Z Wang

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.

Micro-angiographic detector with fluoroscopic capability

New neuro-interventional devices such as stents require high spatial-resolution image guidance to enable accurate localization both along the vessel axis as well as in a preferred rotational orientation around the axis. A new high-resolution angiographic detector has been designed with capability for micro-angiography at rates exceeding the 5 fps of our current detector and, additionally, with noise low enough and gain high enough for fluoroscopy. Although the performance requirements are demanding and the detector must fit within practical clinical space constraints, image guidance is only needed within a approximately 5 cm region of interest at the site of the intervention. To achieve the design goals, the new detector is being assembled from available components which include a CsI(Tl) phosphor module coupled to a fiber-optic taper assembly with a two stage light image intensifier and a mirror between the output of the fiber taper and the input to a conventional high performance optical CCD camera. Resulting acquisition modes include 50-micron effective pixels at up to 30 fps with the capability to adjust sensitivity for both fluoroscopy and angiography. Estimates of signal at the various stages of detection are made with quantum accounting diagrams (QAD).

Angiographic analysis of blood flow modification in cerebral aneurysm models with a new asymmetric stent

We have built new asymmetric stents for minimally invasive endovascular treatment of cerebral aneurysms. Each asymmetric stent consists of a commercial stent with a micro-welded circular mesh patch. The blood flow modification in aneurysm-vessel phantoms due to these stents was evaluated using x-ray angiographic analysis. However, the density difference between the radiographic contrast and the blood gives rise to a gravity effect, which was evaluated using an initial optical dye-dilution experiment. For the radiographic evaluations, curved-vessel phantoms instead of simple straight side-wall aneurysm phantoms were used in the characterization of meshes/stents. Six phantoms (one untreated, one treated with a commercial stent, and four treated with different asymmetric stents) with similar morphologies were used for comparison. We calculated time-density curves of the aneurysm region and then calculated the peak value (Pk) and washout rate (1/τ) after analytical curve fitting. Flow patterns in the angiograms showed reduction of vortex flow and slow washout in the dense mesh patch treated aneurysms. The meshes reduced Pk down to 21% and 1/τ down to 12% of the values for the untreated case. In summary, new asymmetric stents were constructed and their evaluation demonstrates that they may be useful in the endovascular treatment of aneurysms.

Vessel size measurements in angiograms: Manual measurements

Vessel size measurement is perhaps the most often performed quantitative analysis in diagnostic and interventional angiography. Although automated vessel sizing techniques are generally considered to have good accuracy and precision, we have observed that clinicians rarely use these techniques in standard clinical practice, choosing to indicate the edges of vessels and catheters to determine sizes and calibrate magnifications, i.e., manual measurements. Thus, we undertook an investigation of the accuracy and precision of vessel sizes calculated from manually indicated edges of vessels. Manual measurements were performed by three neuroradiologists and three physicists. Vessel sizes ranged from 0.1-3.0 mm in simulation studies and 0.3-6.4 mm in phantom studies. Simulation resolution functions had full-widths-at-half-maximum (FWHM) ranging from 0.0 to 0.5 mm. Phantom studies were performed with 4.5 in., 6 in., 9 in., and 12 in. image intensifier modes, magnification factor = 1, with and without zooming. The accuracy and reproducibility of the measurements ranged from 0.1 to 0.2 mm, depending on vessel size, resolution, and pixel size, and zoom. These results indicate that manual measurements may have accuracies comparable to automated techniques for vessels with sizes greater than 1 mm, but that automated techniques which take into account the resolution function should be used for vessels with sizes smaller than 1 mm.

Image quality evaluation of a selenium-based flat-panel digital x-ray detector system based on animal studies

The x-ray flat-panel detector (FPD) will be a key component of the coming generation of x-ray imaging systems. FPD systems applicable to both fluoroscopy and radiography especially, will be the prime candidate to replace current image intensifier x-ray (IIXR-TV) systems. Nevertheless, IIXR-TV systems which have recently been improved by the addition of CCD cameras, have established themselves over time by offering good image quality which in most cases clinicians appear to be satisfied with. It will thus take a substantial improvement in image quality combined with a new ease of use due to reduced physical size for new FPDs to replace those systems that have evolved over many decades. Our group has been developing a selenium-based FPD which has superior spatial resolution characteristics. The purpose of this research is to elucidate the FPDs potential to replace IIXR-TV systems by offering improved image quality. Detailed measurements of physical characteristics were made and extensive in vivo animal studies were conducted. It can be concluded that the FPDs demonstrated superior image quality appears to have the potential to improve clinical performance.

Direct-conversion flat-panel detector for region-of-interest angiography

Minimally invasive image-guided interventions require very high image resolution and quality, specifically over regions-of-interest (ROI) crucial to the procedure. An ROI high quality image allows limited patient radiation deposition while permitting rapid frame transfer rates. Considering current developments in direct conversion Flat Panel Detectors (FPD), advantages of such an imager for ROI angiography were investigated. The performance of an amorphous-selenium based FPD was simulated to evaluate improvements in MTF and DQE under various angiographic imaging conditions. The detector envisioned incorporates the smallest pixel size of 70 mm, reported to date, and a photoconductor thickness of 1000 mm to permit angiography. The MTF of the FPD is calculated to be 60% at the Nyquist frequency of 7.1 lp/mm compared to 6% for a previously reported CsI(Tl)-based ROI CCD camera. The DQE(0) of the FPD at 0.7 mR and 70 kVp is 74% while for the CCD camera is 70%. At 7.1 lp/mm, the FPDs DQE is 26% while for the CCD camera it is 12%. Images of an undeployed stent with 70 mm pixel mammography FPD prototype, compare favorably with images acquired with the CCD camera. Thus a practical direct flat-panel ROI detector with both improved performance and physical size is proposed.

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‐T‐445: Variations of Rectal Dose Estimation Using Rectal Markers in HDR Cervical Brachytherapy

Purpose: In the treatment of cervical cancer using high‐dose‐rate (HDR) brachytherapy, high rectal dose often limits the tumorcidal dose. In film based brachytherapy, rectal dose is usually computed using rectal markers. However, positions of these markers can vary relative to the anterior rectal wall. This retrospective study analyzes the potential inaccuracy in rectal dose estimation due to variations of marker placement in a multi‐fractionated HDR treatment regimen. Method and Materials: Five Patients treated with multiple‐fraction tandem and ovoid HDR brachytherapy were selected. By adequate packing the vagina, there was little inter‐fractional variation of the actual rectum position. Therefore, variations of rectal makers were due to their locations relative to the wall. For each patient (with 3–4 fractions), the plan of the first fraction was selected as the reference plan. Simulation films of all the other fractions were matched with the films of the reference plan based on bony anatomies. Rectal dose points specified based upon the rectal markers of all different fractions were then digitized into the reference plan after film matching. Dose prescription and dwell weights in the reference plan were set to be identical to the original treatment plan. The rectal doses within each group of rectal points were then analyzed. Results: Our results have shown that inter‐fractional variations of rectal marker locations to specify the rectal dose are up to 0.5–0.8 cm. The resulting differences in rectal dose values are 42.4%, 23.3%, 25.6%, 6.8% and 50.4% for each patient, respectively. Conclusion: Care should be taken when using rectal markers as reference points for estimating rectal dose in HDR cervical brachytherapy. Positions of the markers inside the rectum relative to the wall can vary and cause underestimation of the dose to the rectum. The true rectal dose should be determined by the most anterior point among all fractions.

Accuracy comparison of micro-angiographic detector and image intensifier for an interventional localization task

In order to assess the effect of the use of a high-resolution micro-angiographic detector on the accuracy of performing interventional localization tasks, we set up a simple task to localize a guide wire tip relative to one edge of a thin marker wire. Trained observers were asked to evaluate the micro-angiographic images as well as the comparison standard x-ray image intensifier (XRII) images, both of which were taken with the guidewire in the same positions. The observers were able to get more accurate estimation results using images obtained with the micro-angiographic detector than the XRII; the average distance error and performance variability both were reduced by factors of 2-3.

Improved method of magnification factor calculation for the angiographic measurement of neurovascular lesion dimensions

Accurately evaluating the size of a neurovascular lesion is essential for properly devising treatment strategies. The magnification factor must be considered in order to measure the dimension of a lesion from an angiogram. Although a method to calculate the magnification of the lesion by linear interpolation of the measurable magnification factors of two markers has been in use, this paper shows that it can be inaccurate. By deriving the exact formula for calculating the magnification factor at the level of the lesion, the error generated by the linear interpolation of magnification factor has been evaluated. This error was found to depend on source‐to‐skin distance (SSD), the location of the lesion in the head, and the head size. The closer the head is to the focal spot and the nearer the lesion is to the center of the head, the larger is the error. Since clinicians tend to use high geometric magnification (i.e., small SSD) in interventional procedures, there exists a possible consequential error of more than 3% in lesion sizing if the linear‐interpolation calculation method is used. It is thus recommended that the exact formula derived here be used to calculate the magnification factor to improve accuracy. PACS number(s): 87.57.–s, 87.57.Nk, 87.59.Dj