Multiple-readout selective inversion recovery angiography

Cervical spinal cord angiography and vessel-selective perfusion imaging in the rat

Arterial spin labeling (ASL) has been widely used to evaluate arterial blood and perfusion dynamics, particularly in the brain, but its application to the spinal cord has been limited. The purpose of this study was to optimize vessel-selective pseudocontinuous arterial spin labeling (pCASL) for angiographic and perfusion imaging of the rat cervical spinal cord. A pCASL preparation module was combined with a train of gradient echoes for dynamic angiography. The effects of the echo train flip angle, label duration, and a Cartesian or radial readout were compared to examine their effects on visualizing the segmental arteries and anterior spinal artery (ASA) that supply the spinal cord. Lastly, vessel-selective encoding with either vessel-encoded pCASL (VE-pCASL) or super-selective pCASL (SS-pCASL) were compared. Vascular territory maps were obtained with VE-pCASL perfusion imaging of the spinal cord, and the interanimal variability was evaluated. The results demonstrated that longer label durations (200 ms) resulted in greater signal-to-noise ratio in the vertebral arteries, improved the conspicuity of the ASA, and produced better quality maps of blood arrival times. Cartesian and radial readouts demonstrated similar image quality. Both VE-pCASL and SS-pCASL adequately labeled the right or left vertebral arteries, which revealed the interanimal variability in the segmental artery with variations in their location, number, and laterality. VE-pCASL also demonstrated unique interanimal variations in spinal cord perfusion with a right-sided dominance across the six animals. Vessel-selective pCASL successfully achieved visualization of the arterial inflow dynamics and corresponding perfusion territories of the spinal cord. These methodological developments provide unique insights into the interanimal variations in the arterial anatomy and dynamics of spinal cord perfusion.

Efficient 3D cone trajectory design for improved combined angiographic and perfusion imaging using arterial spin labeling

PURPOSE

To improve the spatial resolution and repeatability of a non-contrast MRI technique for simultaneous time resolved 3D angiography and perfusion imaging by developing an efficient 3D cone trajectory design.

METHODS

A novel parameterized 3D cone trajectory design incorporating the 3D golden angle was integrated into 4D combined angiography and perfusion using radial imaging and arterial spin labeling (CAPRIA) to achieve higher spatial resolution and sampling efficiency for both dynamic angiography and perfusion imaging with flexible spatiotemporal resolution. Numerical simulations and physical phantom scanning were used to optimize the cone design. Eight healthy volunteers were scanned to compare the original radial trajectory in 4D CAPRIA with our newly designed cone trajectory. A locally low rank reconstruction method was used to leverage the complementary k-space sampling across time.

RESULTS

The improved sampling in the periphery of k-space obtained with the optimized 3D cone trajectory resulted in improved spatial resolution compared with the radial trajectory in phantom scans. Improved vessel sharpness and perfusion visualization were also achieved in vivo. Less dephasing was observed in the angiograms because of the short TE of our cone trajectory and the improved k-space sampling efficiency also resulted in higher repeatability compared to the original radial approach.

CONCLUSION

The proposed 3D cone trajectory combined with 3D golden angle ordering resulted in improved spatial resolution and image quality for both angiography and perfusion imaging and could potentially benefit other applications that require an efficient sampling scheme with flexible spatial and temporal resolution.

Optimization of 4D combined angiography and perfusion using radial imaging and arterial spin labeling

PURPOSE

To extend and optimize a non-contrast MRI technique to obtain whole head 4D (time-resolved 3D) qualitative angiographic and perfusion images from a single scan.

METHODS

4D combined angiography and perfusion using radial imaging and arterial spin labeling (CAPRIA) uses pseudocontinuous labeling with a 3D golden ratio ("koosh ball") readout to continuously image the blood water as it travels through the arterial system and exchanges into the tissue. High spatial/temporal resolution angiograms and low spatial/temporal resolution perfusion images can be flexibly reconstructed from the same raw k-space data. Constant and variable flip angle (CFA and VFA, respectively) excitation schedules were optimized through simulations and tested in healthy volunteers. A conventional sensitivity encoding (SENSE) reconstruction was compared against a locally low rank (LLR) reconstruction, which leverages spatiotemporal correlations. Comparison was also made with time-matched time-of-flight angiography and multi-delay EPI perfusion images. Differences in image quality were assessed through split-scan repeatability.

RESULTS

The optimized VFA schedule (2-9°) resulted in a significant (p < 0.001) improvement in image quality (up to 84% vs. CFA), particularly for the lower SNR perfusion images. The LLR reconstruction provided effective denoising without biasing the signal timecourses, significantly improving angiographic and perfusion image quality and repeatability (up to 143%, p < 0.001). 4D CAPRIA performed well compared with time-of-flight angiography and had better perfusion signal repeatability than the EPI-based approach (p < 0.001).

CONCLUSION

4D CAPRIA optimized using a VFA schedule and LLR reconstruction can yield high quality whole head 4D angiograms and perfusion images from a single scan.

Time-encoded pseudo-continuous arterial spin labeling: Increasing SNR in ASL dynamic angiography

PURPOSE

Dynamic angiography using arterial spin labeling (ASL) can provide detailed hemodynamic information. However, the long time-resolved readouts require small flip angles to preserve ASL signal for later timepoints, limiting SNR. By using time-encoded ASL to generate temporal information, the readout can be shortened. Here, the SNR improvements from using larger flip angles, made possible by the shorter readout, are quantitatively investigated.

METHODS

The SNR of a conventional protocol with nine Look-Locker readouts and a 4 × $$ \times $$ 3 time-encoded protocol with three Look-Locker readouts (giving nine matched timepoints) were compared using simulations and in vivo data. Both protocols were compared using readouts with constant flip angles (CFAs) and variable flip angles (VFAs), where the VFA scheme was designed to produce a consistent ASL signal across readouts. Optimization of the background suppression to minimize physiological noise across readouts was also explored.

RESULTS

The time-encoded protocol increased in vivo SNR by 103% and 96% when using CFAs or VFAs, respectively. Use of VFAs improved SNR compared with CFAs by 25% and 21% for the conventional and time-encoded protocols, respectively. The VFA scheme also removed signal discontinuities in the time-encoded data. Preliminary data suggest that optimizing the background suppression could improve in vivo SNR by a further 16%.

CONCLUSIONS

Time encoding can be used to generate additional temporal information in ASL angiography. This enables the use of larger flip angles, which can double the SNR compared with a non-time-encoded protocol. The shortened time-encoded readout can also lead to improved background suppression, reducing physiological noise and further improving SNR.

Arterial spin labeling for the measurement of cerebral perfusion and angiography

Arterial spin labeling (ASL) is an MRI technique that was first proposed a quarter of a century ago. It offers the prospect of non-invasive quantitative measurement of cerebral perfusion, making it potentially very useful for research and clinical studies, particularly where multiple longitudinal measurements are required. However, it has suffered from a number of challenges, including a relatively low signal-to-noise ratio, and a confusing number of sequence variants, thus hindering its clinical uptake. Recently, however, there has been a consensus adoption of an accepted acquisition and analysis framework for ASL, and thus a better penetration onto clinical MRI scanners. Here, we review the basic concepts in ASL and describe the current state-of-the-art acquisition and analysis approaches, and the versatility of the method to perform both quantitative cerebral perfusion measurement, along with quantitative cerebral angiographic measurement.

Optimization of 4D vessel-selective arterial spin labeling angiography using balanced steady-state free precession and vessel-encoding

Vessel-selective dynamic angiograms provide a wealth of useful information about the anatomical and functional status of arteries, including information about collateral flow and blood supply to lesions. Conventional x-ray techniques are invasive and carry some risks to the patient, so non-invasive alternatives are desirable. Previously, non-contrast dynamic MRI angiograms based on arterial spin labeling (ASL) have been demonstrated using both spoiled gradient echo (SPGR) and balanced steady-state free precession (bSSFP) readout modules, but no direct comparison has been made, and bSSFP optimization over a long readout period has not been fully explored. In this study bSSFP and SPGR are theoretically and experimentally compared for dynamic ASL angiography. Unlike SPGR, bSSFP was found to have a very low ASL signal attenuation rate, even when a relatively large flip angle and short repetition time were used, leading to a threefold improvement in the measured signal-to-noise ratio (SNR) efficiency compared with SPGR. For vessel-selective applications, SNR efficiency can be further improved over single-artery labeling methods by using a vessel-encoded pseudo-continuous ASL (VEPCASL) approach. The combination of a VEPCASL preparation with a time-resolved bSSFP readout allowed the generation of four-dimensional (4D; time-resolved three-dimensional, 3D) vessel-selective cerebral angiograms in healthy volunteers with 59 ms temporal resolution. Good quality 4D angiograms were obtained in all subjects, providing comparable structural information to 3D time-of-flight images, as well as dynamic information and vessel selectivity, which was shown to be high. A rapid 1.5 min dynamic two-dimensional version of the sequence yielded similar image features and would be suitable for a busy clinical protocol. Preliminary experiments with bSSFP that included the extracranial vessels showed signal loss in regions of poor magnetic field homogeneity. However, for intracranial vessel-selective angiography, the proposed bSSFP VEPCASL sequence is highly SNR efficient and could provide useful information in a range of cerebrovascular diseases. © 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

A fast analysis method for non-invasive imaging of blood flow in individual cerebral arteries using vessel-encoded arterial spin labelling angiography

Arterial spin labelling (ASL) MRI offers a non-invasive means to create blood-borne contrast in vivo for dynamic angiographic imaging. By spatial modulation of the ASL process it is possible to uniquely label individual arteries over a series of measurements, allowing each to be separately identified in the resulting angiographic images. This separation requires appropriate analysis for which a general Bayesian framework has previously been proposed. Here this framework is adapted for clinical dynamic angiographic imaging. This specifically addresses the issues of computational speed of the algorithm and the robustness required to deal with real patient data. An algorithm is proposed that can incorporate planning information about the arteries being imaged whilst adapting for subsequent patient movement. A fast maximum a posteriori solution is adopted and shown to be only marginally less accurate than Monte Carlo sampling under simulation. The final algorithm is demonstrated on in vivo data with analysis on a time scale of the order of 10min, from both a healthy control and a patient with a vertebro-basilar occlusion.

Balanced SSFP transient imaging using variable flip angles for a predefined signal profile

Variable flip angles are used in steady-state free precession (SSFP) acquisitions (e.g., time-of-flight) but to a lesser extent than in spin echo acquisitions. In balanced steady-state free precession, imaging is often assumed to occur during the steady state, which has been well described in the literature. However, in many cases, imaging occurs during the transient stage, and the use of variable flip angles can improve signal and thus image quality. Here, we present the calculation of flip angles in transient balanced steady-state free precession to generate a predefined signal profile. The signal profile was iteratively optimized to maximize the integral of the signal versus time curve. The key contribution of this work is the formulation of the flip angle as a deterministic function of the preceding and desired magnetization. Catalyzation schemes, e.g., Kaiser-windowed ramp, can be combined with variable flip angles balanced steady-state free precession to reduce signal oscillations. A uniform signal profile was used as an example to demonstrate the variable flip angle algorithm. Accuracy of the algorithm and Bloch simulations were verified with MRI phantom acquisitions. Renal angiograms were acquired using an inflow-based balanced steady-state free precession MR angiography technique; improved small-vessel depiction was observed in volunteer examinations.

Vessel-encoded dynamic magnetic resonance angiography using arterial spin labeling

A new noninvasive MRI method for vessel selective angiography is presented. The technique combines vessel-encoded pseudocontinuous arterial spin labeling with a two-dimensional dynamic angiographic readout and was used to image the cerebral arteries in healthy volunteers. Time-of-flight angiograms were also acquired prior to vessel-selective dynamic angiography acquisitions in axial, coronal, and/or sagittal planes, using a 3-T MRI scanner. The latter consisted of a vessel-encoded pseudocontinuous arterial spin labeling pulse train of 300 or 1000 ms followed by a two-dimensional thick-slab flow-compensated fast low angle shot readout combined with a segmented Look-Locker sampling strategy (temporal resolution = 55 ms). Selective labeling was performed at the level of the neck to generate individual angiograms for both right and left internal carotid and vertebral arteries. Individual vessel angiograms were reconstructed using a bayesian inference method. The vessel-selective dynamic angiograms obtained were consistent with the time-of-flight images, and the longer of the two vessel-encoded pseudocontinuous arterial spin labeling pulse train durations tested (1000 ms) was found to give better distal vessel visibility. This technique provides highly selective angiograms quickly and noninvasively that could potentially be used in place of intra-arterial x-ray angiography for larger vessels.

Vessel-encoded dynamic magnetic resonance angiography using arterial spin labeling

A new noninvasive MRI method for vessel-selective angiography is presented. The technique combines vessel-encoded pseudocontinuous arterial spin labeling with a two-dimensional dynamic angiographic readout and was used to image the cerebral arteries in healthy volunteers. Time-of-flight angiograms were also acquired prior to vessel-selective dynamic angiography acquisitions in axial, coronal, and/or sagittal planes, using a 3-T MRI scanner. The latter consisted of a vessel-encoded pseudocontinuous arterial spin labeling pulse train of 300 or 1000 ms followed by a two-dimensional thick-slab flow-compensated fast low-angle shot readout combined with a segmented Look-Locker sampling strategy (temporal resolution = 55 ms). Selective labeling was performed at the level of the neck to generate individual angiograms for both right and left internal carotid and vertebral arteries. Individual vessel angiograms were reconstructed using a bayesian inference method. The vessel-selective dynamic angiograms obtained were consistent with the time-of-flight images, and the longer of the two vessel-encoded pseudocontinuous arterial spin labeling pulse train durations tested (1000 ms) was found to give better distal vessel visibility. This technique provides highly selective angiograms quickly and noninvasively that could potentially be used in place of intra-arterial x-ray angiography for larger vessels.

3D hyperpolarized He-3 MRI of ventilation using a multi-echo projection acquisition

A method is presented for high-resolution 3D imaging of the whole lung using inhaled hyperpolarized (HP) He-3 MR with multiple half-echo radial trajectories that can accelerate imaging through undersampling. A multiple half-echo radial trajectory can be used to reduce the level of artifact for undersampled 3D projection reconstruction (PR) imaging by increasing the amount of data acquired per unit time for HP He-3 lung imaging. The point spread functions (PSFs) for breath-held He-3 MRI using multiple half-echo trajectories were evaluated using simulations to predict the effects of T(2)* and gas diffusion on image quality. Results from PSF simulations were consistent with imaging results in volunteer studies showing improved image quality with increasing number of echoes using up to 8 half-echoes. The 8-half-echo acquisition is shown to accommodate lost breath-holds as short as 6 sec using a retrospective reconstruction at reduced resolution and also to allow reduced breath-hold time compared with an equivalent Cartesian trajectory. Furthermore, preliminary results from a 3D dynamic inhalation-exhalation maneuver are demonstrated using the 8-half-echo trajectory. Results demonstrate the first high-resolution 3D PR imaging of ventilation and respiratory dynamics in humans using HP He-3 MR.

Multimodale MR-Diagnostik arteriovenöser Malformationen

Arteriovenous malformations (AVM) are complex inborn malformations of the vascular system. In the brain they can lead to severe complications, such as parenchymal haemorrhaging, epileptic seizures or neurologic failure, which necessitates a safe diagnosis and therapy. In addition to conventional angiography, magnetic resonance imaging (MRI) and MR angiography (MRA) have now been established as the gold standard diagnostic procedures. Using MRA the angioarchitecture of malformations can be captured and with a parallel imaging of the parenchyme the method is very well suited for therapy planning and monitoring. This review summarizes the present day possibilities of multimodal MRT diagnosis of AVM and describes the purely morphologic as well as the physiologic and pathophysiologic characteristics. The various MR angiographic techniques will be firstly described with respect to the basic technical principles and the results obtained so far will be summarized. The principle of functional MRT will be described and the diagnostic possibilities with respect to AVM will be discussed.

Combining spin echoes with gradient echoes in the context of the global coherent free precession pulse sequence

To extend the signal longevity of magnetically excited spins in flowing fluids while in a state of global coherent free precession (GCFP), a refocusing radiofrequency (RF) pulse and bipolar gradient waveforms were combined with the GCFP sequence. The data demonstrate that RF refocusing in the presence of flowing blood is possible, but the improvement in signal amplitude depends on the static magnetic field homogeneity along the direction of motion and the displacement of the spins between the excitation and the RF refocusing pulse, as well as displacement during subsequent RF refocusing pulses. The least amount of phase dispersion and thus the longest lasting signal is obtained with the shortest echo spacing where only one line of data is recorded between two RF refocusing pulses. This approach was successfully used in a phantom and in vivo to image fast and slow blood flow. Depending on the experimental conditions, signal persistence is improved significantly compared to playing the same sequence without RF refocusing, but the improvement is limited by the product of blood flow velocity and the time between RF refocusing pulses.

Non-invasive visualization of collateral blood flow patterns of the circle of Willis by dynamic MR angiography

The circle of Willis plays an important role in the distribution of blood flow in the brain. To obtain dynamic information of the blood flow through the circle of Willis, a dynamic MRA technique based on arterial spin labeling (ASL) is introduced as a non-invasive technique. When the ASL labeling slab is restricted to a single artery, it is possible to visualize selectively the flow distribution of that specific artery. However, because of the decay of the label and the presence of noise it is difficult to extract functional information from these images. In the present study we propose three visualization and post-processing methods for the interpretation of these images. Firstly, the passage of labeled blood was corrected for decay of the label and hereafter shown as a movie. Secondly, by calculating the time of arrival at every location in the arteries of the circle of Willis, a 2D image was reconstructed summarizing the information of the movie. Finally, quantitative flow values were obtained by relating the arterial input function to the passage of labeled blood through a region of interest encompassing the vessel under investigation. Experiments in a circle of Willis phantom showed a high linear relation between measured flow and true flow, although the measured values were 10-15% lower than the true flow values. Measurements in healthy volunteers showed the potential to quantify the flow in all major arteries of the circle of Willis.

Renal arteries: navigator-gated balanced fast field-echo projection MR angiography with aortic spin labeling: initial experience

A cardiac-triggered free-breathing three-dimensional balanced fast field-echo projection magnetic resonance (MR) angiographic sequence with a two-dimensional pencil-beam aortic labeling pulse was developed for the renal arteries. For data acquisition during free breathing in eight healthy adults and seven consecutive patients with renal artery disease, real-time navigator technology was implemented. This technique allows high-spatial-resolution and high-contrast renal MR angiography and visualization of renal artery stenosis without exogenous contrast agent or breath hold. Initial promising results warrant larger clinical studies.

Comparison of fat suppression strategies in 3D spiral coronary magnetic resonance angiography

PURPOSE

In the present study, the impact of the two different fat suppression techniques was investigated for free breathing 3D spiral coronary magnetic resonance angiography (MRA). As the coronary arteries are embedded in epicardial fat and are adjacent to myocardial tissue, magnetization preparation such as T(2)-preparation and fat suppression is essential for coronary discrimination.

MATERIALS AND METHODS

Fat-signal suppression in three-dimensional (3D) thin- slab coronary MRA based on a spiral k-space data acquisition can either be achieved by signal pre-saturation using a spectrally selective inversion recovery pre-pulse or by spectral-spatial excitation. In the present study, the performance of the two different approaches was studied in healthy subjects.

RESULTS

No significant objective or subjective difference was found between the two fat suppression approaches.

CONCLUSION

Spectral pre-saturation seems preferred for coronary MRA applications due to the ease of implementation and the shorter cardiac acquisition window.

Selective three-dimensional visualization of the coronary arterial lumen using arterial spin tagging

Conventional coronary magnetic resonance angiography (MRA) techniques display the coronary blood-pool along with the surrounding structures, including the myocardium, the ventricular and atrial blood-pool, and the great vessels. This representation of the coronary lumen is not directly analogous to the information provided by x-ray coronary angiography, in which the coronary lumen displayed by iodinated contrast agent is seen. Analogous "luminographic" data may be obtained using MR arterial spin tagging (projection coronary MRA) techniques. Such an approach was implemented using a 2D selective "pencil" excitation for aortic spin tagging in concert with a 3D interleaved segmented spiral imaging sequence with free-breathing, and real-time navigator technology. This technique allows for selective 3D visualization of the coronary lumen blood-pool, while signal from the surrounding structures is suppressed.

Direct comparison of 3D spiral vs. Cartesian gradient-echo coronary magnetic resonance angiography

While 3D thin-slab coronary magnetic resonance angiography (MRA) has traditionally been performed using a Cartesian acquisition scheme, spiral k-space data acquisition offers several potential advantages. However, these strategies have not been directly compared in the same subjects using similar methodologies. Thus, in the present study a comparison was made between 3D coronary MRA using Cartesian segmented k-space gradient-echo and spiral k-space data acquisition schemes. In both approaches the same spatial resolution was used and data were acquired during free breathing using navigator gating and prospective slice tracking. Magnetization preparation (T(2) preparation and fat suppression) was applied to increase the contrast. For spiral imaging two different examinations were performed, using one or two spiral interleaves, during each R-R interval. Spiral acquisitions were found to be superior to the Cartesian scheme with respect to the signal-to-noise ratio (SNR) and contrast-to-noise-ratio (CNR) (both P < 0.001) and image quality. The single spiral per R-R interval acquisition had the same total scan duration as the Cartesian acquisition, but the single spiral had the best image quality and a 2.6-fold increase in SNR. The double-interleaf spiral approach showed a 50% reduction in scanning time, a 1.8-fold increase in SNR, and similar image quality when compared to the standard Cartesian approach. Spiral 3D coronary MRA appears to be preferable to the Cartesian scheme. The increase in SNR may be "traded" for either shorter scanning times using multiple consecutive spiral interleaves, or for enhanced spatial resolution.

Three-dimensional flow-independent peripheral angiography

A magnetization-prepared sequence, T2-Prep-IR, exploits T1, T2, and chemical shift differences to suppress background tissues relative to arterial blood. The resulting flow-independent angiograms depict vessels with any orientation and flow velocity. No extrinsic contrast agent is required. Muscle is the dominant source of background signal in normal volunteers. However, long-T2 deep venous blood and nonvascular fluids such as edema also contribute background signal in some patients. Three sets of imaging parameters are described to address patient-specific contrast requirements. A rapid, spiral-based, three-dimensional readout is utilized to generate high-resolution angiograms of the lower extremities. Comparisons with x-ray angiography and two-dimensional time-of-flight angiography indicate that this flow-independent technique has unique capabilities to accurately depict stenoses and to visualize slow flow and in-plane vessels.

Magnetic resonance angiography

Magnetic resonance angiography (MRA) permits the non-invasive visualization of blood flow through the effects of moving spins on the magnetic resonance signal. MRA techniques can be divided into two main classifications depending upon the primary effect responsible for contrast in the image. Angiograms can be produced using either the time-of-flight (TOF) or phase contrast (PC) methods. Each method has particular advantages and limitations as an angiographic imaging technique and these are reflected in their respective applications. This review article is intended to outline the scientific and technical development of MRA from its basis in the earliest in vitro nuclear magnetic resonance (NMR) experiments, through the implementation of in vivo angiographic techniques on whole body MRI systems, to the recent rapid expansion in MRA acquisition and processing techniques.

3D MR angiography of pulmonary arteries using real-time navigator gating and magnetization preparation

An ECG-triggered magnetization-prepared segmented 3D fast gradient echo sequence was developed to perform pulmonary arterial MR angiography. A selective inversion recovery pulse was used in the magnetization preparation to suppress venous vasculature. A real-time gating technique based on navigator echoes was implemented to reduce respiration effects. Pencil-beam navigator echoes were acquired immediately before and after the readout train and processed in real-time to dynamically measure the diaphragm position, which was used to control data acquisition with an accept-or-reject-reacquire logic. In a study of 10 volunteers, a gated 3D acquisition with 28 slices required on average approximately 4 min of acquisition time, and six to seven segmental arteries related to the interlobar trunk of the pulmonary artery were depicted. The use of SIR pulse reduced venous signal by 99%. The gated acquisitions were superior to the ungated acquisitions (n = 10, P < 0.005). The real-time navigator gating technique is effective for reduction of respiration effects and thereby makes high resolution 3D MRA of the pulmonary arteries feasible.

Cerebral arteriovenous malformations: improved nidus demarcation by means of dynamic tagging MR-angiography

Our purpose was to further improve the target volume definition for radiosurgical treatment of cerebral arteriovenous malformations (AVMs) by means of dynamic MRA (dMRA) using a blood bolus tagging sequence. We therefore compare this technique with 3D-TOF-MRA and transfemoral high resolution angiography in plain film technique. Twenty patients with angiographically proven cerebral AVMs were investigated by dMRA, TOF-MRA, and conventional angiography during the MR-assisted radiosurgical planning protocol. The patient's head was fixed in an MR-compatible stereotactic device. The different angiography techniques were evaluated by consensus of two radiologists. AVMs were characterized by the number and origin of feeding arteries, the maximum diameter of the AVM nidus, and the venous drainage pattern. Dynamic MRA was able to demonstrate the complete AVM characteristics and hemodynamics in 12 out of 20 patients. In three patients with an AVM nidus smaller than 1 cm in diameter the technique could not reliably depict the malformation. Technical problems due to steel screws and pins in the initially used stereotactic frame occurred in five patients. Due to reduced vessel overlap and the lack of disturbances caused by formations with short T1 time, dMRA was superior to TOF-MRA in the detection and the exact localization of the AVM nidus in four patients. We conclude that dMRA is able to demonstrate reliably AVM characteristics and hemodynamics in AVMs with a nidus larger than 1 cm in diameter. Because of the improved demarcation of the AVM nidus, this technique may be a valuable adjunct to radiosurgery planning of cerebral AVMs.

Nonlinear excitation profiles for three-dimensional inflow MR angiography

An RF excitation pulse for three-dimensional (3D) time-of-flight (TOF) MR angiography (MRA) with a nonlinear excitation profile was numerically calculated under the condition of uniform vessel signal across the excitation volume (slab), and the superiority of the optform pulse as compared with conventional RF pulses and TONE pulses was demonstrated. For this purpose we acquired MRA of the lower leg and of the carotid and vertebral arteries in a 30-year-old healthy volunteer. Although the flow velocity ranges in these two anatomic locations are different by about a factor of 10, in both cases the corresponding optform pulse provided the best signal homogeneity at the highest level.

Multi-shot EPI for improvement of myocardial tag contrast: comparison with segmented SPGR

To assess the potential value of multi-shot EPI relative to segmented k-space SPGR for myocardial tagging, we measured tag contrast for both sequences in a phantom and human study and compared it with theoretical predictions. In the human heart, EPI tag contrast was three times that of SPGR at the end of systole. Tag duration was lengthened with EPI to at least 600 ms. In addition, the entire heart was examined in a total of 32 heartbeats with EPI versus 152 heartbeats with SPGR.

Optimization of parameter values for complex pulse sequences by simulated annealing: application to 3D MP-RAGE imaging of the brain

A number of pulse sequence techniques, including magnetization-prepared gradient echo (MP-GRE), segmented GRE, and hybrid RARE, employ a relatively large number of variable pulse sequence parameters and acquire the image data during a transient signal evolution. These sequences have recently been proposed and/or used for clinical applications in the brain, spine, liver, and coronary arteries. Thus, the need for a method of deriving optimal pulse sequence parameter values for this class of sequences now exists. Due to the complexity of these sequences, conventional optimization approaches, such as applying differential calculus to signal difference equations, are inadequate. We have developed a general framework for adapting the simulated annealing algorithm to pulse sequence parameter value optimization, and applied this framework to the specific case of optimizing the white matter-gray matter signal difference for a T1-weighted variable flip angle 3D MP-RAGE sequence. Using our algorithm, the values of 35 sequence parameters, including the magnetization-preparation RF pulse flip angle and delay time, 32 flip angles in the variable flip angle gradient-echo acquisition sequence, and the magnetization recovery time, were derived. Optimized 3D MP-RAGE achieved up to a 130% increase in white matter-gray matter signal difference compared with optimized 3D RF-spoiled FLASH with the same total acquisition time. The simulated annealing approach was effective at deriving optimal parameter values for a specific 3D MP-RAGE imaging objective, and may be useful for other imaging objectives and sequences in this general class.

Magnetic resonance imaging and spectroscopy of the human heart

Magnetic resonance imaging and spectroscopy have a great potential both for clinical cardiac diagnostics and for research in cardiac physiology, metabolism and disease. At the present time, cardiac MRI already is the method of choice in several clinical conditions, especially in imaging central vasculature and intra- and paracardiac masses. With the recent development of contrast agents and ability to measure both flow velocities and flow volume, the cardiac MRI is likely to have a profound role in evaluating coronary arterial disease as well as valvular heart disease. The limitations due to long imaging times of cardiac MRI-studies are likely to be overcome with the development of ultrafast imaging techniques in the near future. On the other hand, cardiac MRS is still a research tool, which needs technical improvements before it can be widely utilized in clinical work. However, attempts to this aim are highly justified, when the possibility that MRS will provide metabolic information of the heart is considered and bearing in mind, that MR-magnets with sufficient field strength for MRS are increasingly in use in most modern hospitals. The role of magnetic resonance imaging (MRI) and spectroscopy (MRS) in the evaluation of heart diseases is still evolving. Some clear indications for clinical use of cardiac MRI have already become apparent, whereas cardiac MRS is still confined to research applications. The current paper consists of a review of the role of MRI for cardiovascular diagnosis together with a review of the currents status of cardiac MRS.

Shaping the signal response during the approach to steady state in three-dimensional magnetization-prepared rapid gradient-echo imaging using variable flip angles

A theoretical algorithm for shaping the signal response during the approach to steady state in three-dimensional magnetization-prepared rapid gradient-echo (3D MP-RAGE) pulse sequences has been developed and implemented. This algorithm derives the flip angle series required to produce specifically chosen time evolutions of the signal intensities during the data acquisition segment of 3D MP-RAGE sequences. Theoretical predictions for the cases of unshaped, uniform, and mono-exponential decay signal responses were quantitatively validated with a doped-water phantom on a 1.5-T whole-body imager and in all cases there was excellent agreement between the theoretical and experimental values. The effects of RF inhomogeneities and eddy currents on the signal response shaping were also investigated. To demonstrate the potential utility of the technique, the signal response shaping algorithm was applied to a T1-weighted 3D MP-RAGE sequence to derive the acquisition flip angle series which theoretically yields the maximum white matter/gray matter signal difference (WGSD) consistent with the chosen response shape. Images obtained from a healthy volunteer using this variable flip angle sequence were compared with 3D RF-spoiled steady-state gradient-echo images obtained in the same total imaging time. The 3D MP-RAGE images demonstrated a 41% increase in the WGSD-to-noise ratio. These initial very promising results indicate that with further refinement to eliminate some intensity artifacts, the variable flip angle 3D MP-RAGE technique may, with respect to certain image properties, provide considerable improvements over currently available 3D gradient-echo imaging techniques.

Two-dimensional selective adiabatic pulses

Using the technique of separable k-space excitation, we have designed a two-dimensional selective adiabatic pulse that inverts magnetization from a square region in the xy plane with insensitivity to RF variations. We also have designed a two-dimensional adiabatic pulse that inverts selectively in frequency and in one spatial dimension. The pulses should be useful for both MR imaging and spectroscopy. We present experimental results to demonstrate that the two-dimensional adiabatic pulses are feasible on commercial MR imaging systems.

Fast angiography using selective inversion recovery

We have developed an enhancement of selective inversion recovery that allows us to obtain high-resolution angiograms in reduced scan time. By applying several read pulses following each tagging inversion pulse, we can obtain several phase encodes in each cardiac cycle, thereby reducing the total scan time required for a complete image. Using this technique, high-resolution angiograms can be obtained in as little as 15 s. Because the phase encodes are collected in short bursts separated by long pauses, care must be taken to maintain uniform signal weighting across phase-encoding views and avoid ghosting. We use an increasing flip-angle sequence to equalize signal level weighting across the readouts. The phase encodes are collected in a special order to minimize ghosting. A postprocessing technique is used to further reduce signal nonuniformity between phase encodes. This fast angiography technique can significantly reduce artifacts due to patient motion during scanning and is especially useful for imaging vasculature in regions of the body where respiratory motion is a problem.

Multiple inversion recovery reduces static tissue signal in angiograms

Spin label angiography compares two images by subtraction. The first is obtained after blood in one region is labeled by inversion and flows into a region of interest. Labeling is not used for the second image, so only labeled blood remains in the final angiogram after subtraction. This subtraction is never perfect, but with starting images containing less static tissue signal, the remaining background can be reduced. This can be achieved by observing at the zero crossing following an inversion. Multiple inversions allow one to null the signal from tissues with differing T1 simultaneously. We present equations and sample calculations for inversion times and demonstrate the resistance to subject motions (peristalsis, breathing, speaking) resulting from two inversions. Adequate suppression of static tissue signal allows one to dispense with labeling and subtraction, halving the minimum time needed to acquire an image.

Coronary angiography using fast selective inversion recovery

Using a fast version of selective inversion recovery, we have obtained coronary angiograms of normal volunteers showing the proximal portions of the left coronary artery. Blood is tagged in the aortic root at end systole using a 2D inversion pulse. After a wash-in time of 300-600 ms, the coronary vessels are imaged with a 2- to 3-cm-thick slab in either axial or oblique projection. The scan is completed within a breathhold in 24 heartbeats.

Object-based 3-D reconstruction of arterial trees from magnetic resonance angiograms

By exploiting a priori knowledge of arterial shape and smoothness, subpixel accuracy reconstructions are achieved from only four noisy projection images. The method incorporates a priori knowledge of the structure of branching arteries into a natural optimality criterion that encompasses the entire arterial tree. An efficient optimization algorithm for object estimation is presented, and its performance on simulated, phantom, and in vivo magnetic resonance angiograms is demonstrated. It is shown that accurate reconstruction of bifurcations is achievable with parametric models.