Biomedical subjects

D N Firmin

Publications and source records attributed to D N Firmin.

At least 19 recordsLinked to original sources

PubMed2000

Asymmetric redirection of flow through the heart.

Through cardiac looping during embryonic development, paths of flow through the mature heart have direction changes and asymmetries whose topology and functional significance remain relatively unexplored. Here we show, using magnetic resonance velocity mapping, the asymmetric redirection of streaming blood in atrial and ventricular cavities of the adult human heart, with sinuous, chirally asymmetric paths of flow through the whole. On the basis of mapped flow fields and drawings that illustrate spatial relations between flow paths, we propose that asymmetries and curvatures of the looped heart have potential fluidic and dynamic advantages. Patterns of atrial filling seem to be asymmetric in a manner that allows the momentum of inflowing streams to be redirected towards atrio-ventricular valves, and the change in direction at ventricular level is such that recoil away from ejected blood is in a direction that can enhance rather than inhibit ventriculo-atrial coupling. Chiral asymmetry might help to minimize dissipative interaction between entering, recirculating and outflowing streams. These factors might combine to allow a reciprocating, sling-like, 'morphodynamic' mode of action to come into effect when heart rate and output increase during exercise.

Adult↗

PubMed2000

A comparison between segmented k-space FLASH and interleaved spiral MR coronary angiography sequences.

A direct comparison of segmented fast low-angle short (FLASH) imaging and interleaved spiral magnetic resonance coronary angiography (MRCA) during free respiration using navigator echo has been performed. MRCA images were acquired in 30 normal subjects and 15 patients with coronary artery disease (CAD). Images of the right coronary artery were acquired during free respiration using navigator echo gating for both a segmented k-space FLASH sequence (8 views/segment, segment duration 105 msec) and an interleaved spiral sequence (20 interleaves, spiral read-out period 19 msec). Image quality was scored by three independent blinded observers, and coronary artery signal-to-noise ratio (SNR) and coronary artery/epicardial fat contrast-to-noise ratio (CNR) were measured. There was a significant improvement in image quality when coronary images were acquired with the interleaved spiral sequence (spiral 2. 3 vs. FLASH 1.8; P = 0.002). This was associated with an increase in the coronary artery SNR (16.6 +/- 6.9 vs. 11.8 +/- 5.0; P < 0.001), the coronary artery/epicardial fat CNR (12.5 +/- 6.1 vs. 7.4 +/- 4.0, P < 0.001), and the image resolution (256 x 256 vs. 256 x 128). However, there was a 12% increase in acquisition time for the interleaved spiral sequence. Image quality, SNR, CNR, and resolution can be improved using an interleaved spiral sequence. These improvements are secondary to the intrinsic characteristics of spiral imaging and the short acquisition period, which reduces the effects of both cardiac and respiratory motion.

Adult↗

PubMed2000

Left ventricular quantification in heart failure by cardiovascular MR using prospective respiratory navigator gating: comparison with breath-hold acquisition.

Cardiovascular magnetic resonance (CMR) is the reference standard for the assessment of cardiac function. Faster sequences, such as breath-hold (BH) fast low-angle shot, have made CMR more clinically acceptable and cost effective. In a significantly large patient group, however, holding their breath is difficult, resulting in poor-quality images. We compared prospective navigator-echo respiratory gating (NE), which allows image acquisition during free breathing, and BH imaging in 14 patients with heart failure and 10 normal volunteers. There was good agreement between both NE and BH volumes, mass, and ejection fraction. The image quality of both NE basal and apical slices was significantly better than the corresponding BH slices in both the heart failure (P < 0.01) and normal groups (P < 0.05). The NE image acquisition was more time efficient than the BH acquisition in the heart failure group (P < 0. 01), with no difference in the normal group (P = 0.2). Thus, prospective navigator-echo gating, previously only described in coronary artery imaging, can be used in the assessment of cardiac function. It is particularly useful in patients who find it difficult to hold their breath in whom NE provides good-quality, time-efficient images.

Electrocardiography↗

PubMed2000

Phase ordering with automatic window selection (PAWS): a novel motion-resistant technique for 3D coronary imaging.

Navigator acceptance imaging methods are hindered by the loss in scan efficiency which results from the changes in the breathing pattern of a subject over time. The diminishing variance algorithm (DVA), which does not use a predefined acceptance window, is less influenced by such changes. The use of phase ordering and weighting techniques has been shown to significantly improve image quality over nonordered window methods. However, the use of an acceptance window is inherent in all these techniques as a decision to accept or reject data must still be made. A technique is presented which is resistant to changes in breathing while allowing the use of phase ordering to provide effective motion artifact reduction in optimal time. The basic principle is described and illustrated for this automatic window-selection technique with in vitro results to demonstrate the feasibility of this method. Results of an in vivo study are also presented which demonstrate significant improvement in image quality over the DVA (p < 0.01) and hybrid-ordered phase encoding methods (p < 0.05).

Algorithms↗

PubMed2000

The cardiovascular magnetic resonance machine: hardware and software requirements.

The flexibility of cardiac magnetic resonance imaging (MRI) includes faster imaging for applications such as stress tests, ventricular function, myocardial perfusion and coronary artery imaging. Faster imaging makes greater demands on the hardware and software. Although some cardiac imaging can be performed at 0.5 T, some of the faster techniques demand the higher signal-to-noise ratio of higher main field, and fat suppression in cardiac images is more easily achieved at higher field. Main field inhomogeneity affects rapid imaging and performance in open-access magnets. High gradient performance, low eddy currents and surface receiver coils are essential for fast cardiac imaging and the hardware of these systems including interventional imaging is discussed. The use of ECG signals for prospective and retrospective cardiac synchronization of MRI is examined. Techniques for reducing the major problem of respiratory motion in MRI are surveyed. Flexibility in the computer architecture of the scanner and the electronics generating the pulse sequence and controlling data acquisition is vital in cardiac imaging, for retrospective cardiac gating, respiratory navigator-controlled imaging and "real-time interactive" imaging in a similar manner to ultrasound imaging. Automated measurements from MR images remain under development. The pulse sequences and image display functions a cardiovascular MRI system should support for basic cardiac imaging applications and current clinical research areas are summarized.

Cardiovascular System↗

PubMed1999

Safety and preliminary findings with the intravascular contrast agent NC100150 injection for MR coronary angiography.

In this Phase I clinical study, a novel ultrasmall superparamagnetic iron oxide contrast agent, NC100150 Injection (Nycomed Imaging, Oslo, Norway, a part of Nycomed Amersham), was used in two-dimensional magnetic resonance coronary angiography (MRCA). Safety and imaging data were acquired from 18 healthy male volunteers at both 0.5 and 1.5 T, before and after the administration of NC100150 Injection. Through-plane and in-plane images of the right coronary artery were analyzed. The postcontrast imaging sequences used prepulses and a high flip angle, to introduce T1 weighting. At 1.5 T (TE 2.6 msec), the through-plane coronary artery signal-to-noise ratio (SNR) (P = 0.04), coronary artery-to-fat signal difference-to-noise ratio (SDNR) (P = 0.001), coronary artery-to-myocardium SDNR (P<0.001), and coronary artery delineation (P<0.001) were improved by the administration of NC100150 Injection. For in-plane imaging, coronary artery delineation improved, but there were no significant changes in the SNR and SDNR. At 0.5 T, with the longer TE (6.7 msec) imaging sequence used, there was a reduction in the SNR (P = 0.01), the fat SDNR (through-plane P = 0.02; in-plane P = 0.25), and the coronary artery diameter (P<0.01 in both imaging planes). There was a trend toward improvement in the myocardial SDNR and coronary artery delineation. In conclusion, NC 100150 Injection was given safely to 18 healthy subjects, with no major adverse reactions. Coronary artery delineation was improved in both imaging planes at 1.5 T, with a trend toward improvement at 0.5 T. At 1.5 T, with a short TE imaging sequence, the marked T1 shortening effects of NC100150 Injection were dominant, leading to an improvement in the quantitative parameters for the through-plane images. At 0.5 T, with a longer TE imaging sequence, the T2* effects of the contrast agent played a role in reducing the quantitative image parameters. With further optimization of imaging sequences, to take advantage of the long-lived intravascular T1 shortening effect of NC100150 Injection, further improvements in MRCA will be possible.

Adult↗

PubMed1999

Automated monitoring of diaphragm end-expiratory position for real-time navigator echo MR coronary angiography.

Real-time navigator echo (NE)-gated magnetic resonance coronary angiography (MRCA) during free respiration is now possible. However, the mean diaphragm end-expiratory position (DEEP) drifts over time, and this results in a reduction in scanning efficiency and increased artifacts due to the acquisition of data during periods of high diaphragm velocity. To address these problems, a diaphragm monitoring program that follows the mean DEEP over time has been developed. Fifteen subjects with ischemic heart disease underwent continuous NE monitoring of their diaphragm for 30 minutes. Using these diaphragm traces, theoretical MRCA scans were performed. Several diaphragm monitoring algorithms were developed and compared with the simplest case (a stationary 5 mm NE acceptance window placed around the mean DEEP, as measured by NE monitoring at the outset of the scan). An overall scan efficiency was calculated, and the number of completed scans where the mean DEEP lay within the NE acceptance window was recorded. Of the six algorithms considered, the most effective one monitored the mean DEEP and prospectively placed the upper limit of the NE acceptance window on this position for the subsequent acquisition. Using this algorithm in comparison with the simplest stationary scenario, both scan efficiency (47.9% vs. 38.5%, P = 0.01) and the number of completed scans where the mean DEEP lay within the NE acceptance window (71.2 vs. 30.3, P < 0.001) were improved. The implementation of such a monitoring algorithm, in combination with adaptive motion correction techniques, should improve overall scan efficiency while maintaining the end-expiratory position at the top end of the NE acceptance window, to reduce image artifacts.

Adult↗

PubMed1999

Differences between normal subjects and patients with coronary artery disease for three different MR coronary angiography respiratory suppression techniques.

A comparison between three magnetic resonance coronary angiography (MRCA) respiratory motion suppression techniques was performed for both normal subjects and patients with coronary artery disease (CAD). MRCA images were acquired in 17 normal subjects and 15 patients with CAD, using conventional breath-hold MRCA, navigator echo (NE)-guided breath-hold MRCA (LED feedback), and NE-gated MRCA during free respiration. Image quality, diaphragm registration, and total acquisition time were assessed. Overall, there was poor diaphragm registration for conventional breath-holding compared with free respiration (P < 0.001). CAD patients found it significantly more difficult to perform a steady breath-hold (P = 0.04) or attain the same diaphragm position over multiple breath-holds than normal subjects (P = 0.02). All normal subjects, but only 3 of the 15 CAD patients, were able to perform the LED feedback technique (P < 0.001). For normal subjects, image quality was similar between the three respiratory suppression techniques (P = 0.3), while for CAD patients there was an improvement in image quality, for images acquired during free respiration (breath-hold vs. free respiration, P < 0.01). There was no significant difference in the total acquisition times between the breath-hold and free respiration techniques (P = 0.2). There were substantial differences in the effectiveness of MRCA respiratory suppression techniques between normal subjects and CAD patients. In patients, only NE-gated MRCA performed well, requiring minimal cooperation with no increase in total acquisition time. Validation of NE-MRCA techniques should always be performed in patients, as well as normal subjects, to ensure correct evaluation of the technique for the target population.

Adult↗

PubMed1999

First-pass myocardial perfusion imaging and equilibrium signal changes using the intravascular contrast agent NC100150 injection.

In this phase I clinical study, the new ultrasmall superparamagnetic iron oxide contrast agent, NC100150 Injection (Nycomed AS, Oslo, Norway, a part of Nycomed Amersham), was assessed for first-pass magnetic resonance myocardial perfusion studies and its ability to produce equilibrium signal changes, as a possible indicator of myocardial blood volume. Data were acquired in 18 healthy male volunteers at 0.5 T and 1.5 T. At both field strengths, first-pass studies using T1-weighted sequences were acquired. Long TE spin-echo echoplanar imaging (EPI) was used at 0.5 T and short TE fast low-angle shot (FLASH) imaging at 1.5 T. With both sequences, T1 effects dominated the images for low doses, and time intensity curves potentially suitable for perfusion analysis were generated. At higher doses, T2 and T2* effects were observed. At 1.5 T, these predominantly affected the blood pool signal; however, at 0.5 T the myocardial signal was also involved, reflecting the relative T2 and T2* sensitivity of the spin-echo EPI sequence as a result of the long TE and long readout window, respectively. Equilibrium changes were assessed at both field strengths using T1-weighted FLASH sequences and in addition at 1.5 T using T2*-weighted gradient-echo EPI. With the T1-weighted images at both field strengths, signal changes were observed in all subjects; however, no dose-response relationship could be shown. With the T2*-weighted EPI there was significantly lower signal (P < 0.05) with the 3 and 4 mg/kg doses than with the 2 mg/kg dose. In conclusion, NC100150 Injection is useful for first-pass myocardial perfusion using T1-weighted sequences; however, low doses in combination with short TE sequences are required to minimize sensitivity to T2* effects. Equilibrium signal changes can also be induced in the myocardium. More work is required to optimize the imaging sequences and dose of NC100150 Injection for first-pass studies and also to determine whether the equilibrium signal changes can be used to measure myocardial blood volume changes in ischemic heart disease.

Adult↗

PubMed1999

Locally focused 3D coronary imaging using volume-selective RF excitation.

This paper describes a locally focused magnetic resonance (MR) imaging method for three-dimensional (3D) zonal echoplanar coronary angiography using volume-selective radiofrequency (RF) excitation. Spatially variable resolution was used for delineating coronary arteries and reducing the effect of residual signals caused by the imperfect excitation profile of the RF pulse. The use of variable resolution enabled the derivation of basis functions having different spatial characteristics pertaining to regional object details, and a significantly smaller number of phase-encoded signal measurements was needed for image reconstruction. Based on the relative significance of each required phase-encoding step, real-time phase-encode reordering was used to minimize the effect of respiratory motion during coronary imaging. To eliminate Nyquist ghosting in oblique echoplanar imaging, the echoplanar data acquired during forward and reverse echoes were reconstructed separately and then averaged with spatial registration for improving the signal-to-noise ratio. The technique was evaluated with phantom experiments and right coronary artery images of 11 asymptomatic volunteers using a 0.5 T MR system. A marked improvement in image quality has been achieved despite a 30% reduction in imaging time.

Artifacts↗

PubMed1999

Coronary artery imaging in a 0.5-Tesla scanner: implementation of real-time, navigator echo-controlled segmented k-space FLASH and interleaved-spiral sequences.

Coronary angiography techniques have been implemented on a 0.5-Tesla scanner with a view to performing coronary artery imaging. Slice-followed, segmented k-space FLASH sequences and interleaved-spiral sequences have been employed with acquisitions under real-time navigator echo control with patient feed back, enabling poor signal-to-noise levels to be overcome by averaging data acquired over multiple, variable-length, reproducible breath holds. Good-quality, millimetre-resolution coronary images were obtained in ten normal subjects with both techniques. The mean percent of data segments or interleaves acquired with the navigator echo within the 5-mm diaphragm acceptance window was 57% [standard deviation (S.D.), 11%; range, 38-85%], and the average image-acquisition times were 123+/-22 sec and 71+/-14 sec for segmented FLASH and interleaved-spiral imaging, respectively. In addition to shorter acquisition times, the interleaved-spiral sequence has superior temporal resolution, allowing the acquisition of limited, multislice data sets. However, the sequence is particularly sensitive to the off-resonance effects of residual epicardial fat surrounding the artery and to field nonuniformities, both of which lead to image blurring and, unlike segmented FLASH acquisitions (which are very robust), the spiral data sets generally require postprocessing.

Adult↗

PubMed1999

3D coronary artery imaging with phase reordering for improved scan efficiency.

Three-dimensional (3D) coronary imaging has the potential to overcome problems resulting from vessel tortuosity and to reduce partial volume effects. With these techniques, however, acquisition times are long and respiratory motion artifacts problematical. This work describes the development of a method that applies phase encode reordering to 3D acquisitions, allowing larger navigator acceptance windows to be used, with a consequent reduction in acquisition time. This method is compared with navigator acceptance window methods (the acceptance-rejection algorithm and the diminishing variance algorithm) and the retrospective respiratory gating technique, both in vitro and in vivo. The use of phase reordering with a 10 mm acceptance window provided a significant increase in scan efficiency over a non-reordered 5 mm method (P<0.001) with no significant change in image quality, and a significant increase in image quality compared with a non-reordered image acquired in the same time (P<0.05). A significant improvement in both image quality and scan efficiency was demonstrated over the retrospective respiratory gating method (P<0.05).

Adult↗

PubMed1999

Flow distortion and signal loss in spiral imaging.

The effect of in-plane motion on the point spread function (velocity PSF) in spiral imaging is studied experimentally and derived mathematically and is shown to consist of a smoothed, trailing edge and fringes around the leading edge. The velocity PSF remains largely in phase with the static PSF, consistent with the absence of signal loss by motion-related phase shifts in central k space. However, single-shot spiral imaging gives no clear improvement in complex and turbulent flow signal uniformity compared with echo-planar imaging with early, central k-space acquisition, which requires explanation given the spiral's earlier coverage of central k space. Alternate leading-edge fringes of the spiral's velocity PSF are in antiphase to the source, and cancellation may occur when these overlap other in-phase signals. Phase variations toward peripheral k space in turbulent flow also cause distortion. It is concluded that spiral imaging may lose complex and turbulent flow signals because of complex PSF distortion.

Algorithms↗

PubMed1999

Motion-selective encoding for fast cine imaging.

In magnetic resonance (MR) cine imaging applications, it is common that only a fraction of the field of view is used for delineating dynamic features whereas surrounding static structures are merely served as landmarks for three-dimensional orientation. This paper presents a method of using motion-selective encoding for fast cine imaging. By the use of a simple RF saturation pulse, the imaging field of view was divided into two areas, one with motion and the other without. This allowed the static material to be imaged by fast ungated acquisition and dynamic structures by reduced phase encoding. Issues related to phase error correction were addressed and the proposed algorithm was validated using images acquired from five normal subjects. The saving in imaging time was dependent on the size of the moving structures and a 35% reduction in imaging time did not cause visually noticeable changes in image quality. The application of the technique is limited to cine images with large surrounding static structures and a theoretical analysis of maximum savings in imaging time in relation to the size of the moving structures within the image field of view is provided. Magn Reson Med 42:430-435, 1999.

Adult↗

PubMed1998

FID-based lung MRI at 0.5 T: theoretical considerations and practical implications.

Pulse sequences based on FID signals and projection reconstruction (PR) were investigated for lung MRI at 0.5 T and evaluated for artifacts caused by: (1) k-space mismapping due to either delay or distortion of the readout gradient waveform, (2) cardiac motion and pulsatile flow, and (3) respiratory motion. Nonstructured artifacts were described, simulated, and experimentally confirmed for the first time. Nonstructured artifacts did not impair the demonstration of structures of high signal-to-noise ratio (SNR) but generated quantitative errors in the image intensity analysis over the lung parenchyma. The use of FID-based PR techniques for lung MRI is not justified at 0.5 T.

Artifacts↗

PubMed1998

Three-dimensional coronary MR angiography using zonal echo planar imaging.

Using an adapted two-dimensional spatially selective RF excitation scheme, a novel yet practical three-dimensional (3D) zonal echo-planar imaging technique for MR coronary angiography has been developed. The robustness of the technique compared with the two-dimensional (2D) segmented fast low angle shot (FLASH) method was evaluated using the right coronary artery images of 16 asymptomatic volunteers with a 0.5-T mobile scanner. Each 3D acquisition required multiple breath-holds, and real-time navigator echoes were used to ensure consistent breath-holding. Advantages of the technique include an improved signal-to-noise ratio, clearer depiction of tortuous coronary vessels due to decreased partial volume effects, and reduced motion blurring by the use of a short echo-planar readout.

Adult↗

PubMed1998

Motion and deformation tracking for short-axis echo-planar myocardial perfusion imaging.

The assessment of regional myocardial perfusion during the first-pass of a contrast agent bolus requires tracking of the signal time course for each myocardial segment so that a detailed perfusion map can be derived. To obtain such a map in practice, however, is not trivial because deformation of the shape of the myocardium and respiratory-induced motion render a major difficulty in this process. This study describes an automated approach for motion and deformation tracking of functional myocardial perfusion images. The effectiveness of the described method has been evaluated using a numerical phantom and results are compared with those from existing techniques which use deformable models. Preliminary results from applying our approach to 20 patients are discussed and compared with those from SPECT studies.

Aged↗

PubMed1997

Non-breath-hold lung magnetic resonance imaging with real-time navigation.

Magnetic resonance imaging (MRI) with navigating techniques based on consecutive breath-holds demand a level of respiratory control that is often beyond the capability of patients with lung disease. The objectives of this investigation were to develop and evaluate a navigating technique for lung MRI that does not rely on patient cooperation. Navigating techniques were implemented at 0.5 T using conventional imaging techniques of short echo-time and imaging during normal breathing in the diastolic phase of the cardiac cycle. A column of spins, orthogonal to the diaphragm, was excited both immediately before and after the imaging segment. These signals were processed in real time to provide the position of the lung-diaphragm interface. An imaging segment was considered correctly acquired only when the interface position was within the acceptance window both before and after the acquisition of the segment. A distribution of lung-diaphragm interface positions obtained during normal respiration was employed to define the acceptance window. In the case of multislice techniques, the position of the lung-diaphragm interface immediately before the imaging segment was also employed to decide which phase-encoding step to acquire next, therefore reducing the apparent frequency of the respiratory motion. A distribution of interface positions, updated in real time, served as a reference for the allocation of phase-encoding steps according to diaphragm position. The lung images obtained represent a significant advance in image quality, improving further the ability of MR to detect and monitor pulmonary disease. Motion artifacts were reduced, and images reliably demonstrated smaller vessels, which are not normally visible without navigation.

Computer Systems↗