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Biomedical subjects

M S Conradi

Publications and source records attributed to M S Conradi.

8 recordsLinked to original sources
PubMed2000

Dynamic echo planar MR imaging of lung ventilation with hyperpolarized (3)He in normal subjects and patients with severe emphysema.

We applied the rapid imaging capability of echo planar MR pulse sequences and hyperpolarized (3)He ventilation imaging to observe the dynamic distribution of gas in the lungs during breathing. Findings in five normal volunteers (age 19-53 years) and four patients with severe smoking-related emphysema (age 56-71 years) were compared. All studies were performed on a 1.5 T whole body scanner using a 30 cm Helmholtz surface coil and 0.5 l of 20-40% polarized (3)He mixed with 1-2 l nitrogen. Our echo planar imaging pulse sequence allowed acquisition of each image in 0.04 s, with a pixel size of 7 mm(2) (TR = 40.5 ms, TE = 12.1 ms, flip angle = 22 degrees, echo train length = 32, matrix = 32 x 64, field of view = 225 x 450 mm, slice thickness = 10 mm). Imaging was performed in the transaxial plane repeatedly at 3, 10 or 20 evenly spaced levels, immediately before and during breathing of the gas mixture. In normal subjects during the first breath, (3)He appeared throughout each slice first in the mid lungs, then in the lower lungs, then in the upper lungs, with slightly greater signal in the dependent posterior regions. In patients with emphysema, sequential filling of different lung regions was seen during the first breath, with delayed filling of other regions observed during rebreathing and room air washout. We conclude that subsecond dynamic (3)He MR ventilation imaging can reveal normal and abnormal ventilation phenomena not seen with conventional scintigraphic methods, and offers another approach to the study of ventilation physiology and pathophysiology.

Adult↗
PubMed2000

MR imaging of diffusion of (3)He gas in healthy and diseased lungs.

Hyperpolarized (3)He gas MRI was used to form maps of the effective diffusivity of gas in human lungs. Images of diffusion as well as spin density are presented from a study of 11 healthy volunteers and 5 patients with severe emphysema. The effective rate of diffusion, D(e), of the gas is reduced by the alveolar walls; tissue destruction in emphysema is hypothesized to result in larger D(e). Indeed, the mean value of D(e) in the emphysematous lungs is found here to be about 2.5 times that of healthy lungs, although still smaller than the unrestricted diffusivity of (3)He in free air. Histograms of D(e) values across coronal slices are presented. The results are discussed in terms of spatial variations, variations among individuals, healthy and diseased, and variations due to changes in lung volume. Magn Reson Med 44:174-179, 2000.

Adult↗
PubMed2000

Remote tuning of NMR probe circuits.

There are many circumstances in which the probe tuning adjustments cannot be located near the rf NMR coil. These may occur in high-temperature NMR, low-temperature NMR, and in the use of magnets with small diameter access bores. We address here circuitry for connecting a fixed-tuned probe circuit by a transmission line to a remotely located tuning network. In particular, the bandwidth over which the probe may be remotely tuned while keeping the losses in the transmission line acceptably low is considered. The results show that for all resonant circuit geometries (series, parallel, series-parallel), overcoupling of the line to the tuned circuit is key to obtaining a large tuning bandwidth. At equivalent extents of overcoupling, all resonant circuit geometries have nearly equal remote tuning bandwidths. Particularly for the case of low-loss transmission line, the tuning bandwidth can be many times the tuned circuit's bandwidth, f(o)/Q.

Calibration↗
PubMed1999

Rapid imaging of hyperpolarized gas using EPI.

Rapid repetitive MRI of hyperpolarized (HP) gases using echo-planar imaging (EPI) has been theoretically investigated and experimentally demonstrated for (3)He in human lung. A quantitative treatment of signal attenuation and magnetization consumption for the unique circumstance of a rapidly diffusing nonrenewable magnetization source has been performed. Rapid (compared to the human respiratory cycle) and repetitive imaging of the lung gas space with EPI and a single delivered bolus of HP-(3)He is feasible using low flip angles, provided the voxels are not too small. A coarse-grid (32 x 64) EPI pulse sequence has been developed and implemented to image the lungs of healthy volunteers during rebreathing of a HP-(3)He/N(2) gas mixture. A set of three 10-mm axial slices was imaged every 0.12 sec for the 36 sec duration of rebreathing, yielding a real-time visualization of ventilation. Despite some mild artifacts, the images are of good quality and show changes in gas density related to respiratory physiology. Magn Reson Med 42:507-514, 1999.

Echo-Planar Imaging↗
PubMed1999

What are the conditions for exponential time-cubed echo decays?

Diffusion of precessing spins through a constant field gradient is well-known to produce two distinctive features: an exp(-bt(3)) decay of the echo amplitude in response to two pulses and a much slower decay of the Carr-Purcell echo train. These features will appear whenever the spin frequency is described by a continuous random-walk. The present work shows that this may also occur in the presence of motions with long correlation times tau(c)-continuous Gaussian frequency noise with an exponential autocorrelation has the correct properties over time durations smaller than tau(c). Thus, time-cubed echo decays will occur in situations other than physical diffusion. The decay rate of the Carr-Purcell echo train is shown to vary with the pulse spacing tau whenever the correlation time tau(c) is long; the slower Carr-Purcell decay compared to the two-pulse echo decay is not unique to diffusion. Simulations are presented that display time-cubed decays. The simulations confirm two important criteria: the echo time must be less than tau(c) and the frequency noise must consist of nearly continuous variations, as opposed to step-like changes. These criteria define the range of physical parameters for which time-cubed decays will be observable.

Computer Simulation↗
PubMed1998

Low frequency NMR polarimeter for hyperpolarized gases.

An inexpensive and self-contained apparatus for pulsed NMR at 30-250 kHz is described. The intended application is monitoring of the spin polarization of rare gas nuclei in a laser-polarizing apparatus in fields of order 30 G. In addition, the device provides a convenient method for following the polarization decay during storage and transport. Some of the features are a flexible pulse generator, splitting of transmitter RF cycles by the RF gate, a Q switch, and a wide range of receiver gains.

Equipment Design↗
PubMed1987

Flow velocity measurement with ac gradients.

Two NMR experiments are described that use ac field gradients to measure flow velocities. Both experiments perform no spacial localization; instead, fluid velocity is converted to a resonance frequency shift. Thus, by Fourier transformation of a single FID or echo train the distribution of velocities is available. The first experiment uses only one rf pulse but requires a homogeneous static field. The second experiment uses the CPMG multiple-pulse sequence to refocus the effects of field inhomogeneity and chemical shifts. Results are shown for both experiments using a simple flowing phantom. Applications of the techniques to time-resolved blood flow measurement are discussed.

Blood Flow Velocity↗