Effects of physiologic waveform variability in triggered MR imaging: Theoretical analysis
One of the assumptions inherent in most forms of triggered magnetic resonance (MR) imaging is that the pulsatile waveform (be it cardiac, respiratory, or some other) is purely periodic. In reality, the periodicity condition is rarely met. Physiologic waveform variability may lead to image artifacts and errors in velocity or volume flow rate estimates. The authors analyze the effects of physiologic waveform variability in triggered MR imaging. They propose that this variability be treated as a modulation of the underlying motion waveform. This report concentrates on amplitude modulation of the velocity waveform, which results in amplitude and phase modulation of the transverse magnetization. Established Fourier and modulation theory and the recently described principles of (k,t)-space were used to derive the appearance of physiologic waveform variability artifacts in triggered MR images and to predict errors in time-averaged and instantaneous velocity estimates that may result from such motion effects, including effects such as ghost overlap. Simulations and experimental results are provided to confirm the theory.
Available from: Jongmin Lee
- "We believe that if the duration of the main bolus were greater than venous enhancement time, arteriovenous circulation time would be the main parameter. Although some reports have suggested possibly insufficient coverage of K-space during a rapid short bolus injection (17, 22-24), we assumed that the standard dose of 0.1 mmol / kg during an injection lasting 3 - 5 seconds would more than cover the total length of arteriovenous circulation and image acquisition time. All second arterial-phase images and some 50-second delayed venous-phase images in our study revealed that arteries were still strongly enhanced, thus demonstrating that the extent by which arterial bolus length exceeded arteriovenous circulation time was sufficiently great. "
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To determine the optimal scan timing for contrast-enhanced magnetic resonance angiography and to evaluate a new timing method based on the arteriovenous circulation time.
Materials and Methods
Eighty-nine contrast-enhanced magnetic resonance angiographic examinations were performed mainly in the extremities. A 1.5T scanner with a 3-D turbo-FLASH sequence was used, and during each study, two consecutive arterial phases and one venous phase were acquired. Scan delay time was calculated from the time-intensity curve by the traditional (n = 48) and/or the new (n = 41) method. This latter was based on arteriovenous circulation time rather than peak arterial enhancement time, as used in the traditional method. The numbers of first-phase images showing a properly enhanced arterial phase were compared between the two methods.
Mean scan delay time was 5.4 sec longer with the new method than with the traditional. Properly enhanced first-phase images were found in 65% of cases (31/48) using the traditional timing method, and 95% (39/41) using the new method. When cases in which there was mismatch between the target vessel and the time-intensity curve acquisition site are excluded, erroneous acquisition occurred in seven cases with the traditional method, but in none with the new method.
The calculation of scan delay time on the basis of arteriovenous circulation time provides better timing for arterial phase acquisition than the traditional method.
Available from: Richard Frayne
- "Variability is especially important in ungated procedures such as magnetic resonance angiography (MRA) (Nishimura 1990, Dumoulin 1989) and cardiac-gated cine MRI flow measurements (Pelc et al 1991). In these investigations, data collection may occur over hundreds of cardiac intervals, and cycle-to-cycle variations can lead to artefacts in the final data (Hangiandreou et al 1993, Hofman et al 1993, Lauzon et al 1994, Rogers and Shapiro 1993). We are interested in experimental investigations and numerical simulations of blood flow through the carotid bifurcation. "
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ABSTRACT: Knowledge of human blood-flow waveforms is required for in vitro investigations and numerical modelling. Parameters of interest include: velocity and flow waveform shapes, inter- and intra-subject variability and frequency content. We characterized the blood-velocity waveforms in the left and right common carotid arteries (CCAs) of 17 normal volunteers (24 to 34 years), analysing 3560 cardiac cycles in total. Instantaneous peak-velocity (Vpeak) measurements were obtained using pulsed-Doppler ultrasound with simultaneous collection of ECG data. An archetypal Vpeak waveform was created using velocity and timing parameters at waveform feature points. We report the following timing (post-R-wave) and peak-velocity parameters: cardiac interbeat interval (T(RR)) = 0.917 s (intra-subject standard deviation = +/- 0.045 s); cycle-averaged peak-velocity (V(CYC)) = 38.8 cm s(-1) (+/-1.5 cm s(-1)); maximum systolic Vpeak = 108.2 cm s(-1) (+/-3.8 cm s(-1)) at 0.152 s (+/-0.008 s); dicrotic notch Vpeak = 19.4 cm s(-1) (+/-2.9 cm s(-1)) at 0.398 s (+/-0.007 s). Frequency components below 12 Hz constituted 95% of the amplitude spectrum. Flow waveforms were computed from Vpeak by analytical solution of Womersley flow conditions (derived mean flow = 6.0 ml s(-1)). We propose that realistic, pseudo-random flow waveform sequences can be generated for experimental studies by varying, from cycle to cycle, only T(RR) and V(CYC) of a single archetypal waveform.
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ABSTRACT: Magnetic resonance imaging (MRI) has been increasingly recognized for its role in the diagnosis, treatment planning, and clinical
management of patients with cardiovascular disease and has several important advantages over alternative imaging modalities,
including electrocardiogram (ECG) synchronized and direct three-dimensional (3D) volumetric imaging unrestricted by imaging
depth. In addition, the intrinsic sensitivity of MRI to flow, motion, and diffusion offers the unique possibility to acquire
spatially registered functional information simultaneously with the morphological data within a single experiment (1–13,16–19,31,36,38). As a result, flow-sensitive MRI techniques, also known as phase contrast (PC) MRI, provide noninvasive methods for the
accurate and quantitative assessment of blood flow or tissue motion. Characterizations of the dynamic components of blood
flow and cardiovascular function provide insight into normal and pathological physiology and have made considerable progress
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