A. O. Rodriguez

Cardiovascular magnetic resonance evaluation of aortic stenosis severity using single plane measurement of effective orifice area

BackgroundTransthoracic echocardiography (TTE) is the standard method for the evaluation of the severity of aortic stenosis (AS). Valve effective orifice area (EOA) measured by the continuity equation is one of the most frequently used stenotic indices. However, TTE measurement of aortic valve EOA is not feasible or not reliable in a significant proportion of patients. Cardiovascular magnetic resonance (CMR) has emerged as a non-invasive alternative to evaluate EOA using velocity measurements. The objectives of this study were: 1) to validate a new CMR method using jet shear layer detection (JSLD) based on acoustical source term (AST) concept to estimate the valve EOA; 2) to introduce a simplified JSLD method not requiring vorticity field derivation.Methods and resultsWe performed an in vitro study where EOA was measured by CMR in 4 fixed stenoses (EOA = 0.48, 1.00, 1.38 and 2.11 cm2) under the same steady flow conditions (4-20 L/min). The in vivo study included eight (8) healthy subjects and 37 patients with mild to severe AS (0.72 cm2 ≤ EOA ≤ 1.71 cm2). All subjects underwent TTE and CMR examinations. EOA was determinated by TTE with the use of continuity equation method (TTECONT). For CMR estimation of EOA, we used 3 methods: 1) Continuity equation (CMRCONT); 2) Shear layer detection (CMRJSLD), which was computed from the velocity field of a single CMR velocity profile at the peak systolic phase; 3) Single plane velocity truncation (CMRSPVT), which is a simplified version of CMRJSLD method. There was a good agreement between the EOAs obtained in vitro by the different CMR methods and the EOA predicted from the potential flow theory. In the in vivo study, there was good correlation and concordance between the EOA measured by the TTECONT method versus those measured by each of the CMR methods: CMRCONT (r = 0.88), CMRJSLD (r = 0.93) and CMRSPVT (r = 0.93). The intra- and inter- observer variability of EOA measurements was 5 ± 5% and 9 ± 5% for TTECONT, 2 ± 1% and 7 ± 5% for CMRCONT, 7 ± 5% and 8 ± 7% for CMRJSLD, 1 ± 2% and 3 ± 2% for CMRSPVT. When repeating image acquisition, reproducibility of measurements was 10 ± 8% and 12 ± 5% for TTECONT, 9 ± 9% and 8 ± 8% for CMRCONT, 6 ± 5% and 7 ± 4% for CMRJSLD and 3 ± 2% and 2 ± 2% for CMRSPVT.ConclusionThere was an excellent agreement between the EOA estimated by the CMRJSLD or CMRSPVT methods and: 1) the theoretical EOA in vitro, and 2) the TTECONT EOA in vivo. The CMRSPVT method was superior to the TTE and other CMR methods in terms of measurement variability. The novel CMR-based methods proposed in this study may be helpful to corroborate stenosis severity in patients for whom Doppler-echocardiography exam is inconclusive.

Game Motivated and Constraint Induced Therapy in Late Stroke with fMRI Studies Pre and Post Therapy

20 patients with stroke more than one year earlier were evaluated, admitted to a novel therapy including constraint-induced and computer game-motivated therapy. Statistically significant improvements after 4 weeks of late therapy were seen in all 20 patients on nine out of eleven quantified clinical evaluation scales. The patients looked forward to and enjoyed the therapy. These same late stroke patients were studied via fMRI BOLD immediately before therapy and post therapy. fMRI BOLD studies confirm brain functional reorganization; 3 of the 20 fMRI cases are presented here. We propose that fMRI can help in the process of designing effective stroke therapy programs based on biological principles of brain plasticity

Embedded NMR Sensor to Monitor Compressive Strength Development and Pore Size Distribution in Hydrating Concrete.

In cement-based materials porosity plays an important role in determining their mechanical and transport properties. This paper describes an improved low–cost embeddable miniature NMR sensor capable of non-destructively measuring evaporable water loss and porosity refinement in low and high water-to-cement ratio cement-based materials. The sensor consists of two NdFeB magnets having their North and South poles facing each other, separated by 7 mm to allow space for a Faraday cage containing a Teflon tube and an ellipsoidal RF coil. To account for magnetic field changes due to temperature variations, and/or the presence of steel rebars, or frequency variation due to sample impedance, an external tuning circuit was employed. The sensor performance was evaluated by analyzing the transverse magnetization decay obtained with a CPMG measurement from different materials, such as a polymer phantom, fresh white and grey cement pastes with different w/c ratios and concrete with low (0.30) and high (0.6) w/c ratios. The results indicated that the sensor is capable of detecting changes in water content in fresh cement pastes and porosity refinement caused by cement hydration in hardened materials, even if they are prepared with a low w/c ratio (w/c = 0.30). The short lifetime component of the transverse relaxation rate is directly proportional to the compressive strength of concrete determined by destructive testing. The r2 (0.97) from the linear relationship observed is similar to that obtained using T2 data from a commercial Oxford Instruments 12.9 MHz spectrometer.

A multi-slot surface coil for MRI of dual-rat imaging at 4 T

A slotted surface coil inspired by the hole-and-slot cavity magnetron was developed for magnetic resonance imaging of obese rats at 4 T. Full-wave analysis of the magnetic field was carried out at 170 MHz for both the slotted and circular-shaped coils. The noise figure values of two coils were investigated via the numerical calculation of the quality factors. Fat simulated phantoms to mimic overweight rats were included in the analysis with weights ranging from 300 to 900 g. The noise figures were 1.2 dB for the slotted coil and 2.4 dB for the circular coil when loaded with 600 g of simulated phantom. A slotted surface coil with eight circular slots and a circular coil with similar dimensions were built and operated in the transceiver mode, and their performances were experimentally compared. The imaging tests in phantoms demonstrated that the slotted surface coil has a deeper RF-sensitivity and better field uniformity than the single-loop RF-coil. High quality images of two overweight Zucker rats were acquired simultaneously with the slotted surface coil using standard spin-echo pulse sequences. Experimental results showed that the slotted surface coil outperformed the circular coil for imaging considerably overweight rats. Thus, the slotted surface coil can be a good tool for MRI experiments in rats on a human whole-body 4 T scanner.

A new index measured by cardiovascular magnetic resonance imaging to detect mechanical heart valve malfunction

More than two thirds of valve replacement operations performed each year used mechanical heart valve. These valves are subject to complications such: pannus and/or thrombus formation. One other potential complication is a malfunction in one of the valve leaflets. It is then important to develop parameters that will allow a non-invasive diagnosis of such valve malfunction. In the present study, we evaluated under steady low flow (1–8 L/min) and pulsatile flow (3, 5 and 7 L/min) a bileafleat mechanical heart valve with normal function, 50% and 100% of one valve leaflet malfunction. Image analysis was performed using cardiovascular magnetic resonance imaging to evaluate transvalvular pressure gradients (TPG), effective orifice area and a new index given by central/lateral velocity ratio downstream of the valve. Our results showed that the flow upstream and downstream of the defective valve is highly influenced by malfunction severity. TPG did not allow differentiating valve malfunction at low flow under steady and pulsatile conditions. However the new index given by central/lateral ratio allowed differentiating the presence of valve malfunction using a single transverse velocity measurement.

Year 2000 status of MRI in Mexico

The development of clinical magnetic resonance imaging in Mexico has followed a different course from that in the U.S. and Europe. The first clinical unit was installed in Mexico in the late 1980s at the very beginning of clinical applications worldwide. Since then, installations have proceeded at a sedate pace that now brings the installed total to 66 imagers. The largest fraction of these units (28, or 42%) is in Mexico City, with a broad distribution across the remaining 23 Mexican states. There is a noticeable increase of the number of units (20, or 30%) in the states bordering the U.S., while the states bordering nations to the south have no units. More than half the units (38, or 58%) are 0.5 T units, while a further 35% are 1.0 T or higher. Slower addition of units in Mexico relative to the U.S. is attributed to the higher fraction of public‐funded hospitals and the inherent conservatism of such institutions when considering new technologies. Present public planning for health care development suggests that the coming decade will see more rapid installation of units to meet growing demand in Mexico for the latest medical technology. Experience over the past two decades indicates the need for more systematic training of technical and clinical personnel to implement these additions. The National University (Universidad Nacional Autónoma de México (UNAM) and the Metropolitan University (Universidad Autónoma Metropolitana (UAM) are collaborating with diverse clinical facilities to create such a program. J. Magn. Reson. Imaging 2001;13:813–817.

Slotted cage resonator for high-field magnetic resonance imaging of rodents

A variation of the high-frequency cavity resonator coil was experimentally developed according to the theoretical frame proposed by Mansfield in 1990. Circular slots were used instead of cavities to form the coil endplates and it was called the slotted cage resonator coil. The theoretical principles were validated via a coil equivalent circuit and also experimentally with a coil prototype. The radio frequency magnetic field, B1, produced by several coil configurations was numerically simulated using the finite-element approach to investigate their performances. A transceiver coil, 8 cm long and 7.6 cm in diameter, and composed of 4 circular slots with a 15 mm diameter on both endplates, was built to operate at 300 MHz and quadrature driven. Experimental results obtained with the slotted cage resonator coil were presented and showed very good agreement with the theoretical expectations for the resonant frequency as a function of the coil dimensions and slots. A standard birdcage coil was also built for performance comparison purposes. Phantom images were then acquired to compute the signal-to-noise ratio of both coils showing an important improvement of the slotted cage coil over the birdcage coil. The whole-body images of the mouse were also obtained showing high-quality images. Volume resonator coils can be reliably built following the physical principles of the cavity resonator design for high-field magnetic resonance imaging applications of rodents.

Improved SNR of phased-array PERES coils via simulation study

A computational comparison of signal-to-noise ratio (SNR) was performed between a conventional phased array of two circular-shaped coils and a petal resonator surface array. The quasi-static model and phased-array optimum SNR were combined to derive an SNR formula for each array. Analysis of mutual inductance between coil petals was carried out to compute the optimal coil separation and optimum number of petal coils. Mutual interaction between coil arrays was not included in the model because this does not drastically affect coil performance. Phased arrays of PERES coils show a 114% improvement in SNR over that of the simplest circular configuration.

Quantification of Vena Cava Blood Flow with Half Fourier Echo-Planar Imaging

BACKGROUND Human hemodynamics occurs in very short periods of time. To quantify blood flow under these circumstances, a fast-scan imaging technique is required. Echo-planar imaging can be a good candidate because it is able to acquire images in <50-100 msec. An imaging scheme with these properties can produce real-time images as well as overcome motion artifacts such as blurring and ghosting, which alter image quality. Additionally, echo-planar imaging does not require a calibration protocol to perform flow experiments in the human cardiovascular system. Consequently, echo-planar imaging appears to be the best imaging tool available to quantify blood flow in the vena cava. From a clinical point of view, echo-planar imaging has become a widespread commodity to produce magnetic resonance images in real-time. METHODS Flow-encoded half Fourier echo-planar imaging is proposed to determine blood flow in the arteries. This flow sequence was used to investigate vena cava blood flow in healthy volunteers and compared with other diagnostic imaging modalities. RESULTS Two-dimensional flow maps were obtained by using the two components (sine and cosine images) resulting from the flow-encoded echo-planar imaging sequence. Velocity profiles of vena cava of two healthy volunteers were calculated from the previous bidimensional blood flow maps. CONCLUSIONS We proved that real-time flow imaging of the cardiovascular system can be achieved with flow-encoded echo-planar imaging and a partial Fourier method. It is possible to quantify blood flow in the superior vena cava in humans. We believe that this imaging tool might offer relevant anatomic and physiologic information of the vena cava as well as of the cardiovascular system.

Numerical analysis of the concentric ring number for electric field sensing with a Split-Ring Resonator metamaterial

Wireless communications have experienced a tremendous development during the last decade. The interaction of the electromagnetic waves as a function of the frequency and intensity with physical objects remains largely unknown. This poses the challenge to study the physical mechanisms involved to actually quantify the impact of the interaction with other objects caused by various electromagnetic waves-emitting sources. Here, we proposed the design of a plane SRR (Split-Ring Resonator) with metamaterial circular rings for S-Band frequency range. To guide the construction of the SRR prototype, simulations of the bandwidth were conducted using the Finite Integral technique. A logarithmic function describes the dependence of the simulated resonant frequency. The maximum simulated bandwidth of the SRR can be reached with an even number of concentric rings.