Biomedical subjects

O P Simonetti

Publications and source records attributed to O P Simonetti.

6 recordsLinked to original sources

PubMed1992

Signal-to-noise, resolution, and bias function analysis of asymmetric sampling with zero-padded magnitude FT reconstruction.

This report describes NMR image effects due to sampling asymmetry when using zero-padded magnitude FT reconstruction. With this method, the MTF is not flat over the spatial frequency passband, so resolution cannot be accurately described by a single variable such as voxel size. At small to moderate asymmetry, shortened (reduced window duration) asymmetry provides increased S/N and decreased resolution, whereas shifted (constant window duration) asymmetry yields essentially constant S/N with simultaneously increased and decreased resolution. A bias function expression describes image distortion due to sampling in terms separable from the imaged object. The analyses are consistent with previous descriptions of perceived image differences related to data asymmetry.

Algorithms↗

PubMed1990

Multiecho multimoment refocussing of motion in magnetic resonance imaging: MEM-MO-RE.

Gradient moment nulling techniques for refocussing of spin dephasing resulting from movement during application of magnetic resonance imaging gradients have gained widespread application. These techniques offer advantages over conventional imaging gradients by reducing motion artifacts due to intraview motion, and by recovering signal lost from spin dephasing. This paper presents a simple technique for designing multiecho imaging gradient waveforms that refocus dephasing from the interaction of imaging gradients and multiple derivatives of position. Multiple moments will be compensated at each echo. The method described relies on the fact that the calculation of time moments for nulled moment gradient waveforms is independent of the time origin chosen. Therefore, waveforms used to generate the second echo image for multiple echo sequences with echo times given by TEn = TE1 + (n - 1) * (TE2 - TE1) may also be used for generation of the third and additional echo images. All echoes will refocus the same derivatives of position. Multiecho, multimoment refocussing (MEM-MO-RE) images through the liver in a patient with ampullary adenocarcinoma metastatic to the liver demonstrate the application of the method in clinical scanning.

Adenocarcinoma↗

PubMed1990

Modified gradients for motion suppression: variable echo time and variable bandwidth.

A linear algebra based deprivation is presented to demonstrate that linearly time scaling an entire gradient waveform by a factor "R" exponentially increases its sensitivity to time derivatives of position by R(i + 1), where i refers to the i-th derivative of position (e.g., i = 1 is velocity). Thus, time scaling will preserve zero valued refocussing moments associated with artifact reduction techniques designed for motion occurring between excitation and detection. Typically, gradient waveforms for artifact reduction techniques are derived for use only at specific echo times. The time scaling described here allows for simple modification of refocussing gradient waveforms for use at variable echo times. Motion sensitivity associated with non-zero moment gradient waveforms can be easily predicted and modified using this technique, with consideration for field of view, resolution, and bandwidth. A clinical example is presented showing the predicted changes in sensitivity to nonrefocussed derivatives of position as the imaging gradients are time scaled. Further, trade-offs and alternatives in sensitivity to motion, slice thickness, image bandwidth, field of view and resolution will be discussed in conjunction with time scaling. This technique will have applicability in many situations involving MRI of moving tissue and a clinical example in cardiac imaging is presented.

Aorta, Abdominal↗

PubMed1988

Rate of elimination of excess CO2 in humans.

This study examined the ability of the respiratory system of awake normal subjects to correct an acute disturbance in body CO2 stores produced by rebreathing. Thirteen subjects, after 10 min of O2 breathing, rebreathed CO2 for 4 min in order to increase CO2 stores. The rate of CO2 elimination (VelCO2) after rebreathing was measured breath by breath for the next 10 min. The VelCO2 was highest immediately after the end of rebreathing and then decreased non-linearly toward the pre-rebreathing level as stored CO2 decreased. The time for 90% of this change in VelCO2 to occur (T90) was measured as an index of the rate of correction of body CO2 imbalance. The T90 was independent of the peak PCO2 obtained by rebreathing, and changes in CO2 storage did not produce significant changes in T90. The value of T90 was 120 +/- 48 sec (mean +/- SD) and there was a significant negative correlation between T90 and the slope of the ventilatory response to CO2 among subjects (r = -0.864, P less than 0.001). These results suggest that the capacity to eliminate the acutely stored CO2 in awake normal subjects is dependent on the ventilatory response to CO2. The data were further analyzed by using a mathematical model. Mathematical analysis confirmed the inverse relationship between CO2 sensitivity and T90. Model simulations also suggested that the restoration speed of CO2 balance under hyperoxic conditions is affected by brain blood flow but mainly determined by the sensitivity of central chemoreceptors.

Adult↗

PubMedDate not supplied

Significance of the point of expansion in interpretation of gradient moments and motion sensitivity.

The relationship between magnetic field gradient waveform moments and the motion sensitivity of magnetic resonance imaging was explored analytically and by computer simulation. The analysis and simulations revealed several key points. In general, waveform time moments define sensitivity to the time derivatives of position of moving material only at a single time point: the time about which the moments are computed. A Taylor series description of instantaneous position is expanded about this same time point to compute the phase acquired due to specific derivatives of position. A moment is proportional to phase sensitivity to a particular derivative of position throughout the waveform only when sensitivity to all lower-order derivatives is zero. Under restricted conditions of waveform symmetry and motion characteristics, the phase due to motion may be expressed in terms of the average value of a derivative of position over the duration of the waveform. The choice of the moment center, or point of expansion, adds a degree of freedom that may be used advantageously in the design of motion-compensating and motion phase-encoding gradient waveforms. These results facilitate a more complete understanding of the effects of motion through a magnetic field gradient.

Computer Simulation↗

PubMedDate not supplied

Theoretical aspects of motion sensitivity and compensation in echo-planar imaging.

Magnetic resonance (MR) imaging can be performed on or below the time scale of most anatomic motion via echo-planar imaging (EPI) techniques and their derivatives. The goal is to image rapidly and reduce artifacts that typically result from view-to-view changes in the spatial distribution of spins due to motion. However, the required time-dependent magnetic field gradient waveforms remain sensitive to the dephasing effects of motion. Sources of motion artifact are simulated for spins moving along the imaging axes and are shown to be an important source of reduced image quality in EPI. A novel method of EPI is proposed that (a) refocuses single or multiple derivatives of motion at all echoes and (b) prevents accumulation of velocity (or higher derivative)--induced dephasing along the phase-encoding axis by moment nulling all phase-encoding-step waveforms about a single instant of time. Theoretical EPI sequences with considerable reductions in ghosts, blurring, and signal loss due to motion sensitivity are produced and compared with other EPI methods. Their time efficiency is presented as a function of available (relative) gradient strength for a variety of sequence waveforms.

Artifacts↗