Peter Møller Hansen

Volume Flow in Arteriovenous Fistulas Using Vector Velocity Ultrasound

Volume flow in arteriovenous fistulas for hemodialysis was measured using the angle-independent ultrasound technique Vector Flow Imaging and compared with flow measurements using the ultrasound dilution technique during dialysis. Using an UltraView 800 ultrasound scanner (BK Medical, Herlev, Denmark) with a linear transducer, 20 arteriovenous fistulas were scanned directly on the most superficial part of the fistula just before dialysis. Vector Flow Imaging volume flow was estimated with two different approaches, using the maximum and the average flow velocities detected in the fistula. Flow was estimated to be 242 mL/min and 404 mL/min lower than the ultrasound dilution technique estimate, depending on the approach. The standard deviations of the two Vector Flow Imaging approaches were 175.9 mL/min and 164.8 mL/min compared with a standard deviation of 136.9 mL/min using the ultrasound dilution technique. The study supports that Vector Flow Imaging is applicable for volume flow measurements.

New technology - demonstration of a vector velocity technique.

With conventional Doppler ultrasound it is not possible to estimate direction and velocity of blood flow, when the angle of insonation exceeds 60-70°. Transverse oscillation is an angle independent vector velocity technique which is now implemented on a conventional ultrasound scanner. In this paper a few of the possibilities with transverse oscillation are demonstrated.

First report on intraoperative vector flow imaging of the heart among patients with healthy and diseased aortic valves

The vector velocity method Transverse Oscillation (TO) implemented on a conventional ultrasound (US) scanner (ProFocus, BK Medical, Herlev, Denmark) can provide real-time, angle-independent estimates of the cardiac blood flow. During cardiac surgery, epicardial US examination using TO was performed on (A) 3 patients with healthy aortic valve and (B) 3 patients with aortic valve stenosis. In group B, the systolic flow of the ascending aorta had higher velocities, was more aliased and chaotic. The jet narrowed to 44% of the lumen compared to 75% in group A and with a vector concentration, a measure of flow complexity, of 0.41 compared to 0.87 in group A. The two groups had similar secondary flow of the ascending aorta with an average rotation frequency of 4.8 Hz. Simultaneous measurements were obtained with spectral Doppler (SD) and a thermodilution technique (TD). The mean difference in peak systolic velocity compared to SD in group A was 22% and 45% in B, while the mean difference in volume flow compared to TD in group A was 30% and 32% in B. TO can potentially reveal new information of cardiac blood flow, and may become a valuable diagnostic tool in the evaluation of patients with cardiovascular diseases.

Recent advances in blood flow vector velocity imaging

A number of methods for ultrasound vector velocity imaging are presented in the paper. The transverse oscillation (TO) method can estimate the velocity transverse to the ultrasound beam by introducing a lateral oscillation in the received ultrasound field. The approach has been thoroughly investigated using both simulations, flow rig measurements, and in-vivo validation against MR scans. The TO method obtains a relative accuracy of 10% for a fully transverse flow in both simulations and flow rig experiments. In-vivo studies performed on 11 healthy volunteers comparing the TO method with magnetic resonance phase contrast angiography (MRA) revealed a correlation between the stroke volume estimated by TO and MRA of 0.91 (p<;0.01) with an equation for the line of regression given as: MRA = 1.1 · TO-0.4 ml. Several clinical examples of complex flow in e.g. bifurcations and around valves have been acquired using a commercial implementation of the method (BK Medical ProFocus Ultraview scanner). A range of other methods are also presented. This includes synthetic aperture imaging using either spherical or plane waves with velocity estimation performed with directional beamforming or speckle tracking. The key advantages of these techniques are very fast imaging that can attain an order of magnitude higher precision than conventional methods. SA flow imaging was implemented on the experimental scanner RASMUS using an 8-emission spherical emission sequence and reception of 64 channels on a BK Medical 8804 transducer. This resulted in a relative standard deviation of 1.2% for a fully transverse flow. Plane wave imaging was also implemented on the RASMUS scanner and a 100 Hz frame rate was attained. Several vector velocity image sequences of complex flow were acquired, which demonstrates the benefits of fast vector flow imaging. A method for extending the 2D TO method to 3D vector velocity estimation is presented and the implications for future vector velocity imaging is indicated.

Arterial secondary blood flow patterns visualized with vector flow ultrasound

This study presents the first quantification and visualization of secondary flow patterns with vector flow ultrasound. The first commercial implementation of the vector flow method Transverse Oscillation was used to obtain in-vivo, 2D vector fields in real-time. The hypothesis of this study was that the rotational direction is constant within each artery. Three data sets of 10 seconds were obtained from three main arteries in healthy volunteers. For each data set the rotational flow patterns were identified during diastole. Each data set contains a 2D vector field over time using the vector angles and velocity magnitudes the blood flow patterns were visualized using streamlines in Matlab (Mathworks, Natick, MA, USA). The rotational flow was quantified by the angular frequency for each cardiac cycle, and the mean rotational frequencies and standard deviations were calculated for the abdominal aorta {-1.3±0.4;-1.0±0.3;-0.9±0.2}Hz, the common iliac artery {-0.4±0.1;-1.0±0.2;-0.4±0.1}Hz, and the common carotid artery {0.8±0.3;1.4±0.3;0.4±0.1}Hz. A positive sign indicates an anti-clockwise rotation, and a negative sign indicates clockwise rotation. The sign of the rotational directions within each artery were constant.

Preliminary study of synthetic aperture tissue harmonic imaging on in-vivo data

A method for synthetic aperture tissue harmonic imaging is investigated. It combines synthetic aperture sequen- tial beamforming (SASB) with tissue harmonic imaging (THI) to produce an increased and more uniform spatial resolution and improved side lobe reduction compared to conventional B-mode imaging. Synthetic aperture sequential beamforming tissue harmonic imaging (SASB-THI) was implemented on a commercially available BK 2202 Pro Focus UltraView ultrasound system and compared to dynamic receive focused tissue harmonic imag- ing (DRF-THI) in clinical scans. The scan sequence that was implemented on the UltraView system acquires both SASB-THI and DRF-THI simultaneously. Twenty-four simultaneously acquired video sequences of in-vivo abdominal SASB-THI and DRF-THI scans on 3 volunteers of 4 different sections of liver and kidney tissues were created. Videos of the in-vivo scans were presented in double blinded studies to two radiologists for image quality performance scoring. Limitations to the systems transmit stage prevented user defined transmit apodization to be applied. Field II simulations showed that side lobes in SASB could be improved by using Hanning transmit apodization. Results from the image quality study show, that in the current configuration on the UltraView system, where no transmit apodization was applied, SASB-THI and DRF-THI produced equally good images. It is expected that given the use of transmit apodization, SASB-THI could be further improved.

Implementation of tissue harmonic synthetic aperture imaging on a commercial ultrasound system

This paper presents an imaging technique for synthetic aperture (SAI) tissue harmonic imaging (THI) on a commercial ultrasound system. Synthetic aperture sequential beamforming (SASB) is combined with a pulse inversion (PI) technique on a commercial BK 2202 UltraView system. An interleaved scan sequence that performs dynamic receive focused (DRF) imaging and SASB, both using PI, is implemented. From each acquisition four images can be created: DRF image, SASB image, tissue harmonic DRF image (DRF-THI), and tissue harmonic SASB image (SASB-THI). For SASB imaging, a fixed transmit and receive focus at 80 mm and an F# of 3 is applied. For DRF imaging, default scanner settings are used, which are a focus at 85 mm and F# of 5.7 in transmit and a dynamic receive aperture with an F# of 0.8. In all cases a 2.14 MHz one-and-a-half cycle excitation transmit waveform is used. A BK 8820e 192 element convex array transducer is used to conduct scans of wire phantoms. The -6 dB and -20 dB lateral resolution is measured for each wire in the phantom. Results show that the -6 dB lateral resolution for SASB-THI is as good as for DRF-THI except at the point of the virtual source. SASB-THI even shows 7% reduction in -6 dB lateral resolution for the deepest wire at 100 mm. The -20 dB resolution for SASB-THI at [25, 50, 75, 100] mm was reduced by [5, 0 -34, 11] % compared to DRF-THI, which shows, that except for the point of the virtual source, the lateral resolution was improved by SASB-THI. A successful implementation of SASB-THI was achieved on a commercial system, which can be used for future pre-clinical trials.

Preliminary in-vivo evaluation of synthetic aperture sequential beamformation using a multielement convex array

This paper presents a preliminary in-vivo study of synthetic aperture sequential beamforming (SASB) in comparison with conventional imaging. The advantage of SASB compared to conventional imaging, is the ability to obtain a more range independent point spread function, without any loss in lateral resolution or frame rate. The objective of this study is to evaluate whether SASB imaging is feasible in-vivo and whether the image quality obtained is comparable with traditional scanned imaging in terms of penetration depth, spatial resolution, contrast, and unwanted artifacts. Acquisition was performed using a ProFocus ultrasound scanner and a 5 MHz convex array transducer. First stage beamformed SASB radio frequency (RF) data were acquired using a transmit and receive focal depth of 70 mm and 63-element sub-apertures. Subsequently the data were off-line processed to generate second stage SASB RF data. For conventional imaging, beamformed RF data was acquired using 63-element sub-apertures in transmit with a focal depth of 105 mm, in receive an expanding aperture using dynamic focusing with a F-number of 0.8 was used. Both modalities used the same standard manufacturer specified pulse. Conventional and SASB RF data were acquired interleaved, ensuring that the exact same anatomical location was scanned. RF data were recorded in real time and processed off-line to generate image sequences. Two male volunteers were scanned abdominally resulting in 34 image sequence pairs. Evaluation of image quality and penetration was performed by five medical doctors. Results showed no significantly (p = 0.98) increase nor decrease in penetration using SASB. Overall image quality was highly significantly (p <; 0.001) increased. Results show that in-vivo ultrasound imaging using SASB is feasible for abdominal imaging without severe motion artifacts.

New developments in vector velocity imaging using the transverse oscillation approach

Vector velocity imaging using the Transverse Oscillation (TO) approach has recently been FDA approved for linear array transducers on a commercial platform. It can now be used clinically for studying the complex ow at e.g. bifurcations, valves, and the heart in real time. Several clinical examples from venous ow to rotational ow in the heart will be shown. The technique is also being further developed and adapted for convex and phased array probes, for spectral velocity estimation, pressure estimation, and for three dimensional velocity tensor imaging. It is shown how the methods are optimized using Field II simulations along with several examples of their performance.

Clinical evaluation of Synthetic Aperture Sequential Beamforming and Tissue Harmonic Imaging

This study determines if the data reduction achieved by the combination Synthetic Aperture Sequential Beamforming (SASB) and Tissue Harmonic Imaging (THI) affects image quality. SASB-THI was evaluated against the combination of Dynamic Received Focusing and Tissue Harmonic Imaging (DRF-THI). A BK medical UltraView 800 ultrasound scanner equipped with a research interface and an abdominal 3.5 MHz 3.5CL192-3ML convex array transducer was used and connected to a stand alone PC. SASB-THI and DRF-THI scan sequences were recorded interleaved and processed offline. Nineteen patients diagnosed with focal liver pathology were scanned to set a clinical condition, where ultrasonography is often performed. A total of 114 sequences were recorded and evaluated by five radiologists. The evaluators were blinded to the imaging technique, and each sequence was shown twice with different left-right positioning, resulting in 1140 evaluations. The program Image Quality Assessment Program (IQap) and a Visual Analog Scale (VAS) were applied for the evaluation. The scale ranged from -50 to 50, where positive values favored SASB-THI. SASB-THI and DRF-THI were evaluated alike in 49% of the evaluations, 28% favored SASB-THI and 23% favored DRF-THI. The average rating was 0.70 (Cl: -0.80 to 2.19). The statistical analysis, where the hypothesis of no differences between the techniques was tested, yielded a p-value of p=0.64, indicating no preference to any technique. This study demonstrates that SASB-THI and DRF-THI have equally good image quality although a data reduction of 64 times is achieved with SASB-THI.