Technical exhibits.
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Exhibitions as Topic↗
Biomedical subjects
N J Hangiandreou
Publications and source records attributed to N J Hangiandreou.
Interactive selection of optimal section orientations using real-time MRI.
In applications where precise image section positioning is vital, the interactive section rotation and offset capabilities of interactive MRI should be valuable. However, due to the independent nature of these two adjustments, the desired structure may often not be visible in the image after a rotation. Valuable time is wasted during relocation. An algorithm is presented that automatically alters the section offset after a rotation to provide continuous viewing of a marked structure, greatly improving section orientation efficiency. The technique is illustrated in the determination of double oblique angulation for through-plane imaging of the portal vein. This algorithm is expected to prove useful in applications of interactive MRI requiring precise positioning.
Algorithms↗
Error in MR volumetric flow measurements due to ordered phase encoding in the presence of flow varying with respiration.
Respiratory ordered phase encoding is often employed in MRI studies to reduce image artifacts due to breathing motion. The purpose of this work was to evaluate error caused by the use of respiratory ordering of phase encoding in MR cine phase-contrast (CPC) volumetric flow measurements when the flow rate is sensitive to respiration. It was hypothesized that this effect is due to the systematic biasing of a respiratory-induced phase modulation function in k-space. A theoretical model for the effects of respiration was developed and then tested in flow phantom studies and in normal volunteer studies. In phantom experiments, the use of respiratory ordering induced an error of as much as 13% in CPC volumetric flow measurements. In preliminary volunteer studies, error was as high as 26% in superior vena cava flow measurements versus less than 1% error in the ascending aorta. It is concluded that a potential for error exists in CPC volumetric flow measurements obtained with the use of respiratory ordering schemes. Volunteer studies with larger numbers are warranted. Clinical applications in which this effect may be important include flow measurements in vessels subject to variations in flow due to respiration, such as the venae cavae, pulmonary vasculature, and portal vein.
Aorta↗
Sensory motor cortex: correlation of presurgical mapping with functional MR imaging and invasive cortical mapping.
PURPOSE: To describe a clinically useful application of functional magnetic resonance (MR) imaging--presurgical mapping of the sensory motor cortex--and to validate the results with established physiologic techniques. MATERIALS AND METHODS: Functional MR mapping of the sensory motor cortex was performed in two women, aged 24 and 38 years. Both had intractable, simple partial motor seizures due to tumors located in or near the sensory motor cortex. They subsequently underwent invasive cortical mapping--direct cortical stimulation and/or sensory-evoked-potential recording--to localize the affected sensory motor area prior to tumor resection. RESULTS: In both patients, the functional MR study demonstrated task activation of the sensory motor cortex. In both cases, results of cortical functional mapping with invasive techniques matched those obtained with functional MR imaging. CONCLUSION: Presurgical mapping of the sensory motor cortex is a potentially useful clinical application of functional MR imaging.
Adult↗
Imaging of cerebral activation at 1.5 T: optimizing a technique for conventional hardware.
PURPOSE: To empirically optimize a two-dimensional magnetic resonance (MR) imaging technique for detecting changes in signal intensity during cerebral activation with a standard clinical imager. MATERIALS AND METHODS: Visual activation experiments were performed while imaging parameters were manipulated in a serial fashion, to test their effect on the percentage change in signal intensity (PCSI). The parameters tested were section thickness, echo time (TE), field of view (FOV), flip angle, radio-frequency (RF) spoiling, number of readout points, and number of signals averaged. RESULTS: The PCSI for visual activation experiments was typically in the 1%-5% range. The best results were achieved with a small section thickness, long TE, and large FOV. Variations in other acquisition parameters had a negligible influence on the PCSI. CONCLUSION: Good results can be obtained at functional MR imaging with standard 1.5-T hardware. Markedly improved results can be obtained by optimizing several key variables, namely, section thickness, TE, and FOV.
Adult↗
Technical exhibits.
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Diagnostic Imaging↗
Phase velocity mapping with a real time line scan technique.
A real-time, 20-Hz, one-dimensional MR velocity imaging technique is described. A two-dimensional RF pulse excites a 3-cm diameter column. Velocity maps are formed from the phase difference between successive flow encoded and compensated acquisitions. A three-point subtraction variation provides reduced sensitivity to static spins.
Magnetic Resonance Imaging↗
Real-time interactive color flow MR imaging.
Real-time interactive color flow magnetic resonance (MR) imaging is a combination of real-time MR imaging and color encoding of velocity-induced phase angle. Flow-compensated (FC) and flow-encoded (FE) images are acquired continuously by using gradient echoes and a 12-msec repetition time. Each image is reconstructed within 200 msec of acquisition, and the FC magnitude image is displayed in gray-scale format. The phase difference between the reconstructed FC and FE images, a difference proportional to velocity along the flow-encoding direction, is encoded in color and superimposed on the gray-scale FC image. Magnitude and phase information are thus presented simultaneously. The viewer may interactively adjust many acquisition parameters during data acquisition. Experimental results of phantom and in vivo human studies validate the method. Characteristics of the color flow MR imaging technique are compared with those of duplex color ultrasound.
Blood Flow Velocity↗
Investigation of the performance of two types of the Doppler catheter in vitro.
There is considerable interest in the use of Doppler catheters for measuring coronary flow reserve in humans. Two types are currently available, these being models having side-mounted or tip-mounted transducers. The performance of these catheters was carefully observed in Silastic tubing perfused with blood by a roller pump. Each catheter was used with two types of positioning wire: a rigid wire and a standard J-tip guidewire. Linear regressions of velocity vs. flow rate were performed. Both catheters performed well with the rigid wire (r greater than or equal to 0.992). When used with the J-tip, the end-mounted catheter suffered decreased performance because of positioning difficulties, while the side-mounted catheter performance was within 6% of the ideal. Measurements made with the side-mounted catheter at low flow rates underestimated the expected response for forward flow. Because of the ease of positioning, the side-mounted catheter is judged to be more useful in measuring flow ratios.
Catheterization↗
Digital subtraction angiographic imaging of coronary flow reserve.
Recent studies have demonstrated that subjective assessment of the severity of coronary artery stenoses results in poor interobserver concordance and poor correlation with physiologic significance as determined by Doppler measurements of coronary flow reserve. Use of the coronary flow reserve as an integrated measure of the effect of stenosis geometry has been emphasized within the context of quantitative cinemetric analysis. The comparison of two parametric digital subtraction angiographic flow images obtained before and after hyperemic intervention has led to calculation of flow reserve values that correlate well with electromagnetic flowmeter data in dogs. By means of a similar model relating blood flow and image variables, single flow ratio images have been formed. These parametric images provide a two-dimensional display of the ratio of hyperemic flow to baseline flow. Linear temporal interpolation of data from a sequence of cardiac phase-matched subtraction images is used to improve the resolution of the displayed flow ratios. Summation of flow variables measured within the perfusion bed was used to calculate a value for the overall coronary flow reserve and to characterize the significance of isolated lesions in an open-chest canine preparation. A linear regression calculation relating parametric image flow ratio values to electromagnetic flowmeter measurements resulted in a linear fit of y = .96x - 0.19 with a correlation coefficient of .90. The direct visual representation of flow ratio distribution provided by the parametric imaging method may aid in the interpretation of multiple complex lesions as well as of single lesions.
Angiography↗
Digital beam attenuator technique for compensated chest radiography.
The feasibility of producing patient-specific beam attenuators for chest radiography has been investigated using an anthropomorphic phantom and a human volunteer. A low-dose test exposure is digitized, processed, and used to print a small cerium filter, which is placed in the x-ray beam near the collimator. The final radiograph is recorded on film. The technique results in relatively uniform film exposure, so that structures in all regions of the chest are simultaneously displayed with optimal film contrast. The equalized exposure improves image quality in the normally underpenetrated regions and reduces the role of cross-scatter from the lungs. The image is analogous to optical or computer-processed unsharp masking techniques, but the processing is accomplished in the x-ray beam and results in an improved exposure distribution, giving advantages that cannot be achieved with image processing techniques alone.
Humans↗
Analysis of MR phase-contrast measurements of pulsatile velocity waveforms.
Errors in the measurement of the mean velocity of pulsatile velocity waveforms with ungated phase-contrast techniques were studied theoretically and experimentally. Waveforms consisting of a constant and two sinusoidal components were analyzed. Variations in magnitude and phase of the vascular magnetic resonance (MR) signal resulted in errors, the severity of which increased when either factor increased. Magnitude variations always resulted in overestimation. The general shape of the waveform greatly influenced the error, with certain waveforms producing greater inherent error than others. Experimental measurements were performed, validating the predicted sensitivity of these errors to changes in imaging parameters, including TR and flow-encoding sensitivity. Errors generally became more severe with increased flow-encoding sensitivity. The theoretical and experimental results suggest that accurate mean velocity measurements in many vessels of the body--with acquisition times of less than 15 seconds--should be attainable with ungated imaging techniques and with careful selection of relevant imaging parameters.
Aorta, Abdominal↗
Coronary blood flow measurement using an angiographic first pass distribution technique: a feasibility study.
Due to the well-documented problems associated with visual interpretation of coronary angiograms, more physiologic means of assessing coronary artery stenosis are being investigated. One physiologic parameter that has been suggested is coronary flow reserve (CFR). A digital subtraction angiographic technique based on first pass distribution analysis (FPA) is proposed as a means of measuring CFR and absolute coronary flow. The theory of the FPA method is first outlined, and the implementation of a preliminary version of the FPA algorithm is described. Experiments verifying the utility of this algorithm for measuring absolute flow through a flow phantom, and through the canine circumflex artery are reported. It was determined that the preliminary FPA algorithm is capable of measuring canine coronary flow ratios (R) with accuracy and precision characteristics meeting or exceeding those reported for the parametric imaging technique (RFPA = 0.933.Rtrue, SEE = 0.16, r = 0.984). Accurate absolute flow (Q) measurements were obtained in all of the phantom experiments (QFPA = 1.054.Qtrue, r = 0.993), and in one of the three dogs that were studied (QFPA = 0.977.Qtrue, r = 0.935). The difficulty encountered in the other two dog experiments is attributed to the effects of system temporal lag, and would likely be corrected through the use of improved cameras. The feasibility of the general FPA method for measuring relative flow is established, and the potential for routine, absolute flow measurement is demonstrated.
Algorithms↗