Doppler String Phantom Research Articles

Investigation of Intrinsic Spectral Broadening on the Aixplorer ultrafast ultrasound system

For Doppler velocity measurements, Intrinsic Spectral Broadening (ISB) is a broadening of the Doppler spectrum due to the finite spacing between the transducer elements, leading to a range of insonation angles. This effect leads to overestimation in velocity and a decrease in quantification precision. There have been limited attempts to quantify the Intrinsic Spectral Broadening (ISB) of the Aixplorer system under a wide range of imaging conditions. This could be of clinical relevance as it remains unclear whether ultrafast Doppler suffers from the same degree of imprecision as conventional Doppler imaging. In this work, the Aixplorer system ISB was measured using a Doppler string phantom for a range of velocities (10–200 cm/s) and vessel depths (1–4 cm). The results of this analysis showed that, for all depths, the ISB could be modelled using a power law fit with respect to velocity, with a percentage ISB in the range of 3–10 ± 2%. With respect to depth, the ISB did not appear to change significantly, with a percentage ISB in the range of 3–4 ± 2% measured. When utilising ultrafast acquisitions, the percentage ISB was 0.44–0.63 ± 0.07%, affecting an ISB reduction of a factor of 6.5 ± 0.3. These results indicate that the velocity dependence of ISB is strong, with low velocities suffering a high degree of overestimation. Despite the expected result of a strong dependence of ISB on depth, none was observed. Finally, by utilising ultrafast acquisitions, the percentage of ISB can be dramatically reduced.

Inaccuracies in Measuring Velocities and Timing of Flow and Tissue Motion Using High-End Ultrasound Systems

To evaluate the accuracy of measurements of the velocities and timing of blood flow and of tissue motion, we tested four commercially available ultrasound systems. Pulsed wave, continuous wave and tissue Doppler recordings acquired across a range of angles of insonation were compared against a calibrated Doppler string phantom. Temporal delays were measured from the onset of the simulated electrocardiogram to the start of each Doppler signal. Across all modalities and angles, the mean errors of measurements of velocity using the four systems were 0.2%, 1.7%, 6.0% and 3.1%. The largest errors occurred using tissue Doppler at 5 cm/s (mean overestimation: 5.8%, range: +1.1%–12.5%). Mean delays in displaying the onset of flow were –3, 6, 11 and –16 ms. All differences between machines were statistically significant. The accuracy of high-end ultrasound systems for measuring velocities is better than in earlier reports, but the reporting of routine testing against standard phantoms should be mandatory.

Performance tests of a new non-invasive sensor unit and ultrasound electronics

Industrial applications involving pulsed ultrasound instrumentation require complete non-invasive setups due to high temperatures, pressures and possible abrasive fluids. Recently, new pulser-receiver electronics and a new sensor unit were developed by Flow-Viz. The complete sensor unit setup enables non-invasive Doppler measurements through high grade stainless steel. In this work a non-invasive sensor unit developed for one inch pipes (22.5mm ID) and two inch pipes (48.4mm ID) were evaluated. Performance tests were conducted using a Doppler string phantom setup and the Doppler velocity results were compared to the moving string target velocities. Eight different positions along the pipe internal diameter (22.5mm) were investigated and at each position six speeds (0.1–0.6m/s) were tested. Error differences ranged from 0.18 to 7.8% for the tested velocity range. The average accuracy of Doppler measurements for the 22.5mm sensor unit decreased slightly from 1.3 to 2.3% across the ultrasound beam axis. Eleven positions were tested along the diameter of the 48.4mm pipe (eight positions covered the pipe radius) and five speeds were tested (0.2–0.6m/s). The average accuracy of Doppler measurements for the 48.4mm sensor unit was between 2.4 and 5.9%, with the lowest accuracy at the point furthest away from the sensor unit. Error differences varied between 0.07 and 11.85% for the tested velocity range, where mostly overestimated velocities were recorded. This systematic error explains the higher average error difference percentage when comparing the 48.4mm (2.4–5.9%) and 22.5mm (1.3–2.3%) sensor unit performance. The overall performance of the combined Flow-Viz system (electronics, software, sensor) was excellent as similar or higher errors were typically reported in the medical field. This study has for the first time validated non-invasive Doppler measurements through high grade stainless steel pipes by using an advanced string phantom setup.

Doppler string phantom for assessment of clinical doppler ultrasound velocity measurement

Purpose: The Doppler string phantom provides accurate velocity of the string motion; it can be used to calibrate Doppler ultrasound (US) velocity measurements and to evaluate variations due to intrinsic spectral broadening. We developed a semi‐automated method to estimate the mode velocity (Vmode) and peak velocity (Vmax) based on duplex US images from a string phantom, and use them to assess clinical Doppler US velocity measurement. Methods : Steady motion of a rubber O‐ring (20 – 110 cm/s) in a CIRS Doppler String phantom (Model 043) was studied using GE LOGIQ E9 system with a 9L probe. 5 s of Doppler spectral data was averaged to generate a mean spectral profile. It was fitted by a Gaussian function and Vmode was defined as the velocity of the Gaussian peak, while Vmax is defined as the velocity at which the spectral profile falls to within 1 SD of the background. Vmode and Vmax were evaluated against the prescribed motor velocity. Repeatability and variation to scanning parameters were analyzed and reported in % range, i.e. (max – min) / mean. Results: Vmode and Vmax had good repeatability over six days (6.0% for Vmode, 2.9% for Vmax). Gain, compression, scale, sample volume (SV) depth and length, frequency and beam steering all had minimal impact on Vmode and Vmax (variations ≤ 4.4%). Doppler angle θ had minimal effect on Vmode (2.2%) but a strong effect on Vmax (26% increase as θ increased from 10° to 60°). Vmode was linearly correlated with but overestimated the motor velocity (Pearson’s r = 1.05, R 2 = 1). Conclusion : This study developed a simple yet robust Vmode and Vmax estimation method. Combined with a string phantom, these velocity estimators are shown to be a useful tool to evaluate clinical Doppler US system performance. For the tested system, only Doppler angle has an appreciable impact on Vmax estimation. -------------------------------------------- Cite this article as : Zhang Y, Lynch T, Hangiandreou NJ. Doppler string phantom for assessment of clinical doppler ultrasound velocity measurement. Int J Cancer Ther Oncol 2014; 2(2):020246. DOI:10.14319/ijcto.0202.46

An audit of a hospital-based Doppler ultrasound quality control protocol using a commercial string Doppler phantom

Results from a four-year audit of a Doppler quality assurance (QA) program using a commercially available Doppler string phantom are presented. The suitability of the phantom was firstly determined and modifications were made to improve the reliability and quality of the measurements. QA of Doppler ultrasound equipment is very important as data obtained from these systems is used in patient management. It was found that if the braided-silk filament of the Doppler phantom was exchanged with an O-ring rubber filament and the velocity range below 50 cm/s was avoided for Doppler quality control (QC) measurements, then the maximum velocity accuracy (MVA) error and intrinsic spectral broadening (ISB) results obtained using this device had a repeatability of 18 ± 3.3% and 19 ± 3.5%, respectively. A consistent overestimation of the MVA of between 12% and 56% was found for each of the tested ultrasound systems. Of more concern was the variation of the overestimation within each respective transducer category: MVA errors of the linear, curvilinear and phased array probes were in the range 12.3–20.8%, 32.3–53.8% and 27–40.7%, respectively. There is a dearth of QA data for Doppler ultrasound; it would be beneficial if a multicentre longitudinal study was carried out using the same Doppler ultrasound test object to evaluate sensitivity to deterioration in performance measurements.

Accuracy of spectral Doppler flow and tissue velocity measurements in ultrasound systems

Blood and tissue velocity are measured and analysed in cardiac, vascular and other applications of diagnostic ultrasound (US). An error in system calibration is a potential risk for misinterpretation of the measurements. To determine the accuracy in velocity calibration, we tested three common commercial US systems using a Doppler string phantom. We tested pulsed and continuous-wave Doppler modes for velocities relevant to both cardiac blood flow and tissue-velocity estimation. The US systems were tested with settings and transducers commonly used in cardiac applications. One system consistently overestimated velocity by about 5%, whereas the other two systems were quite accurate in velocity estimation. These findings emphasize the importance of continuous quality control of US equipment. (E-mail: andrew.walker@ltvastmanland.se)

Time delays in ultrasound systems can result in fallacious measurements

Even short time delays (less than 30 ms) in cardiac motion pattern may have clinical relevance. These delays can be measured with echocardiography, using techniques such as flow and tissue Doppler and M-mode together with external signals (e.g., ECG and phonocardiography). If one or more of these signals are delayed in relation to the other signals (asynchronous), an incorrect definition of cardiac time intervals can occur, the consequence of which is invalid measurement. To determine if this time delay in signal processing is a problem, we tested three common ultrasound (US) systems using the ECG as the reference signal. We used a digital ECG simulator and a Doppler string phantom to obtain test signals for flow and tissue pulsed Doppler, M-mode, phonocardiography, auxiliary and ECG signals. We found long time delays of up to 90 ms in one system, whereas delays were mostly short in the two other systems. The time delays varied relative to system settings. Consequently, to avoid these errors, precise knowledge of the characteristics of the system being used is essential. (E-mail:andrew.walker@ltvastmanland.se)

Two-dimensional color Doppler flow velocity profiles can be time corrected with an external ECG-delay device.

Although two-dimensional ultrasound color flow imaging is often considered to be a real-time technique, the acquisition time for two-dimensional color images may be up to 200 msec. Time correction is therefore necessary to obtain correct flow velocity profiles. We have developed a time-correction method in which a specially designed unit detects the QRS complex from the patient and creates a trig pulse that is delayed incrementally in relation to the QRS complex. This trig pulse controls the acquisition of the ultrasound images. A number of consecutively delayed images, with known incremental delay between the sweeps, can thus be stored in the memory of the echocardiograph and transferred digitally to a computer. The time-corrected flow velocity profile is obtained by interpolation of data from the time-delayed profiles. The system was evaluated in a Doppler string phantom test. With this technique it is possible to study time-corrected flow velocity profiles without the need to alter existing ultrasound Doppler equipment.

Effect of tank liquid acoustic velocity on Doppler string phantom measurements.

The quantitative effects of degassed water in string phantom tank Doppler measurements are derived theoretically. The Doppler parameter measurements considered are range gate registration, range gate profile, image flow angle measurements, and velocity calculation. The equipment velocity calculation is demonstrated to have an appreciable error which is due to the water acoustic velocity and the transducer acquisition geometry. A velocity calibration technique is proposed that only needs a simple multiplicative factor to compensate for the water in the tank.