Mostafa Shakeri

In vitro validation of flow measurement with phase contrast MRI at 3 tesla using stereoscopic particle image velocimetry and stereoscopic particle image velocimetry-based computational fluid dynamics.

To validate conventional phase‐contrast MRI (PC‐MRI) measurements of steady and pulsatile flows through stenotic phantoms with various degrees of narrowing at Reynolds numbers mimicking flows in the human iliac artery using stereoscopic particle image velocimetry (SPIV) as gold standard.

Using PIV to determine relative pressures in a stenotic phantom under steady flow based on the Pressure-Poisson equation

Pressure gradient across a Gaussian-shaped 87% area stenosis phantom was estimated by solving the pressure Poisson equation (PPE) for a steady flow mimicking the blood flow through the human iliac artery. The velocity field needed to solve the pressure equation was obtained using particle image velocimetry (PIV). A steady flow rate of 46.9 ml/s was used, which corresponds to a Reynolds number of 188 and 595 at the inlet and stenosis throat, respectively (in the range of mean Reynolds number encountered in-vivo). In addition, computational fluid dynamics (CFD) simulation of the same flow was performed. Pressure drops across the stenosis predicted by PPE/PIV and CFD were compared with those measured by a pressure catheter transducer. RMS errors relative to the measurements were 17% and 10% for PPE/PIV and CFD, respectively.

Accuracy of flow measurement with phase contrast MRI in a stenotic phantom: where should flow be measured?

Background Phase-contrast magnetic resonance imaging (PC-MRI) provides a powerful method for the quantification of blood velocity. Accuracy of flow measurement with PCMRI has been validated with several techniques such as Doppler ultrasound and electromagnetic flowmeters. However, these methods suffer from low accuracy, especially in pulsating flows where short response times are required. Methods Herein, a series of detailed experiments are reported for validation of MR measurements of steady and pulsatile flows with stereoscopic particle image velocimetry (SPIV) on three different stenotic models with 50%, 74%, and 87% area occlusions. Mean inlet Reynolds number was 190 for both steady and pulsatile cases, mimicking the flow of the human common iliac artery. Axial PC-MRI images were acquired at three sites: inlet (two diameters proximal to the stenosis), throat, and outlet (two diameters distal to the stenosis) using a 3T TX Achieva Philips MRI scanner with slice thickness = 4 mm, resolution = 1 × 1 mm, TE/TR = 3.0/4.0 ms, field of view =6 4 ×6 4 mm, and velocity encoding (Venc) = 30-200 cm/s depending on the imaging section. For SPIV purposes, a laser light sheet was passed perpendicular to the axis of the phantom to illuminate the flowing fluorescent particles (Fig 1). A set of image pairs were captured using two cameras looking at the phantom at different angles and the fluid velocity was extracted using a cross-correlation scheme, yielding a nominal spatial resolution of 0.2 mm for the velocity data. The temporal resolution of pulsatile flow measurements was 25 ms, corresponding to 40 measurements per second. Results Agreement between PC-MRI and SPIV was demonstrated for both steady and pulsatile flow measurements at the inlet by evaluating the linear regression between the two methods, which showed a correlation coefficient of >0.99 and >0.96 for steady and pulsatile flows, respectively. The difference between SPIV and PC-MRI measurements for steady and pulsatile mean flows was less than 5% for both inlet and throat and showed good agreement in all cases (Fig 2). The agreement, however, was weaker at the outlet especially for the 87% stenosis. The flow rate error distal to the stenosis was shown to be a function of narrowing severity. Conclusions Our experiments revealed that the most accurate measures of flow by PC-MRI are found at the throat of the stenosis. This study also illustrates that SPIV provides an excellent approach to in-vitro validation of new or existing PC-MRI flow measurement techniques. Funding

Optical imaging of steady flow in a phantom model of iliac artery stenosis: comparison of CFD simulations with PIV measurements

A flexible flow phantom system was designed and fabricated for the purpose of validation of i) CFD models proposed in conjunction with vascular imaging and ii) medical imaging techniques (such as MRI) that can produce flow velocities. In particular, one of the most challenging flows for both CFD models when modeling flow velocities and imaging techniques when measuring flow velocities are stenotic flows. Particle Image Velocimetry (PIV) is an optical technique for accurate measurement of in-vitro flow velocities and visualization of fluid flow. The fluid is seeded with tracer particles and the motion of the particles, illuminated with a laser light sheet, reveal particle velocities. Particle Image Velocimetry (PIV) was used to measure the flow fields across a Gaussian-shaped 90% area stenosis phantom. The flow parameters were adjusted to the phantom geometry to mimic the blood flow through the human common iliac artery. In addition, Computational Fluid Dynamics (CFD) simulation of the same flow was performed and the results were validated with those from PIV measurements. Steady flow rate of 46.9 ml/s was used, which corresponds to a Reynolds number of 188 and 595 at the inlet and stenosis throat, respectively. A maximum discrepancy of 15% in peak velocity was observed between the two techniques.

Efficiency of Solar Electricity Production With Long-Term Storage

Solar electric production systems with energy storage were simulated and compared, including an ammonia thermochemical cycle, compressed air energy storage (CAES), pumped hydroelectric energy storage (PHES), vanadium flow battery, and thermal energy storage (TES). All systems used the same parabolic concentrator to collect solar energy and Stirling engine to produce electricity. Efficiency and storage losses were modeled after existing experiments. At receiver and ammonia synthesis temperatures of 800 K, efficiencies of all systems except TES were initially similar at 17–19%, while TES provided ∼23%. Further, TES was most efficient for diurnal-scale storage. However, lower time-dependent storage losses caused the ammonia system to have the highest efficiency after one month of storage and to be increasingly favored as time of storage increased. Solar electric production with full capacity factor may be most efficient with a combination of systems including direct solar-electric production and systems with both diurnal and long-term storage.

A multimodal (MRI/ultrasound) cardiac phantom for imaging experiments

A dynamic cardiac phantom can play a significant role in the evaluation and development of ultrasound and cardiac magnetic resonance (MR) motion tracking and registration methods. A four chamber multimodal cardiac phantom has been designed and built to simulate normal and pathologic hearts with different degrees of “infarction” and “scar tissues”. In this set up, cardiac valves have been designed and modeled as well. The four-chamber structure can simulate the asymmetric ventricular, atrial and valve motions. Poly Vinyl Alcohol (PVA) is used as the principal material since it can simulate the shape, elasticity, and MR and ultrasound properties of the heart. The cardiac shape is simulated using a four-chamber mold made of polymer clay. An additional pathologic heart phantom containing stiff inclusions has been manufactured in order to simulate an infracted heart. The stiff inclusions are of different shapes and different degrees of elasticity and are able to simulate abnormal cardiac segments. The cardiac elasticity is adjusted based on freeze-thaw cycles of the PVA cryogel for normal and scarred regions. Ultrasound and MRI markers were inserted in the cardiac phantom as landmarks for validations. To the best of our knowledge, this is the first multimodal phantom that models a dynamic four-chamber human heart including the cardiac valve.

Effect of V enc on accuracy of velocity profiles in multi-slice phase-contrast MR imaging of stenotic flow

Phase-contrast MRI (PC MRI) is a powerful technique for imaging the flow velocities in-vivo. In PC MRI, the velocity encoding parameter, Venc, plays a central role in fidelity of velocities acquired with this technique. Typically Venc is chosen to be the highest velocity in the flow. However, when acquiring velocity images at a number of slice locations, it may be advantageous to change the Venc as a function of position. The goal of this study was to determine the effect of Venc on the accuracy of the estimated velocity profiles in a range of locations in a phantom model of a 90% area stenosis. To this end, velocities from PC MRI were compared with velocities from computational fluid dynamics (CFD) simulations for identical geometry and flow conditions. Our results confirm that although using smaller Vencs leads to wrapping artifact, it provides more accurate velocity profiles. However, there is a lower limit to setting the Venc, beyond which phase-unwrapping becomes uncertain.

Pressure gradients calculated from PC-MRI, SPIV and CFD velocity data in a phantom model: comparison with catheter-based pressure measurement.

Background Peripheral arterial disease (PAD) is a common manifestation of atherosclerosis and is defined as any pathologic process causing obstruction to blood flow in the arteries outside the heart; mainly the arteries supplying the lower extremities. Phase-contrast MRI (PC-MRI) provides ap owerful and non-invasive method to acquire spatially registered blood velocity. The velocity field, then, can be used to derive other clinically useful hemodynamic parameters, such as blood pressure gradients. Methods Herein, pressure gradient across an axisymmetric Gaussian-shaped 87% area stenosis phantom was estimated by solving the pressure-Poisson equation (PPE). The velocity field needed to solve the pressure equation was obtained using PC-MRI and Stereoscopic Particle Image Velocimetry (SPIV). Steady flow rate of 46.9 ml/s, corresponding to an inlet Reynolds number of 160, was used which mimics the Reynolds number of human common iliac artery. Sagittal PC-MRI images were acquired using

Cine phase-contrast MRI measurement of CSF flow in the cervical spine: a pilot study in patients with spinal cord injury

MRI velocimetry (also known as phase-contrast MRI) is a powerful tool for quantification of cerebrospinal fluid (CSF) flow in various regions of the brain and craniospinal junction and has been accepted as a diagnostic tool to assist with the diagnosis of certain conditions such as hydrocephalus and chiari malformations. Cerebrospinal fluid is continually produced in the ventricles of the brain, flows through the ventricular system and then out and around the brain and spinal cord and is reabsorbed over the convexity of the brain. Any disease process which either impedes the normal pattern of flow or restricts the area where flow occurs can change the pattern of these waveforms with the direction and velocity of flow being determined by the pressure transmitted from the pulsation of the heart and circulation of blood within the central nervous system. Therefore, we hypothesized that phase-contrast MRI could eventually be used as a diagnostic aid in determining the degree of spinal cord compression following injury to the cervical or thoracic spine. In this study, we examined CSF flow in 3 normal subjects and 2 subjects with non-acute injuries in the cervical spine using Cine phasecontrast MRI. CSF flow analysis was performed using an in-house developed software. The flow waveform was calculated in both normal subjects (n=3) as well as subjects with spinal cord injury in the cervical spine (n=2). The bulk flow at C2 was measured to be 0.30 +/- 0.05 cc, at 5 cm distal to C2, it was 0.19+/- 0.07 cc, and at 10 cm distal to C2, it was 0.17+/- 0.05 cc. These results were in good agreement with previously published results. In patients with spinal cord injury, at the site of injury in the cervical spine, bulk flow was found to be 0.08 +/- 0.12 cc, at 5 cm proximal to the site of injury it was found to be 0.18 +/- 0.07 cc, and at 5 cm distal to the site of injury, it was found to be 0.12 +/- 0.01 cc.

Comparison of Energy Storage Methods for Solar Electric Production

Energy storage is key to expanding the capacity factor for electric power from solar energy. To accommodate variable weather patterns and electric demand, storage may be needed not just for diurnal cycles, but for variations as long as seasonal. Five solar electric systems with energy storage were simulated and compared, including an ammonia thermochemical energy storage cycle, compressed air energy storage (CAES), pumped hydroelectric energy storage (PHES), vanadium flow battery, and thermal energy storage (TES). To isolate the influence of the storage system, all systems used the same parabolic concentrator and Stirling engine. For CAES, PHES and battery, the engine directly produced electricity, which was then converted and stored. For TES, heat transfer fluid was heated by the dish and stored, and later used to drive the engine to produce electricity. For ammonia, the dish heated an ammonia dissociation reactor to produce nitrogen and hydrogen, which was stored. Heat was recovered to drive the engine by reforming ammonia from the stored gases. Each system was simulated in TRNSYS with weather data for Louisville, KY and Phoenix, AZ with subsystem efficiencies and storage losses estimated from previous experimental results. All systems including the ammonia cycle involved time dependent storage losses. Losses from the receiver included convection and emitted radiation, both of which depend on receiver temperature.Overall (solar-storage-electric) efficiency of the ammonia cycle depended strongly on synthesis reactor temperature, ranging from less than 1% to ∼18% for both Louisville, KY and Phoenix, AZ, at 500 K to 800 K, respectively. In contrast, the effect of dissociation reactor temperature was less. Overall (solar-electric-storage-electric) efficiencies of the CAES, systems in the limit of zero storage time ranged from ∼10% to ∼18% for solar receiver temperature of 500 K to 800 K. The vanadium flow battery and PHES efficiencies ranged from ∼9% to ∼17% for the same conditions. TES initially provided 12 to 23% efficiency over the same range of temperature. When time-dependent storage losses were included, however, efficiencies for all systems declined rapidly except the ammonia cycle in both locations and PHES in Louisville. The ammonia system had the highest efficiency after one month of storage, an advantage that increased with time of storage.The simulations showed that TES was most efficient for diurnal-scale storage and the ammonia cycle for longer storage. Full capacity factor for solar electric production may be most efficiently accomplished with a combination of direct solar-electric production and systems with both diurnal and long-term storage, the proportions of which depending on weather conditions and electric demand profiles.Copyright