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Objective: The aim of this study was to construct an anthropomorphic maxillofacial phantom for dental imaging and dosimetry purposes using three-dimensional (3D) printing technology and materials that simulate the radiographic properties of tissues., Methods: Stereolithography photoreactive resins, polyurethane rubber and epoxy resin were modified by adding calcium carbonate and strontium carbonate powders or glass bubbles. These additives were used to change the materials' CT numbers to mimic various body tissues. A maxillofacial phantom was designed using CT images of a head., Results: Commercial 3D printing resins were found to have CT numbers near 120 HU and were used to print intervertebral discs and an external skin for the maxillofacial phantom. By adding various amounts of calcium carbonate and strontium carbonate powders the CT number of the resin was raised to 1000 & 1500 HU and used to print bone mimics. Epoxy resin modified by adding glass bubbles was used in assembly and as a cartilaginous mimic. Glass bubbles were added to polyurethane rubber to reduce the CT number to simulate soft tissue and filled spaces between the printed anatomy and external skin of the phantom., Conclusion: The maxillofacial phantom designed for dental imaging and dosimetry constructed using 3D printing, polyurethane rubbers and epoxy resins represented a patient anatomically and radiographically. The results of the designed phantom, materials and assembly process can be applied to generate different phantoms that better represent diverse patient types and accommodate different ion chambers.

Purpose: In real-time electronic portal imaging device (EPID) dosimetry applications where on-treatment measured transmission images are compared to an ideal predicted image, ideally a tight tolerance should be set on the quantitative image comparison in order to detect a wide variety of possible delivery errors. However, this is currently not possible due to the appearance of banding artifacts in individual frames of the measured EPID image sequences. The purpose of this work was to investigate simulating banding artifacts in our cine-EPID predicted image sequences to improve matching of individual image frames to the acquired image sequence. Increased sensitivity of this method to potential treatment delivery errors would represent an improvement in patient safety and treatment accuracy., Methods: A circuit board was designed and built to capture the target current (TARG-I) and forward power signals produced by the linac to help model the discrete beam-formation process of the linac. To simulate the temporal-spatial nature of the EPID readout, a moving read out mask was applied with the timing of the application of the readout mask synchronized to the TARG-I pulses. Since identifying the timing of the first TARG-I pulse affected the location of the banding artifacts throughout the image sequence, and furthermore the first several TARG-I pulses at the beginning of "beam on" are not at full height yet (i.e., dose rate is ramping up), the forward-power signal was also used to assist in reliable detection of the first radiation pulse of the beam delivery. The predicted EPID cine-image sequence obtained using a comprehensive physics-based model was modified to incorporate the discrete nature of the EPID frame readout. This modified banding predicted EPID (MBP-EPID) image sequence was then compared to its corresponding measured EPID cine-image sequence on a frame-by-frame basis. The EPID was mounted on a Clinac 2100ix linac (Varian Medical Systems, Palo Alto, CA). The field size was set to 21.4  ×  28.6 cm 2 with no MLC modulation, beam energy of 6 MV, dose rate of 600 MU/min, and 700 MU were delivered for each clockwise (CW) and counter-clockwise (CCW) arc. No phantoms were placed in the beam., Results: The dose rate ramp up effect was observed at the beginning irradiations, and the identification and timing of the radiation pulses, even during the dose rate ramp up, were able to be quantified using the TARG-I and forward power signals. The approach of capturing individual dose pulses and synchronizing with the mask image applied to the original predicted EPID image sequence was demonstrated to model the actual EPID readout. The MBP-EPID image sequences closely reproduced the location and magnitude of the banding features observed in the acquired (i.e., measured) image sequence, for all test irradiations examined here., Conclusions: The banding artifacts observed in the measured EPID cine-frame sequences were reproduced in the predicted EPID cine-frames by simulating the discrete temporal-spatial nature of the EPID read out. The MBP-EPID images showed good agreement qualitatively to the corresponding measured EPID frame sequence of a simple square test field, without any phantom in the beam. This approach will lead to improved image comparison tolerances for real-time patient dosimetry applications., (© 2020 American Association of Physicists in Medicine.)

Evidence suggests that women who are naturally resistant to HIV infection exhibit low baseline immune activation at the female genital tract (FGT). This "immune quiescent" state is associated with lower expression of T-cell activation markers, reduced levels of gene transcription and pro-inflammatory cytokine or chemokine production involved in HIV infection while maintaining an intact immune response against pathogens. Therefore, if this unique immune quiescent state can be pharmacologically induced locally, it will provide an excellent women-oriented strategy against HIV infection To our knowledge, this is the first research article evaluating in vivo, an innovative trackable implant that can provide controlled delivery of hydroxychloroquine (HCQ) to successfully attenuate vaginal T lymphocyte activation and inflammation in a rabbit model as a potential strategy to induce an "immune quiescent" state within the FGT for the prevention of HIV infection. This biocompatible implant can deliver HCQ above therapeutic concentrations in a controlled manner, reduce submucosal immune cell recruitment, improve mucosal epithelium integrity, decrease protein and gene expression of T-cell activation markers, and attenuate the induction of key pro-inflammatory mediators. Our results suggest that microbicides designed to maintain a low level of immune activation at the FGT may offer a promising new strategy for reducing HIV infection., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Purpose: Three-dimensional printing has been implemented at our institution to create customized treatment accessories, including lead shields used during radiation therapy for facial skin cancer. To effectively use 3-dimensional printing, the topography of the patient must first be acquired. We evaluated a low-cost, structured-light, 3-dimensional, optical scanner to assess the clinical viability of this technology., Methods and Materials: For ease of use, the scanner was mounted to a simple gantry that guided its motion and maintained an optimum distance between the scanner and the object. To characterize the spatial accuracy of the scanner, we used a geometric phantom and an anthropomorphic head phantom. The geometric phantom was machined from plastic and included hemispherical and tetrahedral protrusions that were roughly the dimensions of an average forehead and nose, respectively. Polygon meshes acquired by the optical scanner were compared with meshes generated from high-resolution computed tomography images. Most optical scans contained minor artifacts. Using an algorithm that calculated the distances between the 2 meshes, we found that most of the optical scanner measurements agreed with those from the computed tomography scanner within approximately 1 mm for the geometric phantom and approximately 2 mm for the head phantom. We used this optical scanner along with 3-dimensional printer technology to create custom lead shields for 10 patients receiving orthovoltage treatments of nonmelanoma skin cancers of the face. Patient, tumor, and treatment data were documented., Results: Lead shields created using this approach were accurate, fitting the contours of each patient's face. This process added to patient convenience and addressed potential claustrophobia and medical inability to lie supine., Conclusions: The scanner was found to be clinically acceptable, and we suggest that the use of an optical scanner and 3-dimensional printer technology become the new standard of care to generate lead shielding for orthovoltage radiation therapy of nonmelanoma facial skin cancer.

Technological advances have influenced practice patterns and innovation in many health disciplines, including respiratory therapy. Collaborative approaches and knowledge-sharing environments are vital in addressing problems and adopting emerging technology. This article illustrates how the emergence of low-cost three-dimensional printing technology to physically reproduce the results of computed tomography imaging data can provide ways to assess airway abnormalities and symptomology not explained by traditional diagnostic methods.

Purpose: To develop a fast patient-specific analytical estimator of first-order Compton and Rayleigh scatter in cone-beam computed tomography, implemented using graphics processing units., Methods: The authors developed an analytical estimator for first-order Compton and Rayleigh scatter in a cone-beam computed tomography geometry. The estimator was coded using NVIDIA's CUDA environment for execution on an NVIDIA graphics processing unit. Performance of the analytical estimator was validated by comparison with high-count Monte Carlo simulations for two different numerical phantoms. Monoenergetic analytical simulations were compared with monoenergetic and polyenergetic Monte Carlo simulations. Analytical and Monte Carlo scatter estimates were compared both qualitatively, from visual inspection of images and profiles, and quantitatively, using a scaled root-mean-square difference metric. Reconstruction of simulated cone-beam projection data of an anthropomorphic breast phantom illustrated the potential of this method as a component of a scatter correction algorithm., Results: The monoenergetic analytical and Monte Carlo scatter estimates showed very good agreement. The monoenergetic analytical estimates showed good agreement for Compton single scatter and reasonable agreement for Rayleigh single scatter when compared with polyenergetic Monte Carlo estimates. For a voxelized phantom with dimensions 128 × 128 × 128 voxels and a detector with 256 × 256 pixels, the analytical estimator required 669 seconds for a single projection, using a single NVIDIA 9800 GX2 video card. Accounting for first order scatter in cone-beam image reconstruction improves the contrast to noise ratio of the reconstructed images., Conclusion: The analytical scatter estimator, implemented using graphics processing units, provides rapid and accurate estimates of single scatter and with further acceleration and a method to account for multiple scatter may be useful for practical scatter correction schemes.

EPID images acquired in cine mode during arc therapy have inaccurate gantry angles recorded in their image headers. In this work, methods were developed to assess the accuracy of the gantry potentiometer for linear accelerators. As well, assessments of the accuracy of other, more accessible, sources of gantry angle information (i.e., treatment log files, analysis of EPID image headers) were investigated. The methods used in this study are generally applicable to any linear accelerator unit, and have been demonstrated here with Clinac/Trilogy systems. Gantry angle data were simultaneously acquired using three methods: i) a direct gantry potentiometer measurement, ii) an incremental rotary encoder, and iii) a custom-made radiographic gantry-angle phantom which produced unique wire intersections as a function of gantry angle. All methods were compared to gantry angle data from the EPID image header and the linac MLC DynaLog file. The encoder and gantry-angle phantom were used to validate the accuracy of the linac's potentiometer. The EPID image header gantry angles and the DynaLog file gantry angles were compared to the potentiometer. The encoder and gantry-angle phantom mean angle differences with the potentiometer were 0.13° ± 0.14° and 0.10°± 0.30°, respectively. The EPID image header angles analyzed in this study were within ± 1° of the potentiometer angles only 35% of the time. In some cases, EPID image header gantry angles disagreed by as much as 3° with the potentiometer. A time delay in frame acquisition was determined using the continuous acquisition mode of the EPID. After correcting for this time delay, 75% of the header angles, on average, were within ± 1° of the true gantry angle, compared to an average of only 35% without the correction. Applying a boxcar smoothing filter to the corrected gantry angles further improved the accuracy of the header-derived gantry angles to within ± 1° for almost all images (99.4%). An angle accuracy of 0.11° ± 0.04° was determined using a point-by-point comparison of the gantry angle data in the MLC DynaLog file and the potentiometer data. These simple correction methods can be easily applied to individual treatment EPID images in order to more accurately define the gantry angle.

Purpose: The aim of this work is to characterize a new online dose verification device, COMPASS transmission detector array (IBA Dosimetry, Schwarzenbruck, Germany). The array is composed of 1600 cylindrical ionization chambers of 3.8 mm diameter, separated by 6.5 mm center-to-center spacing, in a 40 × 40 arrangement., Methods: The line spread function (LSF) of a single ion chamber in the detector was measured with a narrow slit collimator for a 6 MV photon beam. The 0.25 × 10 mm(2) slit was formed by two machined lead blocks. The LSF was obtained by laterally translating the detector in 0.25 mm steps underneath the slit over a range of 24 mm and taking a measurement at each step. This measurement was validated with Monte Carlo simulation using BEAMnrc and DOSXYZnrc. The presampling modulation transfer function (MTF), the Fourier transform of the line spread function, was determined and compared to calculated (Monte Carlo and analytical) MTFs. Two head-and-neck intensity modulated radiation therapy (IMRT) fields were measured using the device and were used to validate the LSF measurement. These fields were simulated with the BEAMnrc Monte Carlo model, and the Monte Carlo generated incident fluence was convolved with the 2D detector response function (derived from the measured LSF) to obtain calculated dose. The measured and calculated dose distributions were then quantitatively compared using χ-comparison criteria of 3% dose difference and 3 mm distance-to-agreement for in-field points (defined as those above the 10% maximum dose threshold)., Results: The full width at half-maximum (FWHM) of the measured detector response for a single chamber is 4.3 mm, which is comparable to the chamber diameter of 3.8 mm. The pre-sampling MTF was calculated, and the resolution of one chamber was estimated as 0.25 lp∕mm from the first zero crossing. For both examined IMRT fields, the χ-comparison between measured and calculated data show good agreement with 95.1% and 96.3% of in-field points below χ of 1.0 for fields 1 and 2, respectively (with an average χ of 0.29 for IMRT field 1 and 0.24 for IMRT field 2)., Conclusions: The LSF for a new novel online detector has been measured at 6 MV using a narrow slit technique, and this measurement has been validated by Monte Carlo simulation. The detector response function derived from line spread function has been applied to recover measured IMRT fields. The results have shown that the device measures IMRT fields accurately within acceptable tolerance.

Purpose: The purpose of this work was to develop a quality assurance (QA) tool for high-dose-rate (HDR) brachytherapy that would quickly and easily verify both source positioning (dwell positions) and durations (dwell times)., Methods: The authors constructed a QA tool that combined radiochromic film to verify position with four photodiode detectors to verify dwell times. To characterize the temporal accuracy of the tool, a function generator powered four red light-emitting diodes that were optically coupled to the four photodiode detectors. The QA tool was used to verify the dwell positions and times of a commercial brachytherapy afterloader. Measurements of dwell time were independently verified by a one-dimensional optical camera that acquired 1000 lines/s., Results: The temporal accuracy of the QA tool was found to be about 1 ms. For visual assessment, the source position could be located within about 0.5 mm. Evaluating the accuracy and precision of an HDR brachytherapy afterloader, the authors found that the bias in dwell time can exceed 60 ms and the dwell time associated with the first dwell position had an unexpectedly large standard deviation of 30 ms. They found that the source locations were much easier to locate on the film if a plastic catheter was used instead of a metal treatment tube. Scanning the films enabled the dwell positions to be determined within about 0.2 mm., Conclusions: For pretreatment QA, the authors found that this tool allowed verification of dwell positions and dwell times in about 6 min.

We investigated the physical characteristics of two complementary metal oxide semiconductor (CMOS) mammography detectors. The detectors featured 14-bit image acquisition, 50 microm detector element (del) size and an active area of 5 cm x 5 cm. One detector was a passive-pixel sensor (PPS) with signal amplification performed by an array of amplifiers connected to dels via data lines. The other detector was an active-pixel sensor (APS) with signal amplification performed at each del. Passive-pixel designs have higher read noise due to data line capacitance, and the APS represents an attempt to improve the noise performance of this technology. We evaluated the detectors' resolution by measuring the modulation transfer function (MTF) using a tilted edge. We measured the noise power spectra (NPS) and detective quantum efficiencies (DQE) using mammographic beam conditions specified by the IEC 62220-1-2 standard. Our measurements showed the APS to have much higher gain, slightly higher MTF, and higher NPS. The MTF of both sensors approached 10% near the Nyquist limit. DQE values near dc frequency were in the range of 55-67%, with the APS sensor DQE lower than the PPS DQE for all frequencies. Our results show that lower read noise specifications in this case do not translate into gains in the imaging performance of the sensor. We postulate that the lower fill factor of the APS is a possible cause for this result.

We describe the components of a bench-top megavoltage computed tomography (MVCT) scanner that uses an 80-element detector array consisting of CdWO4 scintillators coupled to photodiodes. Each CdWO4 crystal is 2.75 x 8 x 10 mm3. The detailed design of the detector array, timing control, and multiplexer are presented. The detectors show a linear response to dose (dose rate was varied by changing the source to detector distance) with a correlation coefficient (R2) nearly unity with the standard deviation of signal at each dose being less than 0.25%. The attenuation of a 6 MV beam by solid water measured by this detector array indicates a small, yet significant spectral hardening that needs to be corrected before image reconstruction. The presampled modulation transfer function is strongly affected by the detector's large pitch and a large improvement can be obtained by reducing the detector pitch. The measured detective quantum efficiency at zero spatial frequency is 18.8% for 6 MV photons which will reduce the dose to the patient in MVCT applications. The detector shows a less than a 2% reduction in response for a dose of 24.5 Gy accumulated in 2 h; however, the lost response is recovered on the following day. A complete recovery can be assumed within the experimental uncertainty (standard deviation <0.5%); however, any smaller permanent damage could not be assessed.

In this paper we describe techniques for measuring the dimensions and position reproducibility of an 192Ir brachytherapy stepping source. Measurements were carried out using a 0.25x10x152 mm3 collimator placed in front of a detector of our own design. The brachytherapy source was translated past the collimator in 0.025 mm increments using a stepper motor. The source was found to be 3.58 mm long and 0.69 mm wide, which is in good agreement with the manufacturer's values of 3.5x0.6 mm2. The source position was reproducible to within 0.12 mm.

We assessed the usefulness of an animal model, the axolotl (Ambystoma mexicanum), in comparing 3-D ultrasound (3-D US) and plain film radiographs. Hindlimbs were amputated from 5 animals, at either the zeugopodial or stylopodial level, and each regenerating limb was imaged 16 times with 3-D US and 14 times with plain film X ray over 315 days. US images were acquired with a Siemens Sonoline Versa Pro and a 10-MHz linear array transducer. For 3-D US images, the probe was translated in a motor-driven linear stage while images were digitized. The regenerating tibia and fibula bones were detected on 3-D US an average of 37 days earlier than on plain film radiography, and regenerating phalangeal bones were detected on 3-D US an average of 18 days earlier. After 120 days, both imaging modalities consistently showed the bones. The average bone growth rates for the tibia and fibula were 0.019 +/- 0.001 mm/day and 0.019 +/- 0.001 mm/day, respectively.

An adaptive real-time Doppler peak-frequency tracing algorithm was evaluated in vitro and compared to manual peak-frequency traces. A computer-controlled pump was used to generate physiological flow waveforms in a vasculature-mimicking phantom. Spectral waveforms were obtained on an ATL HDI along with real-time estimates of diagnostic parameters, including maximum systolic, minimum diastolic, time-averaged peak frequencies and pulsatility and resistance indices. The effect of the signal-to-noise ratio on the measured parameters was investigated. The imprecision in the measured parameters was found to depend somewhat on the waveform shape; e.g., the imprecision in PI was 4.1% for a normal renal waveform and 8.5% for a waveform having reverse diastolic flow. The peak frequency envelopes of the same waveform data were traced manually by nine operators, and the resulting diagnostic parameters were compared to ones obtained from automated peak-frequency traces of the same waveform data. The agreement between parameters measured by the automated routine and those measured manually was found to depend somewhat on the waveform shape; e.g., the bias in the PI was 1.3% for a renal waveform lacking diastolic flow, and 12% for a waveform with reverse diastolic flow. The between-observer variations in the manual measurements ranged from 0.8% up to 9.4%. The overall variations associated with the automated traces were found to be smaller than or equal to those of the manual traces.

We describe two variations of a phantom designed to evaluate the wall filters implemented on colour and spectral Doppler instruments. Both variations use an acoustic beam splitter to place the same Doppler sample volume within a motor-driven clutter belt and a flow source, which is either a second belt (dual-belt phantom) or a vascular phantom (belt/vascular phantom). We used the dual-belt phantom to evaluate the effects of the clutter belt velocity, flow belt velocity and clutter-to-flow power ratio on the reported colour Doppler shifts. The results show that the choice of wall filter, as well as the amplitudes and velocities of the clutter and flow components, affect the measured Doppler shifts. Results obtained with the belt/vascular phantom show that colour Doppler shifts due to the moving fluid depend strongly on the clutter velocity and choice of wall filter. However, only a small dependence on Doppler signal strength was observed.

A quantitative in vitro flow study was performed by using a three-dimensional colour Doppler imaging system. This system was based on a clinical ultrasound instrument with its transducer mounted on a motor-driven translation stage. A vascular and tissue-mimicking phantom containing two wall-less vessels, one normal and another stenotic, was used to quantify the measurement accuracy of the flow velocity and the flow field. Steady state flows, having Reynolds numbers ranging between 460 and 1300, were generated by a computer-controlled positive displacement pump. Effects of the parameter settings of the ultrasound instrument on results of the estimation of flow field were also studied. Experimental results show that our three-dimensional colour Doppler system's velocity accuracy was better than 7% of the Nyquist velocity and its spatial accuracy was better than 0.5 mm. The system showed a good correlation (r = 0.999) between the estimated and the true mean flow velocity, and a good correlation (r = 0.998) between the estimated maximum and the true mean flow velocity. This study is our first step toward validating the measurement of the three-dimensional velocity and wall shear stress distributions by using three-dimensional colour Doppler ultrasound

Doppler ultrasound flow measurement techniques are often validated using phantoms that simulate the vasculature, surrounding tissue and blood. Many researchers use rubber tubing to mimic blood vessels because of the realistic acoustic impedance, robust physical properties and wide range of available sizes. However, rubber tubing has a very high acoustic attenuation, which may introduce artefacts into the Doppler measurements. We describe the construction of a wall-less vessel phantom that eliminates the highly attenuating wall and reduces impedance mismatches between the vessel lumen and tissue mimic. An agar-based tissue mimic and a blood mimic are described and their acoustic attenuation coefficients and velocities are characterised. The high attenuation of the latex rubber tubing resulted in pronounced shadowing in B-mode images; however, an image of a wall-less vessel phantom did not show any shadowing. We show that the effects of the highly attenuating latex rubber vessels on Doppler amplitude spectra depend on the vessel diameter and ultrasound beam width. In this study, only small differences were observed in spectra obtained from 0.6 cm inside diameter thin-wall latex, thick-wall latex and wall-less vessel phantoms. However, a computer model predicted that the spectrum obtained from a 0.3-cm inside diameter latex-wall vessel would be significantly different than the spectrum obtained from a wall-less vessel phantom, thus resulting in an overestimation of the average fluid velocity. These results suggest that care must be taken to ensure that the Doppler measurements are not distorted by the highly attenuating wall material. In addition, the results show that a wall-less vessel phantom is preferable when measuring flow in small vessels.

A technique for producing accurate models of vascular segments for use in experiments that assess vessel geometry and flow has been developed and evaluated. The models are compatible with x-ray, ultrasound, and magnetic resonance (MR) imaging systems. In this paper, a model of the human carotid artery bifurcation, is evaluated that has been built using this technique. The phantom consists of a thin-walled polyester-resin replica of the bifurcation through which a blood-mimicking fluid may be circulated. The phantom is surrounded by an agar tissue-mimicking material and a series of fiducial markers. The blood- and tissue-mimicking materials have x-ray, ultrasound, and MR properties similar to blood and tissue; fiducial markers provide a means of aligning images acquired by different modalities. The root-mean-square difference between the inner wall geometry of the constructed model and the desired dimensions was 0.33 mm. Static images were successfully acquired using x-ray, ultrasound, and MR imaging systems, and are free of significant artifacts. Flow images acquired with ultrasound and MR agree qualitatively with each other, and with previously published flow patterns. Volume-flow measurements obtained with ultrasound and MR were within 4.4% of the actual values.

We have developed a system to acquire in vivo three-dimensional (3D) colour velocity images of peripheral vasculature. A clinical ultrasound system was modified by mounting the transducer on a motor-driven translation stage, allowing planar ultrasound images to be acquired along a 37 mm long stroke. A 3D velocity image is acquired by digitizing, in synchrony with the cardiac cycle, successive video images as the transducer is moved over the skin surface. 3D images require about 1 min to acquire and 10 min to reconstruct before being viewed interactively. Image acquisition at several points in the cardiac cycle permits a cine-type reconstructed image. Geometrical, temporal and velocity accuracy of the acquisition and reconstruction have been quantified and found not to degrade the image.

A novel computer-controlled flow simulator for use in magnetic resonance (MR) flow experiments was evaluated. The accuracy in constant-flow mode was better than 1%. The accuracy in pulsatile-flow mode was found to be dependent on the interconnecting tubing. The short-term and long-term reproducibilities of pulsatile waveforms were less than or equal to 0.4 mL/sec (1 standard deviation). Increased response times due to the lengths of tubing required in MR flow experiments were surmounted by using a modified tubing configuration and precompensated waveforms. Piston reversal was found not to cause major difficulties in MR flow experiments.

We describe a phantom designed to evaluate the velocity measurements made with colour and pulsed Doppler instruments. Using a belt to translate a large volume of semi-rigid material through the entire Doppler sample volume eliminates many of the problems associated with flow and string phantoms. A servo-motor with feedback circuitry ensures accurate control of the belt velocity with an uncertainty in the mean velocity of 0.14%. The phantom provides velocities with typical variations of 0.07 cm/s. We have demonstrated the usefulness of this phantom by evaluating the linearity and accuracy of three pulsed Doppler instruments over belt velocities ranging from 0 to 80 cm/s. In addition, the measurements show the effects of the wall filter at low belt velocities. Using this phantom, we have quantified the accuracy, linearity and precision of the velocity measurements made by three colour Doppler instruments. The results also show regions where the colour instruments are aliased and where the wall filter dominates.

A computer-controlled pump for use both in the study of vascular haemodynamics and in the calibration of clinical devices which measure blood flow is designed. The novel design of this pump incorporates two rack-mounted pistons, driven into opposing cylinders by a micro-stepping motor. This approach allows the production of nearly uninterrupted steady flow, as well as a variety of pulsatile waveforms, including waveforms with reverse flow. The capabilities of this pump to produce steady flow from 0.1 to 60 ml s-1, as well as sinusoidal flow and physiological flow, such as that found in the common femoral and common carotid arteries are demonstrated. Cycle-to-cycle reproducibility is very good, with an average variation of 0.1 ml s-1 over thousands of cycles.