Biomedical subjects

O M Mueller

Publications and source records attributed to O M Mueller.

5 recordsLinked to original sources

PubMed1990

The NMR phased array.

We describe methods for simultaneously acquiring and subsequently combining data from a multitude of closely positioned NMR receiving coils. The approach is conceptually similar to phased array radar and ultrasound and hence we call our techniques the "NMR phased array." The NMR phased array offers the signal-to-noise ratio (SNR) and resolution of a small surface coil over fields-of-view (FOV) normally associated with body imaging with no increase in imaging time. The NMR phased array can be applied to both imaging and spectroscopy for all pulse sequences. The problematic interactions among nearby surface coils is eliminated (a) by overlapping adjacent coils to give zero mutual inductance, hence zero interaction, and (b) by attaching low input impedance preamplifiers to all coils, thus eliminating interference among next nearest and more distant neighbors. We derive an algorithm for combining the data from the phased array elements to yield an image with optimum SNR. Other techniques which are easier to implement at the cost of lower SNR are explored. Phased array imaging is demonstrated with high resolution (512 x 512, 48-cm FOV, and 32-cm FOV) spin-echo images of the thoracic and lumbar spine. Data were acquired from four-element linear spine arrays, the first made of 12-cm square coils and the second made of 8-cm square coils. When compared with images from a single 15 x 30-cm rectangular coil and identical imaging parameters, the phased array yields a 2X and 3X higher SNR at the depth of the spine (approximately 7 cm).

Humans↗

PubMed1989

Proton-decoupled, Overhauser-enhanced, spatially localized carbon-13 spectroscopy in humans.

Spatially localized, natural abundance, carbon (13C) NMR spectroscopy has been combined with proton (1H) decoupling and nuclear Overhauser enhancement to improve 13C sensitivity up to five-fold in the human leg, liver, and heart. Broadhand-decoupled 13C spectra were acquired in 1 s to 17 min with a conventional 1.5-T imaging/spectroscopy system, an auxiliary 1H decoupler, an air-cooled dual-coil coplanar surface probe, and both depth-resolved surface coil spectroscopy (DRESS) and one-dimensional phase-encoding gradient NMR pulse sequences. The surface coil probe comprised circular and figure-eight-shaped coils to eliminate problems with mutual coupling of coils at high decoupling power levels applied during 13C reception. Peak decoupler RF power deposition in tissue was computed numerically from electromagnetic theory assuming a semi-infinite plane of uniform biological conductor. Peak values at the surface were calculated at 4 to 6 W/kg in any gram of tissue for each watt of decoupler power input excluding all coil and cable losses, warning of potential local RF heating problems in these and related experiments. The average power deposition was about 9 mW/kg per watt input, which should present no systemic hazard. At 3 W input, human 13C spectra were decoupled to a depth of about 5 cm while some Overhauser enhancement was sustained up to about 3 cm depth, without ill effect. The observation of glycogen in localized natural abundance 13C spectra of heart and muscle suggests that metabolites in the citric acid cycle should be observable noninvasively using 13C-labeled substrates.

Carbon Isotopes↗

PubMed1988

Human in vivo phosphate metabolite imaging with 31P NMR.

Phosphorus (31P) spectroscopic images showing the distribution of high-energy phosphate metabolites in the human brain have been obtained at 1.5 T in scan times of 8.5 to 34 min at 27 and 64 cm3 spatial resolution using pulsed phase-encoding gradient magnetic fields and three-dimensional Fourier transform (3DFT) techniques. Data were acquired as free induction decays with a quadrature volume NMR detection coil of a truncated geometry designed to optimize the signal-to-noise ratio on the coil axis on the assumption that the sample noise represents the dominant noise source, and self-shielded magnetic field gradient coils to minimize eddy-current effects. The images permit comparison of metabolic data acquired simultaneously from different locations in the brain, as well as metabolite quantification by inclusion of a vial containing a standard of known 31P concentration in the image array. Values for the NMR visible adenosine triphosphate in three individuals were about 3 mM of tissue. The ratio of NMR detectable phosphocreatine to ATP in brain was 1.15 +/- 0.17 SD in these experiments. Potential sources of random and systematic error in these and other 31P measurements are identified.

Adenosine Triphosphate↗

PubMed1988

Switched surface coil system for bilateral MR imaging.

A method is presented for acquiring magnetic resonance (MR) images in parallel from both members of a pair of organs, such as the temporomandibular joint (TMJ), with use of two surface coil receivers. Interleaved sagittal sections are broken into two groups, one on each side of the head, and the MR receiver is toggled between the two coils. The surface coil that is not receiving at a given time is decoupled from the other surface coil by the active gating of a diode blocking network located across the coil input. This method produces effective decoupling independent of coil loading and positioning and makes it possible to use the same surface coils already employed for single-coil MR reception. Clinical application in 95 patients (190 joints) with symptoms of internal derangement of the TMJ demonstrated a 24% average decrease in total patient examination time, even after the addition of coronal imaging to the procedure. In no case was there a sacrifice in diagnostic quality.

Humans↗

PubMed1986

Surface-coil T1 images.

A method for producing calculated T1 images with high signal-to-noise ratio and good spatial resolution using surface coils is presented. Separate transmit and receive rf coils with electronic decoupling are employed to insure uniform B1 excitation. The sensitivity profile of the receiver coil is normalized out of the image by the T1 calculation. T1 values determined by this method show close agreement with previously reported values.

Extremities↗