Correction of spatial distortion in magnetic resonance angiography for radiosurgical treatment planning of cerebral arteriovenous malformations.

A treatment planning system based on magnetic resonance (MR) angiographic imaging data for the radiosurgery of inoperable cerebral arteriovenous malformations is reported. MR angiography was performed using a three-dimensional (3D) velocity-compensated fast imaging with steady-state precession (FISP) sequence. Depending on the individual MR system, inhomogeneities and nonlinearities induced by eddy currents during the pulse sequence can distort the images and produce spurious displacements of the stereotactic coordinates in both the x-y plane and the z axis. If necessary, these errors in position can be assessed by means of two phantoms placed within the stereotactic guidance system--a "2D-phantom" displaying "pincushion" distortion in the image, and a "3D-phantom" displaying displacement, warp, and tilt of the image plane itself. The pincushion distortion can be "corrected" (reducing displacements from 2-3 mm to 1 mm) by calculations based on modeling the distortion as a fourth order 2D polynomial. Displacement, warp, and tilt of the image plane may be corrected by adjustment of the gradient shimming currents. After correction, the accuracy of the geometric information is limited only by the pixel resolution of the image (= 1 mm). Precise definition of the target volume could be performed by the therapist either directly in the MR images or in calculated projection MR angiograms obtained by a maximum intensity projection algorithm. MR angiography provides a sensitive, noninvasive 3D method for defining target volume and critical structures, and for calculating precise dose distributions for radiosurgery of cerebral arteriovenous malformations.     

Aortoiliac imaging by projective phase sensitive MR angiography: effects of triggering and timing of data acquisition on image quality

To assess the ability of projective phase sensitive magnetic resonance (MR) angiography to visualize the aortoiliac vascular segment, and to determine the effects of triggering and timing of data acquisition om image quality, we studied 18 healthy volunteers, mean age 33.3 +/- 11 years, by color Doppler imaging and by MR angiography. MR angiography was performed at 1.5 T using a flow-adjustable gradient-echo (FLAG) sequence operated in both ECG-triggered and non-triggered acquisition modes. The images were graded in a blinded fashion by two independent observers. The data were analyzed using Pearson's chi-square analysis. Eighteen triggered time-resolved and 17 non-triggered, time-averaged MR angiograms consisting of 252 and 17 angiographic images, (AI) respectively, were analyzed. In the triggered mode 69 (27.4%) AI and in the non-triggered mode 2 (11.8%) AI were diagnostic. At least one triggered diagnostic AI was obtained in each subject. The image grades were not statistically different between observers (kappa = 0.6686). In the triggered mode diagnostic images were acquired within +/- 90 msec of the peak systolic flow velocity determined by Doppler. The proportion of diagnostic images in the triggered mode was highest (73.3%) within a 30-msec interval before the peak flow. In healthy subjects the aortoiliac segment is reliably visualized by FLAG MR angiography. The optimum results are achieved using the triggered acquisition mode and timing acquisition to the initial 180 msec of the abdominal aortic systolic flow pulse.     

Magnetic resonance angiographic demonstration of carotid-cavernous fistula using elliptical centric time resolved imaging of contrast kinetics (EC-TRICKS)

Magnetic resonance angiographic evaluation of the intracranial vasculature has been predominantly carried out using conventional angiographic techniques such as time of flight and phase contrast sequences. These techniques have good spatial resolution but lack temporal resolution. Newer faster angiographic techniques have been developed to circumvent this limitation. Elliptical centric time-resolved imaging of contrast kinetics (EC-TRICKS) is one such technique which has combined the use of elliptical centric ordering of the k-space with multiphase 3D digital subtraction MR angiogram (MRA) to achieve excellent temporal resolution of the arterial and venous circulations. Its applications have been mainly in the peripheral vasculature. We report the use of this technique in a case of a high-flow, direct carotid-cavernous fistula to demonstrate its potential in intracranial MR angiography.     

Cross-term-compensated pulsed-gradient stimulated echo MR with asymmetric gradient pulse lengths

The magic asymmetric gradient stimulated echo (MAGSTE) sequence developed to compensate background-gradient cross-terms in the preparation and readout interval independently, assumes identical lengths for the two gradient pulses applied in each interval. However, this approach is rather inefficient if some extra delay time is present in one half of an interval, e.g. as required for special RF excitations or spatial encoding prior to the stimulated echo in MR imaging. Therefore, a generalized version of the sequence is presented that considers different gradient pulse lengths within an interval. It is shown theoretically that (i) for any pulse lengths a "magic" amplitude ratio exists which ensures the desired cross-term compensation in each interval and that (ii) prolonging one of the gradients can deliver a considerably higher diffusion weighting efficiency. These results are confirmed in MR imaging experiments on phantoms and in vivo in the human brain at 3T using an echo-planar trajectory. In the examples shown, typically 10 times higher b values can be achieved or an echo time reduction with a 40% signal gain in brain white matter. Thus, in case of asymmetric timing requirements, the generalized MAGSTE sequence with different gradient pulse lengths may help to overcome signal-to-noise limitations in diffusion weighted MR.     

A post-processing technique for displaying vessels from routine fast-spin-echo images: MRI-derived angiography.

Fast-spin-echo magnetic resonance (MR) images are routine components of a standard MR brain examination. On these images, blood vessels are visible as black flow void. We report that by applying an enhancement filter to a stack of routine fast-spin-echo MR images, projected angiographic images can be generated. The vascular detail in the projected image is similar to that observed in a phase-contrast image. In addition to its advantage in obtaining vessel information from routine images, the proposed post-processing technique is fast, easy to implement and completely automatic. These images provide additional vessel information that is useful when MR angiography is unavailable or as an aid in planning dedicated MR angiographic studies.     

New technical developments in magnetic resonance imaging of epilepsy

Within the last several years a number of technical developments have been made in magnetic resonance imaging (MRI) that can potentially impact clinical and research MR imaging applications in epilepsy. These include developments in instrumentation and in pulse sequences. Advances in instrumentation include higher capacity gradient systems and multiple receiver coils as directed to brain imaging. Advances in pulse sequence include use of fast or turbo-spin-echo techniques, variants of echo-planar imaging, and sequences such as fluid-attenuation inversion recovery (FLAIR) targeted to specific applications of brain imaging. The purpose of this paper is to review several of these developments.     

MR imaging of coarctation of the aorta and its postoperative complications in adults: assessment with spin-echo and cine-MR imaging

To evaluate the ability of ECG-gated spin-echo (SE)-MR imaging vs. cine-MR imaging to assess coarctation of the aorta, 11 patients aged 15–45 years, with known or suspected coarctation of the aorta, and five patients suspected of re-stenosis or postoperative false-aneurysms after coarctation repair were examined by multisection SE-MR imaging and single-section multiphase cine-MR imaging on a 1.0 Tesla device. Aortography was performed in 15, and surgery in 14 of these 16 patients. Qualitatively, the location, severity, and length of the coarctation were shown in all cases with MR imaging, as well as the relationship with the arteries arising from the aortic arch. The respective sensitivities and specificities in the assessment of severity of stenoses were 86% and 100% for SE MR images, and 100% and 100% for cine-MR images. Cine-MR imaging was superior to SE imaging in stenosis diameter assessment with contrast angiography as reference, as well as to identify the site of leakage in cases of postoperative pseudoaneurysm. Pre-stenotic dilation or post-stenotic aneurysm, collateral channels, and associated malformations were better identified on SE images. Quantitatively, a better stenosis diameter correlation was found between cine-MR images and angiography than between SE-MR images and angiography (r = 0.99 vs. r = 0.78; p = 0.001 vs. p = 0.004), related to overestimation of stenoses with SE-MR imaging. The use of a combination of spin-echo and cine-MR imaging correlates well with conventional angiographic findings in this small series of patients with coarctation of aorta or postoperative pseudoaneurysmal complications. Cine-MRI can provide anatomic information that is equivalent to angiography for surgical planning.     

Generalized MAGSTE with bipolar diffusion-weighting gradient pulses

The generalized magic asymmetric gradient stimulated echo (generalized MAGSTE) sequence compensates background gradient cross-terms and can be adjusted to asymmetric timing boundary conditions which for instance are present in echo-planar MR imaging. However, its efficiency is not optimal because one of the two diffusion-weighting gradients applied in each interval usually must have a reduced amplitude to ensure the desired cross-term compensation. In this work, a modification of generalized MAGSTE is investigated where this gradient pulse is replaced by two gradient pulses with full amplitude but opposite polarities. It is shown that with these bipolar gradients (i) the sequence retains the cross-term compensation capability for an appropriate choice of the gradient pulse durations and (ii) the diffusion-weighting efficiency is improved, i.e. higher k and b values can be achieved without prolonging the echo time. These results are confirmed in MR imaging experiments on phantoms and in vivo in the human brain at 3 T using spin-echo and echo-planar MR imaging. In the examples shown, the b value could be increased between about 30% and 200% when using the bipolar gradient pulses. Thus, bipolar gradients may help to improve the applicability of the generalized MAGSTE sequence.     

Magnetic resonance imaging of a post-traumatic arteriovenous fistula in the lower extremity

Currently, magnetic resonance (MR) is most useful as a means of providing distinct images of gross abnormalities in major blood vessels. While new advances and further refinements will continue, MR has proven its value as a convenient and effective diagnostic tool for the recognition and delineation of vascular abnormalities, such as acquired arteriovenous fistulae (AVFs). We report a case of a chronic, post-traumatic AVF, in which MR angiography provided excellent anatomic detail and angiographic correlation.     

MR imaging and cervical fixation devices: Evaluation of ferromagnetism,heating, and artifacts at 1.5 tesla

The purpose of this study was to assess ferromagnetism, heating, and artifacts for cervical fixation devices exposed to a 1.5 T MR system. Cervical fixation devices (three halos, one tong and two halo vests) were evaluated for compatibility with MR procedures. Ferromagnetism was determined using a previously described technique. Heating was evaluated by measuring temperatures at various positions on the cervical fixation devices while applied to a volunteer subject before and during the use of various pulse sequences, including an magnetization transfer contrast (MTC) sequence. Artifacts associated with routine clinical MR imaging of the cervical spine were qualitatively evaluated with the cervical fixation devices applied to a volunteer subject. None of the devices displayed attraction to the magnetic field. The temperature changes were ±1.5°C in each instance. The MTC pulse sequence produced a sensation of “heating” the skull pins that may have been caused by vibration of the cervical fixation device. The MR images of the cervical spine were obtained without apparent artifacts using each routine, clinical pulse sequence. The lack of ferromagnetism, negligible heating, and capability of obtaining diagnostically acceptable studies of the cervical spine indicate that MR imaging performed at 1.5 T or less may be conducted safely in patients with each of the cervical fixation devices tested using conventional pulse sequences.     

Magnetic resonance imaging of the abdominal aorta and iliac vessels using combined 3-D gadolinium-enhanced MRA and gadolinium-enhanced fat-suppressed spoiled gradient echo sequences.

This study evaluates a combined protocol consisting of breath hold immediate post gadolinium 3-D gradient echo MR angiography and blood pool phase gadolinium-enhanced breath hold 2-D fat-suppressed spoiled gradient echo (SGE) sequences in the examination of diseases of the abdominal aorta and iliac vessels. Thirty-two patients with suspected disease of the abdominal aorta, major aortic branches, or iliac vessels underwent MR angiographic study from January 1996 to January 1997. Examinations were performed on a 1.5 T MR imager using 2-D axial SGE, coronal 3-D fast imaging in steady state precession (3-D FISP) following bolus administration of 40 mL of gadolinium, and axial and coronal blood pool phase gadolinium-enhanced fat-suppressed SGE. Post-processed data, including 3-D reconstructions using maximum intensity projection (MIP), targeted MIP, and multiplanar reconstruction (MPR) were evaluated. MR findings in all patients were correlated as follows: surgery (13 patients), angiography (11 patients), contrast enhanced CT (3 patients), non-contrast enhanced CT (1 patient), color doppler US (2 patients), and previous MR study (2 patients). MR findings correlated closely with findings at surgery or other imaging studies in 31 of 32 patients. One patient had renal artery occlusion that was misinterpreted as mild stenosis. The following vascular diseases were present: aneurysm disease [10 patients: aortic aneurysm (8 patients), inflammatory aneurysm (2 patients)], thoracoabdominal aortic dissection (2 patients), arteriovenous fistula (1 patient), stenoses and/or occlusion of the abdominal aorta, major aortic branches and iliac vessels [12 patients: stenoses and/or occlusion of the abdominal aorta with stenoses of the iliac vessels (9 patients), renal artery stenosis (2 patients), occlusion of the abdominal aorta (1 patient)], and occluded artery to pancreatic transplant artery (1 patient). Five patients had normal studies. The 3-D FISP technique accurately defined the luminal contours of vessels, allowing precise depiction of vessel stenosis (i.e., renal artery stenosis or common iliac artery stenosis) and clear demonstration of relationship of aortic branch vessels (i.e., renal arteries) to underlying aortic pathology (i.e., aortic aneurysm or dissection). Blood pool phase gadolinium-enhanced fat-suppressed SGE images were useful in the evaluation of the external surface of vessel walls, and providing accurate measurement of aneurysm diameter and other associated vascular entities (i.e., inflammatory aneurysm, left-sided IVC). Targeted MIP or MPR reconstruction were important for assessing stenoses of medium sized vessels such as renal arteries and branches of the iliac arteries, and for identifying accessory arteries. The combination of immediate post gadolinium 3-D FISP and blood pool phase gadolinium-enhanced fat-suppressed SGE is useful in the evaluation of the abdominal aorta, major aortic branches and iliac vessels. Immediate post gadolinium 3-D FISP images provides diagnostically useful information regarding vessel luminal contour, while blood pool phase gadolinium-enhanced fat-suppressed SGE provides ancillary information on the vessel wall and surrounding tissue.     

Simulating the effects of time-varying magnetic fields with a realistic simulated scanner

Transient magnetic fields induce changes in magnetic resonance (MR) images ranging from small, visually undetectable effects (caused, for instance, by neuronal currents) to more significant ones, such as those created by the gradient fields and eddy currents. Accurately simulating these effects may assist in correcting or optimising MR imaging for many applications (e.g., diffusion imaging, current density imaging, use of magnetic contrast agents, neuronal current imaging, etc.). Here we have extended an existing MR simulator (POSSUM) with a model for changing magnetic fields at a very high-resolution time-scale. This simulator captures a realistic range of scanner and physiological artifacts by modeling the scanner environment, pulse sequence details and subject properties (e.g., brain geometry and air-tissue boundaries).     

Validation of an adapted MRI pulse sequence for quantification of fatty infiltration in muscle

Purpose

The long-term goal of our research is to develop a patient-specific biomechanical model of the supraspinatus muscle and tendon for analyzing the effects of fatty infiltration and pennation angle changes. One input of the model will be the amount and distribution of fat within the muscle. Therefore, the objective of this project was to adapt and validate a novel magnetic resonance imaging pulse sequence for quantifying the extent of fatty infiltration for the supraspinatus muscle.

Methods

Samples of incremental combinations of muscle and fat from a cow were prepared and scanned with our quantitative MR technique. The MR results were compared to actual fat measurements taken using a Soxtec extraction analyzer.

Results

The MR fat quantification results closely match those values determined by Soxtec analysis (R²=.98, root mean square deviation=4.9).

Conclusions

Our MR fat quantification protocol can be used to accurately quantify fatty infiltration in skeletal muscle. This ability to evaluate skeletal muscle tissue noninvasively will improve the efficacy of evaluation of fatty degeneration in assessing tissue quality preoperatively. This can be important in surgical planning of any musculoskeletal repairs, particularly rotator cuff tears.     

Quantitative evaluation of optimal imaging parameters for single-cell detection in MRI using simulation

Super-paramagnetic iron oxide (SPIO) nanoparticles are actively investigated to enhance disease detection through molecular imaging using magnetic resonance imaging (MRI). Detection of the cells labeled by SPIO depends on the MRI protocols and pulse sequence parameters that can be optimized. To evaluate the sensitivity and specificity of the image acquisition methods and to obtain optimal imaging parameters for single-cell detection, we further developed an MRI simulator. The simulator models an object (tissue) at a microscopic level to evaluate effects of spatial distribution and concentration of nanoparticles on the resulting image. In this study, the simulator was used to evaluate and compare imaging of the labeled cells by the gradient-echo (GE), true-FISP [fast imaging employing steady-state acquisition (FIESTA)] and echo-planar imaging (EPI) pulse sequences. Effects of the imaging and object parameters, such as field strength, imaging protocol and pulse sequence parameters, imaging resolution, cell iron load, position of SPIO within the voxel and cell division within the voxel, were investigated in the work. The results suggest that true-FISP has the highest sensitivity for single-cell detection by MRI.     

Vascular access ports and catheters: Ex vivo testing of ferromagnetism,heating, and artifacts associated with MR imaging

The purpose of this study was to evaluate ferromagnetic qualities, heating, and artifacts associated with MR imaging of implantable vascular access ports (IVAPs, N = 9) and catheters (N = 8). Ferromagnetism was determined using previously described techniques. Heating was assessed for the IVAPs by measuring temperature immediately before and after performing a 3D GRASS, MTC pulse sequence for 60 min at an SAR of 2.8 W/kg. Artifacts were evaluated in association with the use of a fast GRASS pulse sequence and graded according to the severity of image distortion. None of the IVAPs or catheters were attracted by the magnetic field of the MR system. The largest temperature change measured was +0.3°C. Artifacts varied, depending on the component materials used for the construction of the IVAPs and catheters. The lack of ferromagnetic qualities and negligible heating indicates that MR imaging performed at 1.5 T or less may be conducted safely in patients with each of the IVAPs and catheters tested. None of the artifacts produced by the presence of the IVAPs or catheters is considered to impair the diagnostic aspects of MR imaging, especially if the device is not positioned directly in the imaging area of interest.     

Crossing fibers, diffractions and nonhomogeneous magnetic field: correction of artifacts by bipolar gradient pulses

In recent years, diffusion tensor imaging (DTI) and its variants have been used to describe fiber orientations and q-space diffusion MR was proposed as a means to obtain structural information on a micron scale. Therefore, there is an increasing need for complex phantoms with predictable microcharacteristics to challenge different indices extracted from the different diffusion MR techniques used. The present study examines the effect of diffusion pulse sequence on the signal decay and diffraction patterns observed in q-space diffusion MR performed on micron-scale phantoms of different geometries and homogeneities. We evaluated the effect of the pulse gradient stimulated-echo, the longitudinal eddy current delay (LED) and the bipolar LED (BPLED) pulse sequences. Interestingly, in the less homogeneous samples, the expected diffraction patterns were observed only when diffusion was measured with the BPLED sequence. We demonstrated the correction ability of bipolar diffusion gradients and showed that more accurate physical parameters are obtained when such a diffusion gradient scheme is used. These results suggest that bipolar gradient pulses may result in more accurate data if incorporated into conventional diffusion-weighted imaging and DTI.     

Optimization of the ultrafast look-locker echo-planar imaging T1 mapping sequence

The Look–Locker echo-planar imaging (LL-EPI) sequence has been numerically optimized in terms of the signal-to-noise ratio in the measured value of T₁, for both single-shot (repetition time (T_R) = ∞), and dynamically repeated T₁ measurements. The sequence is optimized for the normal biologic range of T₁ (0.2 s to 2.0 s) and for a range of sequence parameters found on most magnetic resonance (MR) scanners. Both linearly and geometrically spaced magnetization sample pulse intervals were considered. For single-shot measurements, the sequence with 24 linearly spaced sample pulses, an inversion time of 0.01 s, an inter-sample pulse delay of 0.10 s, and a sample radiofrequency (RF) pulse flip angle of 25^o was found to be optimum. When the number of sample pulses was limited due to hardware limitations, different pulse sequence parameters were indicated. The optimization procedures used are appropriate for any single-shot T₁ mapping sequence variant and for any rapid T₁ mapping application. The use of an optimized Look–Locker echo-planar imaging sequence is demonstrated by an example of dynamic contrast-enhanced scanning in the brain using fast T₁ mapping.     

Non-CPMG Fast Spin Echo with full signal

The standard Fast Spin Echo sequence used in MR imaging relies on the CPMG condition. A consequence of this condition is that only one component of the transverse magnetization can be measured. To counter this, some phase modulation schemes (XY, MLEV.) for the pulse train have been proposed, but they are useful only over a very restricted range, close to pi, of the refocusing pulse rotation angle. Some other solutions not relying on phase modulation have also been suggested, but they destroy one half the available signal. Revisiting the phase modulation approach, J. Murdoch ("Second SMR Scientific Meeting," p. 1145, 1994) suggested that a quadratic phase modulation could generate a train of classical echoes. We show here that indeed a quadratic phase modulation has a very suitable property: after an adequate change of frame, the dynamic of the system composed of all the protons situated in one pixel can be seen as stationary. If the parameter of the quadratic phase modulation is well chosen, it is then possible to put the dynamic system in a combination of two suitable states and obtain a signal identical to the signal of a classical spin echo, at least for nutation of the refocusing pulse higher than, approximately, two radians.     

Optimizing T2-weighted magnetic resonance sequences for surface coil microimaging of the eye with regard to lid, eyeball and head moving artifacts

PURPOSE: To acquire high-resolution magnetic resonance (MR) images, we developed a new blinking artifact reduced pulse (BARP) sequence with a surface coil specialized for microscopic imaging (47 mm in diameter). MATERIALS AND METHODS: To reduce eye movement, we ascertained that the subjects' eyes were kept open and fixated to the target in the 1.5-T MR gantry. To reduce motion artifacts from blinking, we inserted rest periods for blinking (1.5 s within every 5 s) during MR scanning (T2-weighted fast spin echo; repetition time, 5 s; echo time, 100 ms; echo train, 11; matrix, 256 x 128; field of view, 5 cm; 1-mm thickness x 30 slices). Three scans (100 s x 3) were performed for each normal subject, and they were added together after automatic adjustment for location to reduce quality loss caused by head motion. RESULTS: T2-weighted MR images were acquired with a high resolution and a high signal-to-noise ratio. Motion artifacts were reduced with BARP, as compared with those with random blinking. Intraocular structures such as the iris and ciliary muscles were clearly visualized. Because the whole eye can be covered with a 1-mm thickness by this method, three-dimensional maps can easily be generated from the obtained images. CONCLUSION: The application of BARP with a surface coil of the human eye might become a useful and widely adopted procedure for MR microimaging.     

Intracranial microvascular imaging at 7 T MRI with transceiver RF coils

Purpose

To investigate intracranial microvascular images with transceiver radio-frequency (RF) coils at ultra-high field 7 T magnetic resonance imaging (MRI).

Materials and methods

We designed several types of RF coils for the study of 7 T magnetic resonance angiography and analyzed quantitatively each coil's performance in terms of the signal-to-noise ratio (SNR) profiles to evaluate the usefulness of RF coils for microvascular imaging applications. We also obtained the microvascular images with different resolutions and parallel imaging technique.

Results

The overlapped 6-channel (ch) transceiver coil exhibited the highest performance for angiographic imaging. Although other multi-channel coils, such as 4- or 8-ch, were also suitable for fast imaging, these coils performed poorly in homogeneity or SNR for angiographic imaging. Furthermore, the 8-ch coil was poor in SNR at the center of the brain, while it had the highest SNR at the periphery.

Conclusion

The present study has demonstrated that the overlapped 6-ch coil with large-size loop coils provided the best performance for microvascular imaging or angiography with the ultra-high-field 7 T MRI, mainly because of its long penetration depth together with high SNR.