Biomedical subjects

J R MacFall

Publications and source records attributed to J R MacFall.

24 records · Page 2Linked to original sources

PubMedDate not supplied

An overview of digital spectrometers for MR imaging. Instrumentation Subcommittee of the SMRI Basic Science Council.

Most magnetic resonance (MR) imaging systems were originally designed with analog spectrometers, since that was the "state of the art" in the late 1970s, when they were developed. Advances in technology have allowed the design of radio-frequency electronics with a much larger percentage of digital components while the cost of such components has decreased. This has given manufacturers the incentive to develop new spectrometers that incorporate these electronics for cost reduction and potentially better performance. Upgrades and new models of MR units have become available with these so-called "digital spectrometers." Because of the interest in the new systems, the Instrumentation Subcommittee of the Basic Science Council of the Society for Magnetic Resonance Imaging has produced this report to review the basic features of analog and digital spectrometers to help the MR imaging community better understand the similarities and differences of these systems. Some details of actual commercial implementations were left out to focus on the basic features. Regardless, the authors hope they have provided a readable introduction to this important topic.

Humans↗

PubMedDate not supplied

Magnetization-prepared MR angiography with fat suppression and venous saturation.

Magnetization-prepared magnetic resonance (MR) angiography (MPMRA) is an inflow-based two-dimensional (2D) imaging sequence in which a preparation phase precedes rapid image acquisition. For maximal blood/tissue contrast, an inversion-recovery preparation nulls signal from static tissue. If needed, a second inversion suppresses signal from fat. Fully magnetized blood flows in after the inversion pulse(s), providing high signal intensity. The centric phase-encoding order, which ensures that the initial contrast is reflected in the image set, requires the use of a modified venous saturation technique. The sequence is described and its performance assessed with regard to (a) depiction of in-plane flow, (b) fat suppression, and (c) venous saturation. Phantom and volunteer studies showed good performance in all three areas. MPMRA images, acquired in just 2-4 seconds per image, had a blood/tissue contrast-to-noise ratio nearly twice that of standard 2D time-of-flight MR angiograms, acquired in 5-7 seconds. The technique is promising for restless patients and in anatomic areas plagued by motion degradation.

Adipose Tissue↗

PubMedDate not supplied

Correction of spatially dependent phase shifts for partial Fourier imaging.

Partial Fourier MR images (PFI) are constructed from data that have fewer phase encoding views than are conventionally acquired using direct Fourier transform spin echo acquisition. The PFI data acquisition is structured to obtain the same spatial resolution as conventional acquisition, trading off signal-to-noise reduction for acquisition time improvement. The "missing" views can be zero filled or, if the data are Hermitian, supplied by symmetry (basic algorithm). The effect of spatially dependent phase shifts (SDPS) on PFI constructed with zero-fill or the basic algorithm is illustrated. The causes and typical magnitudes of such SDPS are discussed. In spin echo data only the low order, slowly varying SDPS, is shown to be significant. Through use of simulated and actual data sets these typical SDPS are shown to produce significant artifacts in PFI, when the amount of missing data is close to one-half. The artifacts are reduced when less data are missing. Good images can be generated with the zero-fill algorithm if less than 25% of the data is missing. Several methods of correcting phase shifts in PFI are developed: the basic Hermitian algorithm with frequency (x) direction correction (BAX), basic Fourier correction algorithm (BFC) and an improved iterative Fourier correction algorithm (IFC). The BFC and IFC can produce good images when as much as 46% of the data is missing. Data with rapidly varying SDPS, for example, gradient refocused data, make the phase correction task more difficult. With less than 25% of the data missing, however, acceptable gradient refocused PFI images can be created.

Fourier Analysis↗

PubMedDate not supplied

MR imaging of venous and arterial flow by a selective saturation-recovery spin echo (SSRSE) method.

Flow velocity imaging studies have been conducted by means of a selective saturation-recovery spin echo technique, and the dependence of signal amplitude on interpulse interval, echo delay, slice-selection gradient, and flow velocity was evaluated experimentally. The simple theory predicting a steady increase of signal intensity with increasing interpulse interval until this latter equals the transit time could be verified in phantoms and was shown to permit measurement of blood flow velocity in venous structures such as the femoral vein. The flow phantom experiments further showed that the final intensity, attained when inversion time (TI) = transit time, decreases with increasing flow velocity, an effect that cannot be explained by influx of spins between the 90 degree detection pulse and the 180 degree refocusing pulse. This signal reduction is due to slice-selection gradient-induced phase shifts across the pixel, caused by the intralumenal velocity gradient, leading to destructive interference of the spin isochromats. The velocity distribution can be mapped by plotting signal intensity as a function of interpulse interval for pixels in different radial positions. To highlight arterial flow, gating is required with the acquisition delay selected such that the interpulse period TI falls in a time zone of slow flow within the cardiac cycle. By subtracting images recorded with different acquisition delays, flow images showing arterial enhancement only can be obtained, as illustrated for the femoral artery in the thigh.

Blood Flow Velocity↗

PubMedDate not supplied

An analysis of noise propagation in computed T2, pseudodensity, and synthetic spin-echo images.

Methods are reviewed for estimating the transverse relaxation time T2 and the pseudodensity (PD) from spin-echo measurements acquired at an arbitrary set of echo times [TEi]. Least-squares fitting is applied to the logarithmically processed signals for the case in which the weights are proportional to the inverse of the logarithmically transformed signal variances (the minimum variance case). General formulas are derived for the estimated noise levels in the PD and T2 estimates due to the propagation of uncertainties in the original measurements. It is shown that the T2 and PD estimates are anticorrelated. Additionally, an expression is derived for the variance in a synthetic spin-echo signal subsequently formed from the PD and T2 estimates. It is shown that under many circumstances a signal synthesized at some echo time can have a signal-to-noise ratio superior to that in a signal directly acquired at that time. Experimental measurements made on phantoms match the theoretical predictions to a high degree.

Analysis of Variance↗

PubMedDate not supplied

Cerebral magnetic resonance image synthesis.

The authors previously described magnetic resonance (MR) image synthesis, a process that enables the investigator to manipulate imaging parameters retrospectively and generate or "synthesize" the image that corresponds to various arbitrary scanning factors. They demonstrate the validity and utility of synthetic spin-echo images in cerebral imaging. As a test of their method, spin-echo images are synthesized for echo times identical to those of the original acquired images as well as for alternate values. Subjectively, the quality of synthetic and acquired images is comparable. It is shown quantitatively for several tissue types that the reconstructed synthetic signal matches the acquired signal within the uncertainty of the acquired images. Observed and measured noise levels in the acquired and synthetic images are comparable. Because of a signal-averaging effect, the synthetic images can have a higher signal-to-noise ratio than the source images, thereby providing improved boundary definition. Applications of MR image synthesis are discussed with respect to potential reduction in scanning time. The advantages of image synthesis versus analysis of computed images are discussed.

Adult↗