedure, because such weak magnetic fields are likely able to induce unacceptable S-distortion artifacts in the acquired X-ray images leading to undesirable surgical outcomes.

3D-printed large-area focused grid for scatter reduction in cone-beam CT

BACKGROUND

Cone-beam computed tomography (CBCT) systems acquire volumetric data more efficiently than fan-beam or multislice CT, particularly when the anatomy of interest resides within the axial field-of-view of the detector and data can be acquired in one rotation. For such systems, scattered radiation remains a source of image quality degradation leading to increased noise, image artifacts, and CT number inaccuracies.

PURPOSE

Recent advances in metal additive manufacturing allow the production of highly focused antiscatter grids (2D-ASGs) that can be used to reduce scatter intensity, while preserving primary radiation transmission. We present the first implementation of a large-area, 2D-ASG for flat-panel CBCT, including grid-line artifact removal and related improvements in image quality.

METHODS

A 245 × 194 × 10 mm 2D-ASG was manufactured from chrome-cobalt alloy using laser powder-bed fusion (LPBF) (AM-400; Renishaw plc, New Mills Wotton-under-Edge, UK). The 2D-ASG had a square profile with a pitch of 9.09 lines/cm and 10:1 grid-ratio. The nominal 0.1 mm grid septa were focused to a 732 mm x-ray source to optimize primary x-ray transmission and reduce grid-line shadowing at the detector. Powder-bed fusion ensured the structural stability of the ASG with no need for additional interseptal support. The 2D-ASG was coupled to a 0.139-mm element pitch flat-panel detector (DRX 3543, Carestream Health) and proper alignment was confirmed by consistent grid-line shadow thickness across the whole detector array. A 154-mm diameter CBCT image-quality-assurance phantom was imaged using a rotary stage and a ceiling-mounted, x-ray unit (Proteus XR/a, GE Medical Systems, 80kVp, 0.5mAs). Grid-line artifacts were removed using a combination of exposure-dependent gain correction and spatial-frequency, Fourier filtering. Projections were reconstructed using a Parker-weighted, FDK algorithm and voxels were spatially averaged to 357 × 357 × 595 µm to improve the signal-to-noise characteristics of the CBCT reconstruction. Finally, in order to compare image quality with and without scatter, the phantom was scanned again under the same CBCT conditions but with no 2D-ASG. No additional antiscatter (i.e., air-gap, bowtie filtration) strategies were used to evaluate the effects in image quality caused by the 2D-ASG alone.

RESULTS

The large-area, 2D-ASG prototype was successfully designed and manufactured using LPBF. CBCT image-quality improvements using the 2D-ASG included: an overall 14.5% CNR increase across the volume; up to 48.8% CNR increase for low-contrast inserts inside the contrast plate of the QA phantom; and a 65% reduction of cupping artifact in axial profiles of water-filled cross sections of the phantom. Advanced image processing strategies to remove grid line artifacts did not affect the spatial resolution or geometric accuracy of the system.

CONCLUSIONS

LPBF can be used to manufacture highly efficient, 2D-focused ASGs that can be easily coupled to clinical, flat-panel detectors. The implementation of ASGs in CBCT leads to reduced scatter-related artifacts, improved CT number accuracy, and enhanced CNR with no increased equivalent dose to the patient. Further improvements to image quality might be achieved with a combination of scatter-correction algorithms and iterative-reconstruction strategies. Finally, clinical applications where other scatter removal strategies are unfeasible might now achieve superior soft-tissue visualization and quantitative capabilities.

Cost-effective micro-CT system for non-destructive testing of titanium 3D printed medical components

Micro-CT imaging can be used as an effective method for non-destructive testing (NDT) of metal 3D printed parts-including titanium biomedical components fabricated using laser powder-bed-fusion (LPBF). Unfortunately, the cost of commercially available micro-CT scanners renders routine NDT for biomedical applications prohibitively expensive. This study describes the design, manufacturing, and implementation of a cost-effective scanner tailored for NDT of medium-size titanium 3D printed biomedical components. The main elements of the scanner; which include a low-energy (80 kVp) portable x-ray unit, and a low-cost lens-coupled detector; can be acquired with a budget less than $ 11000 USD. The low-cost detector system uses a rare-earth phosphor screen, lens-coupled to a dSLR camera (Nikon D800) in a front-lit tilted configuration. This strategy takes advantage of the improved light-sensitivity of modern full-frame CMOS camera sensors and minimizes source-to-detector distance to maximize x-ray flux. The imaging performance of the system is characterized using a comprehensive CT quality-assurance phantom, and two titanium 3D-printed test specimens. Results show that the cost-effective scanner can survey the porosity and cracks in titanium parts with thicknesses of up to 13 mm of solid metal. Quantitatively, the scanner produced geometrically stable reconstructions, with a voxel size of 118 μm, and noise levels under 55 HU. The cost-effective scanner was able to estimate the porosity of a 17 mm diameter titanium 3D-printed cylindrical lattice structure, with a 0.3% relative error. The proposed scanner will facilitate the implementation of titanium LPBF-printed components for biomedical applications by incorporating routine cost-effective NDT as part of the process control and validation steps of medical-device quality-management systems. By reducing the cost of the x-ray detector and shielding, the scan cost will be commensurate with the overall cost of the validated component.

Reconstruction of three-dimensional lumbar vertebrae from biplanar x-rays

Objective. Vertebrae models from computer tomographic (CT) imaging are extensively used in image-guided surgical systems to deliver percutaneous orthopaedic operations with minimum risks, but patients may be exposed to excess radiation from the pre-operative CT scans. Generating vertebrae models from intra-operative x-rays for image-guided systems can reduce radiation exposure to the patient, and the surgeons can acquire the vertebrae's relative positions during the operation; therefore, we proposed a lumbar vertebrae reconstruction method from biplanar x-rays.Approach. Non-stereo-corresponding vertebral landmarks on x-rays were identified as targets for deforming a set of template vertebrae; the deformation was formulated as a minimisation problem, and was solved using the augmented Lagrangian method. Mean surface errors between the models reconstructed using the proposed method and CT scans were measured to evaluate the reconstruction accuracy.Main results. The evaluation yielded mean errors of 1.27 mm and 1.50 mm inin vitroexperiments on normal vertebrae and pathological vertebrae, respectively; the outcomes were comparable to other template-based methods.Significance. The proposed method is a viable alternative to provide digital lumbar to be used in image-guided systems, where the models can be used as a visual reference in surgical planning and image-guided applications in operations where the reconstruction error is within the allowable surgical error.

Flat-panel conebeam CT in the clinic: history and current state

Research into conebeam CT concepts began as soon as the first clinical single-slice CT scanner was conceived. Early implementations of conebeam CT in the 1980s focused on high-contrast applications where concurrent high resolution ( < 200    μ m ), for visualization of small contrast-filled vessels, bones, or teeth, was an imaging requirement that could not be met by the contemporaneous CT scanners. However, the use of nonlinear imagers, e.g., x-ray image intensifiers, limited the clinical utility of the earliest diagnostic conebeam CT systems. The development of consumer-electronics large-area displays provided a technical foundation that was leveraged in the 1990s to first produce large-area digital x-ray detectors for use in radiography and then compact flat panels suitable for high-resolution and high-frame-rate conebeam CT. In this review, we show the concurrent evolution of digital flat panel (DFP) technology and clinical conebeam CT. We give a brief summary of conebeam CT reconstruction, followed by a brief review of the correction approaches for DFP-specific artifacts. The historical development and current status of flat-panel conebeam CT in four clinical areas-breast, fixed C-arm, image-guided radiation therapy, and extremity/head-is presented. Advances in DFP technology over the past two decades have led to improved visualization of high-contrast, high-resolution clinical tasks, and image quality now approaches the soft-tissue contrast resolution that is the standard in clinical CT. Future technical developments in DFPs will enable an even broader range of clinical applications; research in the arena of flat-panel CT shows no signs of slowing down.

Are We Being Fooled by Fluoroscopy? Distortion May Affect Limb-Length Measurements in Direct Anterior Total Hip Arthroplasty

BACKGROUND

Distortion is an intrinsic phenomenon associated with image-intensified fluoroscopy that is both poorly understood and infrequently appreciated by orthopedic surgeons. Little information exists regarding its potential influence on intraoperative parameters during orthopedic surgery, let alone during direct anterior (DA) total hip arthroplasty (THA). The purpose of this study was to quantify the amount of potential error caused by fluoroscopic distortion during DA THA.

METHODS

Intra-operative fluoroscopic pelvic images from 74 DA THAs were reviewed by two independent readers. All images were obtained using the same fluoroscopic C-arm unit with a radiopaque grid attached to the image intensifier. The vertical distortion from a straight central horizontal line at the peripheries of images were measured and summed to yield the combined vertical distortion similar to how a surgeon calculates a side to side comparison of limb lengths. Simple linear regression was used to evaluate associations between total distortion and patient demographics, operating theaters, and various operative parameters.

RESULTS

The average combined distortion was 10.0mm (range 2.0-20.0mm). There was a significant difference in the average distortion observed in different theaters (P < .001). There was no association between distortion and patient demographics or fluoroscopic time (all, P > .05).

CONCLUSION

Fluoroscopic distortion is unpredictable and can cause a substantial amount of error when comparing limb lengths during DA THA. This is a critical finding as this amount of inaccuracy could lead to unintended implant positioning and limb-length discrepancies if unaccounted for.

Aneurysm rebleeding before therapy: a predictable disaster?

OBJECTIVE

Current guidelines for subarachnoid hemorrhage (SAH) include early aneurysm treatment within 72 hours after ictus. However, aneurysm rebleeding remains a crucial complication of SAH. The aim of this study was to identify independent predictors allowing early stratification of SAH patients for rebleeding risk.

METHODS

All patients admitted to the authors' institution with ruptured aneurysms during a 14-year period were eligible for this retrospective study. Demographic and radiographic parameters, aneurysm characteristics, medical history, and medications as well as baseline parameters at admission (blood pressure and laboratory parameters) were evaluated in univariate and multivariate analyses. A novel risk score was created using independent risk factors.

RESULTS

Data from 984 cases could be included into the final analysis. Aneurysm rebleeding occurred in 58 cases (5.9%), and in 48 of these cases (82.8%) rerupture occurred within 24 hours after SAH. Of over 30 tested associations, preexisting arterial hypertension (p = 0.02; adjusted odds ratio [aOR] 2.56, 1 score point), aneurysm location at the basilar artery (p = 0.001, aOR 4.5, 2 score points), sac size ≥ 9 mm (p = 0.04, aOR 1.9, 1 score point), presence of intracerebral hemorrhage (p = 0.001, aOR 4.29, 2 score points), and acute hydrocephalus (p < 0.001, aOR 6.27, 3 score points) independently predicted aneurysm rebleeding. A score built upon these parameters (0-9 points) showed a good diagnostic accuracy (p < 0.001, area under the curve 0.780) for rebleeding prediction.

CONCLUSIONS

Certain patient-, aneurysm-, and SAH-specific parameters can reliably predict aneurysm rerupture. A score developed according to these parameters might help to identify individuals that would profit from immediate aneurysm occlusion.

C-Arm Image-Based Surgical Path Planning Method for Distal Locking of Intramedullary Nails

Due to the curvature of the bone marrow cavity, the intramedullary nail used in long bone fracture fixation can be deformed, causing displacement of the locking holes. In this study, an algorithm using only one C-arm image to determine the center positions and axial directions of locking holes was developed for drilling guidance. Based on conventional method that the axial direction of locking hole would be identified when locking hole contour is presented as a circle, the proposed method can locate the circle contour centroid by using one C-arm image including two elliptical contours. Then the two distal locking holes' axial direction and centers would be determined. Three experiments were conducted to verify the performance of the proposed algorithm, which are (1) computer simulation, (2) use of real intramedullary nails, and (3) actual drilling test with the bone model. The experimental results showed that the average error of the axial direction and center position were 0.62 ± 0.6°, 0.73 ± 0.53 mm (simulation) and 3.16 ± 1.36°, 1.10 ± 0.50 mm (actual nail), respectively. The last ten drilling test sets were completed successfully (with an average duration of 48 seconds). Based on the experimental results, the proposed algorithm was feasible for clinic applications.

Geometric calibration and correction for a lens-coupled detector in x-ray phase-contrast imaging

A lens-coupled x-ray camera with a tilted phosphor collects light emission from the x-ray illuminated (front) side of phosphor. Experimentally, it has been shown to double x-ray photon capture efficiency and triple the spatial resolution along the phosphor tilt direction relative to the same detector at normal phosphor incidence. These characteristics benefit grating-based phase-contrast methods, where linear interference fringes need to be clearly resolved. However, both the shallow incident angle on the phosphor and lens aberrations of the camera cause geometric distortions. When tiling multiple images of limited vertical view into a full-field image, geometric distortion causes blurring due to image misregistration. Here, we report a procedure of geometric correction based on global polynomial transformation of image coordinates. The corrected image is equivalent to one obtained with a single full-field flat panel detector placed at the sample plane. In a separate evaluation scan, the position deviations in the horizontal and vertical directions were reduced from 0.76 and 0.028 mm, respectively, to 0.006 and 0.009 mm, respectively, by the correction procedure, which were below the 0.028-mm pixel size of the imaging system. In a demonstration of a phase-contrast imaging experiment, the correction reduced blurring of small structures.

The impact of high-heeled shoes on ankle complex during walking in young women-In vivo kinematic study based on 3D to 2D registration technique

OBJECTIVE

To explore the accurate in vivo kinematic changes in the ankle complex when wearing low- and high-heel shoes (LHS and HHS, respectively).

MATERIALS AND METHODS

Twelve young women were tested unilaterally. Three-dimensional models of the tibia, talus, and calcaneus were first created based on CT scan results. The subjects walked at a self-controlled speed in barefoot, LHS (4cm), and HHS (10cm) conditions. A fluoroscopy system captured the lateral fluoroscopic images of the ankle complex. The images of seven key positions in the stance phase were selected, and 3D to 2D bone model registrations were performed to determine the joint positions. The mean of 6 degree of freedom (DOF) range of motions (ROM), joint positions, and angular displacements of the ankle complex during the gait were then obtained.

RESULTS

For the talocrural joint, the rotational ROMs of the subjects either in LHS or HHS condition displayed no significant difference from those in barefoot condition. For the subtalar joint, all the rotational ROMs in the HHS condition and the internal/external rotations in the LHS condition significantly decreased compared with those in the barefoot condition. The talocrural joint was positioned significantly more plantarflexed, inverted, internally rotated, and posteriorly seated in all seven poses in HHS condition, compared with those in barefoot condition.

CONCLUSION

HHS mainly affected the rotational motion of the ankle complex during walking. The talocrural joint position was abnormal, and the subtalar joint ROM decreased during the gait in HHS condition. Only a few kinematic changes occurred in LHS condition relative to the barefoot condition.

Volumetric limiting spatial resolution analysis of four-dimensional digital subtraction angiography

C-Arm CT three-dimensional (3-D) digital subtraction angiography (DSA) reconstructions cannot provide temporal information to radiologists. Four-dimensional (4-D) DSA provides a time series of 3-D volumes utilizing temporal dynamics in the two-dimensional (2-D) projections using a constraining image reconstruction approach. Volumetric limiting spatial resolution (VLSR) of 4-D DSA is quantified and compared to a 3-D DSA. The effects of varying 4-D DSA parameters of 2-D projection blurring kernel size and threshold of the 3-D DSA (constraining image) of an in silico phantom (ISPH) and physical phantom (PPH) were investigated. The PPH consisted of a 76-micron tungsten wire. An [Formula: see text] scan protocol acquired the projection data. VLSR was determined from MTF curves generated from each 2-D transverse slice of every (248) 4-D temporal frame. 4-D DSA results for PPH and ISPH were compared to the 3-D DSA. 3-D DSA analysis resulted in a VLSR of 2.28 and [Formula: see text] for ISPH and PPH, respectively. Kernel sizes of either [Formula: see text] or [Formula: see text] with a 3-D DSA constraining image threshold of 10% provided 4-D DSA VLSR nearest to the 3-D DSA. 4-D DSA yielded 2.21 and [Formula: see text] with a percent error of 3.1 and 1.2% for ISPH and PPH, respectively, as compared to 3-D DSA. This research indicates 4-D DSA is capable of retaining the resolution of 3-D DSA.

4D digital subtraction angiography: implementation and demonstration of feasibility

BACKGROUND AND PURPOSE

Conventional 3D-DSA volumes are reconstructed from a series of projections containing temporal information. It was our purpose to develop a technique which would generate fully time-resolved 3D-DSA vascular volumes having better spatial and temporal resolution than that which is available with CT or MR angiography.

MATERIALS AND METHODS

After a single contrast injection, projections from the mask and fill rotation are subtracted to create a series of vascular projections. With the use of these projections, a conventional conebeam CT reconstruction is generated (conventional 3D-DSA). This is used to constrain the reconstruction of individual 3D temporal volumes, which incorporate temporal information from the acquired projections (4D-DSA).

RESULTS

Typically, 30 temporal volumes per second are generated with the use of currently available flat detector systems, a factor of ∼200 increase over that achievable with the use of multiple gantry rotations. Dynamic displays of the reconstructed volumes are viewable from any angle. Good results have been obtained by using both intra-arterial and intravenous injections.

CONCLUSIONS

It is feasible to generate time-resolved 3D-DSA vascular volumes with the use of commercially available flat detector angiographic systems and clinically practical injection protocols. The spatial resolution and signal-to-noise ratio of the time frames are largely determined by that of the conventional 3D-DSA constraining image and not by that of the projections used to generate the 3D reconstruction. The spatial resolution and temporal resolution exceed that of CTA and MRA, and the small vessel contrast is increased relative to that of conventional 2D-DSA due to the use of maximum intensity projections.

Image intensifier distortion correction for fluoroscopic RSA: the need for independent accuracy assessment

Fluoroscopic images suffer from multiple modes of image distortion. Therefore, the purpose of this study was to compare the effects of correction using a range of two-dimensional polynomials and a global approach. The primary measure of interest was the average error in the distances between four beads of an accuracy phantom, as measured using RSA. Secondary measures of interest were the root mean squared errors of the fit of the chosen polynomial to the grid of beads used for correction, and the errors in the corrected distances between the points of the grid in a second position. Based upon the two-dimensional measures, a polynomial of order three in the axis of correction and two in the perpendicular axis was preferred. However, based upon the RSA reconstruction, a polynomial of order three in the axis of correction and one in the perpendicular axis was preferred. The use of a calibration frame for these three-dimensional applications most likely tempers the effects of distortion. This study suggests that distortion correction should be validated for each of its applications with an independent "gold standard" phantom.

Sub-Nyquist acquisition and constrained reconstruction in time resolved angiography

In 1980 DSA provided a real time series of digitally processed angiographic images that facilitated and reduced the risk of angiographic procedures. This technique has become an enabling technology for interventional radiology. Initially it was hoped that intravenous DSA could eliminate the need for arterial injections. However the 2D nature of the images resulted in overlap of vessels and repeat injections were often required. Ultimately the use of smaller arterial catheters and reduced iodine injections resulted in significant reduction in complications. During the next two decades time resolved MR DSA angiographic methods were developed that produced time series of 3D images. These 4D displays were initially limited by tradeoffs in temporal and spatial resolution when acquisitions obeying the Nyquist criteria were employed. Then substantial progress was made in the implementation of undersampled non-Cartesian acquisitions such as VIPR and constrained reconstruction methods such as HYPR, which removed this tradeoff and restored SNR usually lost by accelerated techniques. Recently, undersampled acquisition and constrained reconstruction have been applied to generate time series of 3D x-ray DSA volumes reconstructed using rotational C-arm acquisition completing a 30 year evolution from DSA to 4D DSA. These 4D DSA volumes provide a flexible series of roadmaps for interventional procedures and solve the problem of vessel overlap for intravenous angiography. Full time-dependent behavior can be visualized in three dimensions. When a biplane system is used, 4D fluoroscopy is also possible, enabling the interventionalist to track devices in vascular structures from any angle without moving the C-arm gantrys. Constrained reconstruction methods have proved useful in a broad range of medical imaging applications, where substantial acquisition accelerations and dose reductions have been reported.

Hybrid cone-beam tomographic reconstruction: incorporation of prior anatomical models to compensate for missing data

We propose a method for improving the quality of cone-beam tomographic reconstruction done with a C-arm. C-arm scans frequently suffer from incomplete information due to image truncation, limited scan length, or other limitations. Our proposed "hybrid reconstruction" method injects information from a prior anatomical model, derived from a subject-specific computed tomography (CT) or from a statistical database (atlas), where the C-arm X-ray data is missing. This significantly reduces reconstruction artifacts with little loss of true information from the X-ray projections. The methods consist of constructing anatomical models, fast rendering of digitally reconstructed radiograph (DRR) projections of the models, rigid or deformable registration of the model and the X-ray images, and fusion of the DRR and X-ray projections, all prior to a conventional filtered back-projection algorithm. Our experiments, conducted with a mobile image intensifier C-arm, demonstrate visually and quantitatively the contribution of data fusion to image quality, which we assess through comparison to a "ground truth" CT. Importantly, we show that a significantly improved reconstruction can be obtained from a C-arm scan as short as 90° by complementing the observed projections with DRRs of two prior models, namely an atlas and a preoperative same-patient CT. The hybrid reconstruction principles are applicable to other types of C-arms as well.

A spatio-temporal detective quantum efficiency and its application to fluoroscopic systems

PURPOSE

Fluoroscopic x-ray imaging systems are used extensively in spatio-temporal detection tasks and require a spatio-temporal description of system performance. No accepted metric exists that describes spatio-temporal fluoroscopic performance. The detective quantum efficiency (DQE) is a metric widely used in radiography to quantify system performance and as a surrogate measure of patient "dose efficiency". It has been applied previously to fluoroscopic systems with the introduction of a temporal correction factor. However, the use of a temporally-corrected DQE does not provide system temporal information and it is only valid under specific conditions, many of which are not likely to be satisfied by suboptimal systems. The authors propose a spatio-temporal DQE that describes performance in both space and time and is applicable to all spatio-temporal quantum-based imaging systems.

METHODS

The authors define a spatio-temporal DQE (two spatial-frequency axes and one temporal-frequency axis) in terms of a small-signal spatio-temporal modulation transfer function (MTF) and spatio-temporal noise power spectrum (NPS). Measurements were made on an x-ray image intensifier-based bench-top system using continuous fluoroscopy with an RQA-5 beam at 3.9 microR/frame and hardened 50 kVp beam (0.8 mm Cu filtration added) at 1.9 microR/frame.

RESULTS

A zero-frequency DQE value of 0.64 was measured under both conditions. Nonideal performance was noted at both larger spatial and temporal frequencies; DQE values decreased by approximately 50% at the cutoff temporal frequency of 15 Hz.

CONCLUSIONS

The spatio-temporal DQE enables measurements of decreased temporal system performance at larger temporal frequencies analogous to previous measurements of decreased (spatial) performance. This marks the first time that system performance and dose efficiency in both space and time have been measured on a fluoroscopic system using DQE and is the first step toward the generalized use of DQE on clinical fluoroscopic systems.

A small-signal approach to temporal modulation transfer functions with exposure-rate dependence and its application to fluoroscopic detective quantum efficiency

The detective quantum efficiency (DQE) is a metric widely used in radiography to quantify system performance and as a surrogate measure of patient "dose efficiency." It has been applied previously to fluoroscopic systems with the introduction of a temporal correction factor. Calculation of this correction factor relies on measurements of the temporal modulation transfer function (MTF). However, the temporal MTF is often exposure-rate dependent, violating a necessary Fourier linearity requirement. The authors show that a Fourier analysis is appropriate for fluoroscopic systems if a "small-signal" approach is used. Using a semitransparent edge, a lag-corrected DQE is described and measured for an x-ray image intensifier-based fluoroscopic system under continuous (non-pulsed) exposure conditions. It was found that results were equivalent for both rising and falling-edge profiles independent of edge attenuation when effective attenuation was in the range of 0.1-0.6. This suggests that this range is appropriate for measuring the small-signal temporal MTF. In general, lag was greatest at low exposure rates. It was also found that results obtained using a falling-edge profile with a radiopaque edge were equivalent to the small-signal results for the test system. If this result is found to be true generally, it removes the need for the small-signal approach. Lag-corrected DQE values were validated by comparison with radiographic DQE values obtained using very long exposures under the same conditions. Lag was observed to inflate DQE measurements by up to 50% when ignored.

Image guidance for neurovascular intervention: proposed setup for a 3D‐roadmap system

Navigation in neurovascular interventions is currently hindered by the fact that the vessel infrastructure and the instruments are only shown simultaneously in a single real-time image during the use of a roadmap. An image guidance system based on a single C-arm is proposed, which will enable a 3D-roadmap showing a blended image of a 3D-rotational angiography and a real-time fluoroscopy image. The images are combined using machine-based registration, employing sensors mounted on the patient table and the C-arm. The setup of the system and its implications for the interventional procedures are described. The feasibility of the system is discussed with respect to the desired accuracy of matching and speed. The 3D-roadmap is expected to enhance 3D-insight for the interventionist and will facilitate instrument navigation. Implementation of the system will lead to a reduction both of the X-ray dosage and of the use of contrast agent.

Error analysis of marker-based object localization using a single-plane XRII

The role of imaging and image guidance is increasing in surgery and therapy, including treatment planning and follow-up. Fluoroscopy is used for two-dimensional (2D) guidance or localization; however, many procedures would benefit from three-dimensional (3D) guidance or localization. Three-dimensional computed tomography (CT) using a C-arm mounted x-ray image intensifier (XRII) can provide high-quality 3D images; however, patient dose and the required acquisition time restrict the number of 3D images that can be obtained. C-arm based 3D CT is therefore limited in applications for x-ray based image guidance or dynamic evaluations. 2D-3D model-based registration, using a single-plane 2D digital radiographic system, does allow for rapid 3D localization. It is our goal to investigate-over a clinically practical range-the impact of x-ray exposure on the resulting range of 3D localization precision. In this paper it is assumed that the tracked instrument incorporates a rigidly attached 3D object with a known configuration of markers. A 2D image is obtained by a digital fluoroscopic x-ray system and corrected for XRII distortions (+/- 0.035 mm) and mechanical C-arm shift (+/- 0.080 mm). A least-square projection-Procrustes analysis is then used to calculate the 3D position using the measured 2D marker locations. The effect of x-ray exposure on the precision of 2D marker localization and on 3D object localization was investigated using numerical simulations and x-ray experiments. The results show a nearly linear relationship between 2D marker localization precision and the 3D localization precision. However, a significant amplification of error, nonuniformly distributed among the three major axes, occurs, and that is demonstrated. To obtain a 3D localization error of less than +/- 1.0 mm for an object with 20 mm marker spacing, the 2D localization precision must be better than +/- 0.07 mm. This requirement was met for all investigated nominal x-ray exposures at 28 cm FOV, and for all but the lowest two at 40 cm FOV. However, even for those two nominal exposures, the expected 3D localization error is less than +/- 1.2 mm. The tracking precision was +/- 0.65 mm for the out-of-plane translations, +/- 0.05 mm for in-plane translations, and +/- 0.05 degrees for the rotations. The root mean square (RMS) difference between the true and projection-Procrustes calculated location was 1.07 mm. It is believed these results show the potential of this technique for dynamic evaluations or real-time image guidance using a single x-ray source and XRII detector.

Interventional cardiovascular magnetic resonance: still tantalizing

The often touted advantages of MR guidance remain largely unrealized for cardiovascular interventional procedures in patients. Many procedures have been simulated in animal models. We argue these opportunities for clinical interventional MR will be met in the near future. This paper reviews technical and clinical considerations and offers advice on how to implement a clinical-grade interventional cardiovascular MR (iCMR) laboratory. We caution that this reflects our personal view of the "state of the art."

The design and imaging characteristics of dynamic, solid-state, flat-panel x-ray image detectors for digital fluoroscopy and fluorography

Dynamic, flat-panel, solid-state, x-ray image detectors for use in digital fluoroscopy and fluorography emerged at the turn of the millennium. This new generation of dynamic detectors utilize a thin layer of x-ray absorptive material superimposed upon an electronic active matrix array fabricated in a film of hydrogenated amorphous silicon (a-Si:H). Dynamic solid-state detectors come in two basic designs, the indirect-conversion (x-ray scintillator based) and the direct-conversion (x-ray photoconductor based). This review explains the underlying principles and enabling technologies associated with these detector designs, and evaluates their physical imaging characteristics, comparing their performance against the long established x-ray image intensifier television (TV) system. Solid-state detectors afford a number of physical imaging benefits compared with the latter. These include zero geometrical distortion and vignetting, immunity from blooming at exposure highlights and negligible contrast loss (due to internal scatter). They also exhibit a wider dynamic range and maintain higher spatial resolution when imaging over larger fields of view. The detective quantum efficiency of indirect-conversion, dynamic, solid-state detectors is superior to that of both x-ray image intensifier TV systems and direct-conversion detectors. Dynamic solid-state detectors are playing a burgeoning role in fluoroscopy-guided diagnosis and intervention, leading to the displacement of x-ray image intensifier TV-based systems. Future trends in dynamic, solid-state, digital fluoroscopy detectors are also briefly considered. These include the growth in associated three-dimensional (3D) visualization techniques and potential improvements in dynamic detector design.

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.

A moving slanted-edge method to measure the temporal modulation transfer function of fluoroscopic systems

Lag in fluoroscopic systems introduces a frame-averaging effect that reduces measurements of image noise and incorrectly inflates measurements of the detective quantum efficiency (DQE). A correction can be implemented based on measurements of the temporal modulation transfer function (MTF). We introduce a method of measuring the temporal MTF under fluoroscopic conditions using a moving slanted edge, a generalization of the slanted-edge method used to measure the (spatial) MTF, providing the temporal MTF of the entire imaging system. The method uses a single x-ray exposure, constant edge velocity, and assumes spatial and temporal blurring are separable. The method was validated on a laboratory x-ray image intensifier (XRII) system by comparison with direct measurements of the XRII optical response, showing excellent agreement over the entire frequency range tested (+/- 100 Hz). With proper access to linearized data and continuous fluoroscopy, this method can be implemented in a clinical setting on both XRII and flat-panel detectors. It is shown that the temporal MTF of the CsI-based validation system is a function of exposure rate. The rising-edge response showed more lag than the falling edge, and the temporal MTF decreased with decreasing exposure rate. It is suggested that a small-signal approach, in which the range of exposure rates is restricted to a linear range by using a semitransparent moving edge, would be appropriate for measuring the DQE of these systems.

A practical global distortion correction method for an image intensifier based x-ray fluoroscopy system

X-ray images acquired on systems with image intensifiers (II) exhibit characteristic distortion which is due to both external and internal factors. The distortion is dependent on the orientation of the II, a fact particularly relevant to II's mounted on C arms which have several degrees of freedom of motion. Previous descriptions of distortion correction strategies have relied on a dense sampling of the C-arm orientation space, and as such have been limited mostly to a single arc of the primary angle, alpha. We present a new method which smooths the trajectories of the segmented vertices of the grid phantom as a function of a prior to solving the two-dimensional warping problem. It also shows that the same residual errors of distortion correction could be achieved without fitting the trajectories of the grid vertices, but instead applying the previously described global method of distortion correction, followed by directly smoothing the values of the polynomial coefficients as functions of the C-arm orientation parameters. When this technique was applied to a series of test images at arbitrary alpha, the root-mean-square (RMS) residual error was 0.22 pixels. The new method was extended to three degrees of freedom of the C-arm motion: the primary angle, alpha; the secondary angle, beta; and the source-to-intensifier distance, lambda. Only 75 images were used to characterize the distortion for the following ranges: alpha, +/- 45 degrees (Deltaalpha = 22.5 degrees); beta, +/- 36 degrees (Deltabeta = 18 degrees); lambda, 98-118 cm (Deltalambda = 10 cm). When evaluated on a series of test images acquired at arbitrary (alpha, beta, lambda), the RMS residual error was 0.33 pixels. This method is targeted at applications such as guidance of catheter-based interventions and treatment planning for brachytherapy, which require distortion-corrected images over a large range of C-arm orientations.

Targeted ROTational magnetic resonance angiography (TROTA)

An MR angiographic method is presented in which a rotating 2D slice is centered on and targets a region or vessel of interest. Collecting a series of slices rotating about the center of the targeted region yields projection data sufficient for the calculation of 3D volumetric data of the region using conventional backprojection reconstruction techniques. These volumetric data depict the internal structure of the vessel and can be processed and displayed with multiplanar reformation, maximum intensity projections, and 3D rendering algorithms. The rotational angiographic acquisition preserves the high temporal resolution of 2D-MR digital subtraction angiography with the added benefit of 3D reformatting and display. The method is explained in detail and results from phantom and human experiments are presented.

Predictive sensor based x-ray calibration using a physical model

Many computer assisted surgery systems are based on intraoperative x-ray images. To achieve reliable and accurate results these images have to be calibrated concerning geometric distortions, which can be distinguished between constant distortions and distortions caused by magnetic fields. Instead of using an intraoperative calibration phantom that has to be visible within each image resulting in overlaying markers, the presented approach directly takes advantage of the physical background of the distortions. Based on a computed physical model of an image intensifier and a magnetic field sensor, an online compensation of distortions can be achieved without the need of an intraoperative calibration phantom. The model has to be adapted once to each specific image intensifier through calibration, which is based on an optimization algorithm systematically altering the physical model parameters, until a minimal error is reached. Once calibrated, the model is able to predict the distortions caused by the measured magnetic field vector and build an appropriate dewarping function. The time needed for model calibration is not yet optimized and takes up to 4 h on a 3 GHz CPU. In contrast, the time needed for distortion correction is less than 1 s and therefore absolutely acceptable for intraoperative use. First evaluations showed that by using the model based dewarping algorithm the distortions of an XRII with a 21 cm FOV could be significantly reduced. The model was able to predict and compensate distortions by approximately 80% to a remaining error of 0.45 mm (max) (0.19 mm rms).

Novel method for correction of x-ray fluoroscopic image

X-ray fluoroscopic images have been widely used in orthopedic surgery. Unfortunately, the inherent distortion deteriorates the quality of fluoroscopic image. To avoid the discontinuities of local correction techniques and achieve good accuracy in present global correction method, a novel approach for distortion correction is proposed which allows good image quality in relatively acceptable time by combining both global and local methods, and a new local interpolation method is also proposed. Computer simulation and experimental test on fluoroscopic image have been carried out.

Structure-from-motion without correspondence from tomographic projections by Bayesian inversion theory

In conventional tomography, the interior of an object is reconstructed from tomographic projections such as X-ray or transmission electron microscope images. All the current reconstruction methods assume that projection geometry of the imaging device is either known or solved in advance by using e.g., fiducial or nonfiducial feature points in the images. In this paper, we propose a novel approach where the imaging geometry is solved simultaneously with the volume reconstruction problem while no correspondence information is needed. Our approach is a direct application of Bayesian inversion theory and produces the maximum likelihood or maximum a posteriori estimates for the motion parameters under the selected noise and prior distributions. In this paper, the method is implemented for a two-dimensional model problem with one-dimensional affine projection data. The performance of the method is tested with simulated and measured X-ray projection data.

A biplanar fluoroscopic approach for the measurement, modeling, and simulation of needle and soft-tissue interaction

A methodology for modeling the needle and soft-tissue interaction during needle insertion is presented. The approach consists of the measurement of needle and tissue motion using a dual C-arm fluoroscopy system. Our dual C-arm fluoroscopy setup allows real time 3-D extraction of the displacement of implanted fiducials in the soft tissue during needle insertion to obtain the necessary parameters for accurate modeling of needle and soft-tissue interactions. The needle and implanted markers in the tissue are tracked during the insertion and withdrawal of the needle at speeds of 1.016 mm/s, 12.7 mm/s and 25.4 mm/s. Both image and force data are utilized to determine important parameters such as the approximate cutting force, puncture force, the local effective modulus (LEM) during puncture, and the relaxation of tissue. We have also validated the LEM computed from our finite element model with arbitrary needle puncture tasks. Based on these measurements, we developed a model for needle insertion and withdrawal that can be used to generate a 1-DOF force versus position profile that can be experienced by a user operating a haptic device. This profile was implemented on a 7-DOf haptic device designed in our laboratory.

Angle-independent measure of motion for image-based gating in 3D coronary angiography

The role of three-dimensional (3D) image guidance for interventional procedures and minimally invasive surgeries is increasing for the treatment of vascular disease. Currently, most interventional procedures are guided by two-dimensional x-ray angiography, but computed rotational angiography has the potential to provide 3D geometric information about the coronary arteries. The creation of 3D angiographic images of the coronary arteries requires synchronization of data acquisition with respect to the cardiac cycle, in order to minimize motion artifacts. This can be achieved by inferring the extent of motion from a patient's electrocardiogram (ECG) signal. However, a direct measurement of motion (from the 2D angiograms) has the potential to improve the 3D angiographic images by ensuring that only projections acquired during periods of minimal motion are included in the reconstruction. This paper presents an image-based metric for measuring the extent of motion in 2D x-ray angiographic images. Adaptive histogram equalization was applied to projection images to increase the sharpness of coronary arteries and the superior-inferior component of the weighted centroid (SIC) was measured. The SIC constitutes an image-based metric that can be used to track vessel motion, independent of apparent motion induced by the rotational acquisition. To evaluate the technique, six consecutive patients scheduled for routine coronary angiography procedures were studied. We compared the end of the SIC rest period (rho) to R-waves (R) detected in the patient's ECG and found a mean difference of 14 +/- 80 ms. Two simultaneous angular positions were acquired and rho was detected for each position. There was no statistically significant difference (P = 0.79) between rho in the two simultaneously acquired angular positions. Thus we have shown the SIC to be independent of view angle, which is critical for rotational angiography. A preliminary image-based gating strategy that employed the SIC was compared to an ECG-based gating strategy in a porcine model. The image-based gating strategy selected 61 projection images, compared to 45 selected by the ECG-gating strategy. Qualitative comparison revealed that although both the SIC-based and ECG-gated reconstructions decreased motion artifact compared to reconstruction with no gating, the SIC-based gating technique increased the conspicuity of smaller vessels when compared to ECG gating in maximum intensity projections of the reconstructions and increased the sharpness of a vessel cross section in multi-planar reformats of the reconstruction.

2D-3D registration of coronary angiograms for cardiac procedure planning and guidance

We present a completely automated 2D-3D registration technique that accurately maps a patient-specific heart model, created from preoperative images, to the patient's orientation in the operating room. This mapping is based on the registration of preoperatively acquired 3D vascular data with intraoperatively acquired angiograms. Registration using both single and dual-plane angiograms is explored using simulated but realistic datasets that were created from clinical images. Heart deformations and cardiac phase mismatches are taken into account in our validation using a digital 4D human heart model. In an ideal situation where the pre- and intraoperative images were acquired at identical time points within the cardiac cycle, the single-plane and the dual-plane registrations resulted in 3D root-mean-square (rms) errors of 1.60 +/- 0.21 and 0.53 +/- 0.08 mm, respectively. When a 10% timing offset was added between the pre- and the intraoperative acquisitions, the single-plane registration approach resulted in inaccurate registrations in the out-of-plane axis, whereas the dual-plane registration exhibited a 98% success rate with a 3D rms error of 1.33 +/- 0.28 mm. When all potential sources of error were included, namely, the anatomical background, timing offset, and typical errors in the vascular tree reconstruction, the dual-plane registration performed at 94% with an accuracy of 2.19 +/- 0.77 mm.