K Hoffmann

Endovascular image-guided interventions (EIGIs)

Minimally invasive interventions are rapidly replacing invasive surgical procedures for the most prevalent human disease conditions. X-ray image-guided interventions carried out using the insertion and navigation of catheters through the vasculature are increasing in number and sophistication. In this article, we offer our vision for the future of this dynamic field of endovascular image-guided interventions in the form of predictions about (1) improvements in high-resolution detectors for more accurate guidance, (2) the implementation of high-resolution region of interest computed tomography for evaluation and planning, (3) the implementation of dose tracking systems to control patient radiation risk, (4) the development of increasingly sophisticated interventional devices, (5) the use of quantitative treatment planning with patient-specific computer fluid dynamic simulations, and (6) the new expanding role of the medical physicist. We discuss how we envision our predictions will come to fruition and result in the universal goal of improved patient care.

A practical exposure-equivalent metric for instrumentation noise in x-ray imaging systems

The performance of high-sensitivity x-ray imagers may be limited by additive instrumentation noise rather than by quantum noise when operated at the low exposure rates used in fluoroscopic procedures. The equipment-invasive instrumentation noise measures (in terms of electrons) are generally difficult to make and are potentially not as helpful in clinical practice as would be a direct radiological representation of such noise that may be determined in the field. In this work, we define a clinically relevant representation for instrumentation noise in terms of noise-equivalent detector entrance exposure, termed the instrumentation noise-equivalent exposure (INEE), which can be determined through experimental measurements of noise-variance or signal-to-noise ratio (SNR). The INEE was measured for various detectors, thus demonstrating its usefulness in terms of providing information about the effective operating range of the various detectors. A simulation study is presented to demonstrate the robustness of this metric against post-processing, and its dependence on inherent detector blur. These studies suggest that the INEE may be a practical gauge to determine and compare the range of quantum-limited performance for clinical x-ray detectors of different design, with the implication that detector performance at exposures below the INEE will be instrumentation-noise limited rather than quantum-noise limited.

Contrast settling in cerebral aneurysm angiography

During angiography, blood flow is visualized with a radiopaque contrast agent, which is denser than blood. In complex vasculature, such as cerebral saccular aneurysms, the density difference may produce an appreciable gravity effect, where the contrast material separates from blood and settles along the gravity direction. Although contrast settling has been occasionally reported before, the fluid mechanics behind it have not been explored. Furthermore, the severity of contrast settling in cerebral aneurysms varies significantly from case to case. Therefore, a better understanding of the physical principles behind this phenomenon is needed to evaluate contrast settling in clinical angiography. In this study, flow in two identical groups of sidewall aneurysm models with varying parent-vessel curvature was examined by angiography. Intravascular stents were deployed into one group of the models. To detect contrast settling, we used lateral view angiography. Time-intensity curves were analysed from the angiographic data, and a computational fluid dynamic analysis was conducted. Results showed that contrast settling was strongly related to the local flow dynamics. We used the Froude number, a ratio of flow inertia to gravity force, to characterize the significance of gravity force. An aneurysm with a larger vessel curvature experienced higher flow, which resulted in a larger Froude number and, thus, less gravitational settling. Addition of a stent reduced the aneurysmal flow, thereby increasing the contrast settling. We found that contrast settling resulted in an elevated washout tail in the time-intensity curve. However, this signature is not unique to contrast settling. To determine whether contrast settling is present, a lateral view should be obtained in addition to the anteroposterior (AP) view routinely used clinically so as to rule out contrast settling and hence to enable a valid time-intensity curve analysis of blood flow in the aneurysm.

Flow modification in canine intracranial aneurysm model by an asymmetric stent: studies using digital subtraction angiography (DSA) and image-based computational fluid dynamics (CFD) analyses

An asymmetric stent with low porosity patch across the intracranial aneurysm neck and high porosity elsewhere is designed to modify the flow to result in thrombogenesis and occlusion of the aneurysm and yet to reduce the possibility of also occluding adjacent perforator vessels. The purposes of this study are to evaluate the flow field induced by an asymmetric stent using both numerical and digital subtraction angiography (DSA) methods and to quantify the flow dynamics of an asymmetric stent in an in vivo aneurysm model. We created a vein-pouch aneurysm model on the canine carotid artery. An asymmetric stent was implanted at the aneurysm, with 25% porosity across the aneurysm neck and 80% porosity elsewhere. The aneurysm geometry, before and after stent implantation, was acquired using cone beam CT and reconstructed for computational fluid dynamics (CFD) analysis. Both steady-state and pulsatile flow conditions using the measured waveforms from the aneurysm model were studied. To reduce computational costs, we modeled the asymmetric stent effect by specifying a pressure drop over the layer across the aneurysm orifice where the low porosity patch was located. From the CFD results, we found the asymmetric stent reduced the inflow into the aneurysm by 51%, and appeared to create a stasis-like environment which favors thrombus formation. The DSA sequences also showed substantial flow reduction into the aneurysm. Asymmetric stents may be a viable image guided intervention for treating intracranial aneurysms with desired flow modification features.

Is Radiologic Assessment of Alveolar Crest Height Useful to Monitor Periodontal Disease Activity

The mainstay of periodontal assessment is clinical probing. Radiographic assessment provides quantitative information on the status of tooth-supporting bone. This article reviews methods to assess periodontal structures, including basic radiograph acquisition, assessment of alveolar crest levels, and typical patterns of bone loss. Computer technology to objectively assess loss of alveolar crest from radiographs is reviewed. Developments in computer-assisted quantitation of alveolar crest height are described. Although probing measurements continue to be viewed as more practical than radiographic measurements, radiographic assessment can be made quantitative and is likely easier and more precise than probing for routine assessment of periodontal disease activity.

Region of interest (ROI) microangiography: imager development

A new high spatial resolution micro-angiographic camera will enable routine viewing within a region of interest of detailed vascular structure unable to be seen with current full field of view (FOV) angiographic detectors. Such details include perforator vessels, vessel contractility or compliance, and condition and location of 50 micron or smaller stent wires. Although the basic CsI(Tl) phosphor-optical taper-CCD design of the new ROI micro-angiographic camera is essentially the same as that of the pre-clinical prototype, many of the physical parameters are much improved. The FOV is 5 cm X 5 cm vs. the previous 1 cm X 1 cm; the phosphor thickness is 350 - 400 micron vs. the previous 100 micron; the taper ratio is now 1.8 rather than 3.0 (2.8X improvement in light collection). The pixel size is either 25 or 50 micron. Additionally, detector noise may now be carefully considered in the camera design as may mechanical supporting mechanisms, methods to synchronize image acquisition with exposure and the effects of other physical factors such as exposure parameters, tube loading, focal spot size and geometric unsharpness. It is expected that this new capability should allow improved treatments and further development of smaller interventional devices and catheter delivery systems.

SU-FF-I-06: A Portable Test Platform for Image Acquisition and Calibration for Cone Beam Computed Tomography (CBCT) and Region of Interest CBCT (ROI-CBCT) On a Commercial X-Ray C-Arm System

Purpose: We have developed a unique portable test platform (PTP) which enables CBCT for specimens and phantoms on standard commercial clinical x‐ray systems. This PTP can be used to acquire ROI‐CBCT projection images, where a lower resolution, lower dose image peripheral to a high resolution ROI is acquired. This is achieved either by acquiring an image using an Image Intensifier (II) with an ROI filter in the x‐ray beam or by combining images acquired separately with low and high resolution x‐ray detectors.Method and Materials: The CBCTimages are acquired as the object rotates on the computer‐controlled rotary table of the PTP. For ROI‐CBCT, a micro‐angiography (MA) detector or an ROI filter is mounted on the PTP. The PTP also provides for relative X, Y, Z adjustments. After coarse alignment adjustments of the PTP, fine translational and angular adjustments are made based on fluoroscopic imaging of a cylindrical calibration phantom. Results: The PTP allows quick assembly of the parts required for CBCT or ROI‐CBCT reconstruction, reduces initial setup time to < 45 min, and provides for setup reproducibility. The system can be aligned to within one pixel (43 micron for the MA detector), with angular alignments of pitch and roll of the object better than 0.7° and 0.1° respectively. Conclusion:, The PTP allows fast and reliable set‐up and alignment of CBCT specimens, for standard and for ROI‐CBCT applications. The PTP may enable wider use of CBCT and ROI‐CBCT for specimens and phantoms without a costly dedicated system. (Partial support from NIH Grants R01‐NS43924, R01‐EB02873, R01‐HL52567, R01‐EB02916, and Toshiba Medical Systems Corporation).

Geometric tomography: a limited-view approach for computed tomography

Computed tomography(CT), especially since the introduction of helical CT, provides excellent visualization of the internal organs of the body. As a result, CT is used routinely in the clinical arena to obtain threeand four-dimensional data. Data is obtained by exposing patients to a beam of x-rays from a number (about 1000) of different angles (projections). Then, standard CT makes use of the Radon transform to generate 3D data, denoted ~ f , directly from projections, denoted ~g. Thus, the projection relationship can be represented in matrix form by ~g = M ~ f where M represents the projection matrix. Note that techniques based on the Radon transform are in general limited by the Nyquist sampling criteria. The increasing use of CT has resulted in a substantial rise in population-radiation-dose [1], which may lead to an increased incidence of cancer in the population. In addition to increased use, the number of projections in the CT acquisitions is increasing to improve image quality which further increases patient radiation exposure as well as the reconstruction time. This latter issue can be improved by using new technology, e.g., graphical processing units (GPUs) [2], but the problem of radiation dose does remains. Reduction of the number of projections can result in artifacts and reduced image quality. Thus, new approaches are being pursued. This work was supported by The State University of New York at Buffalo Interdisciplinary Research Development Fund, NSF grant IIS-0713489, NSF CAREER Award CCF0546509, and the Toshiba Medical Systems Corporation. Copyright is held by the author/owner(s). SCG’09, June 8–10, 2009, Aarhus, Denmark. ACM 978-1-60558-501-7/09/06. Our novel algorithm is based on a key observation: Standard CT reconstruction techniques(such as filtered backprojection [3] or an algebraic reconstruction technique [4]) converges quickly when the intensity of all voxels are similar. This is because it “evenly” distributes the intensity of each pixel in a projection to all voxels along the corresponding projection ray. When all voxels have similar intensity, the value received by each voxel from one projection ray will be close to its actual intensity. As a results, a few projections will lead to a high quality reconstruction. Therefore, we have taken the approach to mathematically split up ~g = M ~ f into subproblems or Objects-of-Interest (OoI), effectively we are rewriting the general CT problem into: ~ gOoI1 = MOoI1 ~ fOoI1 ~ gOoI2 = MOoI2 ~ fOoI2 (1)

Towards a theory of a solution space for the biplane imaging geometry problem

Biplane angiographic imaging is a primary method for visual and quantitative assessment of the vasculature. In order to reliably reconstruct the three-dimensional (3D) position, orientation, and shape of the vessel structure, a key problem is to determine the rotation matrix R and the translation vector t which relate the two coordinate systems. This so-called Imaging Geometry Determination problem is well studied in the medical imaging and computer vision communities and a number of interesting approaches have been reported. Each such technique determines a solution which yields 3D vasculature reconstructions with errors comparable to other techniques. From the literature, we see that different techniques with different optimization strategies yield reconstructions with equivalent errors. We have investigated this behavior, and it appears that the error in the input data leads to this equivalence effectively yielding what we call the solution space of feasible geometries, i.e., geometries which could be solutions given the error or uncertainty in the input image data. In this paper, we lay the theoretical framework for this concept of a solution space of feasible geometries using simple schematic constructions, deriving the underlying mathematical relationships, presenting implementation details, and discussing implications and applications of the proposed idea. Because the solution space of feasible geometries encompasses equivalent solutions given the input error, the solution space approach can be used to evaluate the precision of calculated geometries or 3D data based on known or estimated uncertainties in the input image data. We also use the solution space approach to calculate an imaging geometry, i.e., a solution.

SU‐FF‐I‐108: Effect of Point Spread Function, X‐Ray Quantum Noise, and Additive Instrumentation Noise On the Accuracy of the Angulated Slit Method for Determination of Pre‐Sampled Detector MTF

Purpose: To evaluate the accuracy of the angulated‐slit method for determining the detectorModulation Transfer Function(MTF) and thus, for the first time to determine the accuracy of the method for various physical circumstances. We quantify the difference between “true” MTFs and “measured” MTFs obtained using the angulated‐slit method. Method and Materials: A series of simulated slit images were initially generated without blur, with different slit‐widths, different slit‐angles relative to the detector matrix, and with and without Poisson distributed x‐ray quantum noise. We used the angled‐slit method to calculate the MTF and compared it with the “true” MTF (a sinc‐function given by the pixel aperture). We then introduced known Gaussian blur and additive instrumentation noise to simulate more realistic slit images. The MTFs “measured” with the slit method and the “true” MTFs were compared. Results: For the ideal slit without blur, there was an increased error in the measured MTF with increasing slit‐angle and slit‐width, with the greatest error at higher spatial frequencies. Larger slit widths and angles resulted in even greater deviations. Slit images simulated using Poisson distributed quanta, known Gaussian blur, and additive instrumentation noise, resulted in an overall increase in the MTF when compared with the “true” MTF, especially at higher spatial frequencies and increasing deviation with increasing width of the Gaussian blur (around 15% difference at 50% of the Nyquist for a PSF with 3 pixels FWHM and 1% additive noise). Conclusion: These results show the angulated‐slit method to be sensitive to slit‐angle and slit‐width. The presence of quantum noise, additive instrumentation noise and the width of the blur function are shown to severely affect the angled‐slit method accuracy, especially at higher spatial frequencies. Although the slit method provides accurate MTF values at low spatial frequencies, it overestimates the MTF at higher spatial frequencies. (Support: NIH‐R01EB002873)