FLAIR imaging using nonselective inversion pulses combined with slice excitation order cycling and k-space reordering to reduce flow artifacts

Advanced design of MRI inversion pulses for inhomogeneous field conditions by optimal control

Non-selective inversion pulses find widespread use in MRI applications, where requirements on them are increasingly demanding. With the use of high and ultra-high field strength systems, robustness to Δ B 0 and B 1 + inhomogeneities, while tackling SAR and hardware limitations, has rapidly become important. In this work, we propose a time-optimal control framework for the optimization of Δ B 0 - and B 1 + -robust inversion pulses. Robustness is addressed by means of ensemble formulations, while allowing inclusion of hardware and energy limitations. The framework is flexible and performs excellently for various optimization goals. The optimization results are analyzed extensively in numerical experiments. Furthermore, they are validated, and compared with adiabatic RF pulses, in various phantom and in vivo measurements on a 3 T MRI system.

Clinical Application of Diagnostic Imaging of Chiari-Like Malformation and Syringomyelia

Chiari-like malformation (CM) and syringomyelia (SM) is a frequent diagnosis in predisposed brachycephalic toy breeds since increased availability of MRI. However, the relevance of that MRI diagnosis has been questioned as CM, defined as identification of a cerebellar herniation, is ubiquitous in some breeds and SM can be asymptomatic. This article reviews the current knowledge of neuroanatomical changes in symptomatic CM and SM and diagnostic imaging modalities used for the clinical diagnosis of CM-pain or myelopathy related to SM. Although often compared to Chiari type I malformation in humans, canine CM-pain and SM is more comparable to complex craniosynostosis syndromes (i.e., premature fusion of multiple skull sutures) characterized by a short skull (cranial) base, rostrotentorial crowding with rostral forebrain flattening, small, and ventrally orientated olfactory bulbs, displacement of the neural tissue to give increased height of the cranium and further reduction of the functional caudotentorial space with hindbrain herniation. MRI may further reveal changes suggesting raised intracranial pressure such as loss of sulci definition in conjunction with ventriculomegaly. In addition to these brachycephalic changes, dogs with SM are more likely to have craniocervical junction abnormalities including rostral displacement of the axis and atlas with increased odontoid angulation causing craniospinal junction deformation and medulla oblongata elevation. Symptomatic SM is diagnosed on the basis of signs of myelopathy and presence of a large syrinx that is consistent with the neuro-localization. The imaging protocol should establish the longitudinal and transverse extent of the spinal cord involvement by the syrinx. Phantom scratching and cervicotorticollis are associated with large mid-cervical syringes that extend to the superficial dorsal horn. If the cause of CSF channel disruption and syringomyelia is not revealed by anatomical MRI then other imaging modalities may be appropriate with radiography or CT for any associated vertebral abnormalities.

Intraventricular cerebrospinal fluid pulsation artifacts on low-field magnetic resonance imaging: Potential pitfall in diagnosis?

Background:

Intraventricular cerebrospinal fluid (CSF) pulsation artifact can pose a diagnostic problem in fluid-attenuated inversion recovery (FLAIR) brain magnetic resonance images (MRI) appearing as intraventricular hyperintensity. The extent of this challenge among radiologists in Africa using low-field MRI systems is relatively sparsely documented in the literature. The purpose of this study was to identify the presence and frequency of ventricular CSF pulsation artifact (VCSFA) on FLAIR axial brain images with a low-field MR system.

Materials and Methods:

FLAIR axial images were obtained on a low-field 0.3T unit (6000 ms/108 ms/2 [repetition time/echo time/excitations], inversion time = 1700 ms, field of view = 28 cm, matrix = 195 × 256, and 6 mm contiguous sections). Two experienced radiologists independently rated VCSFA in the lateral, third, and fourth ventricles in 202 consecutive patients (age range 1–100 years) referred for brain MR for various indications. We reviewed the pattern of artifacts, to determine its relationship to age, gender, and third ventricular size.

Results:

The low-field FLAIR MR brain images of 33 patients (16.3%) showed VCSFA in at least one ventricular cavity. The fourth ventricle was the most common site of VCSFA (n = 10), followed by the third ventricle (n = 8) and the lateral ventricles (n = 7). Eight patients had VCSFA in multiple locations, one of them in all ventricles. A smaller third ventricular size and, to a lesser extent, younger age was significantly associated with VCSFA. CSF Pulsation of VCSFA did not occur across the brain parenchyma in the phase encoding direction.

Conclusion:

VCSFA may mimic pathology on low-field axial FLAIR brain images and are more common in young patients with smaller ventricular size. Although these artifacts are less frequently observed at lower magnetic field strengths, their recognition on low-field MRI systems is important in avoiding a misdiagnosis.

3D fluid-attenuated inversion recovery imaging: reduced CSF artifacts and enhanced sensitivity and specificity for subarachnoid hemorrhage

BACKGROUND AND PURPOSE

FLAIR images are highly sensitive for SAH. However, CSF flow artifacts caused by conventional FLAIR can produce false-positive results. Here, we compare 3D and 3D FLAIR sequences, focusing on their potential for containing these artifacts and their sensitivity and specificity for detection of SAHs.

MATERIALS AND METHODS

We evaluated the following 4 FLAIR sequences: 1) 2D FLAIR at 1.5T, 2) 2D FLAIR, 3) 2D PROPELLER-FLAIR, and 4) 3D Cube-FLAIR at 3T. All sequences were performed in 5 healthy volunteers; sequences 2 and 4 were also performed under routine conditions in 10 patients with focal epilepsy and in 10 patients with SAH. Two neuroradiologists independently conducted the analysis. The presence of flow artifacts in the ventricles and cisterns of healthy volunteers and patients with epilepsy was evaluated and scored on a 4-point scale. Mean values were calculated and compared by using paired t tests. Sensitivity and specificity for SAH detection in sequences 2 and 4 were determined.

RESULTS

Cube-FLAIR showed almost no CSF artifacts in the volunteers and the patients with epilepsy; therefore, it was superior to any other FLAIR (P < .001). Sensitivity and specificity of SAH detection by 3T FLAIR were 58.3% and 89.4%, respectively, whereas Cube-FLAIR had a sensitivity of 95% and a specificity of 100%.

CONCLUSIONS

Cube-FLAIR allows FLAIR imaging with almost no CSF artifacts and is, thus, particularly useful for SAH detection.

Comparison of flow artifacts between 2D-FLAIR and 3D-FLAIR sequences at 3 T

It has been reported that 3D-FLAIR can reduce the flow artifact resulting from cerebrospinal fluid (CSF) at 1.5 T compared to 2D-FLAIR. Flow-related artifacts tend to be worse at 3 T than at 1.5 T. The purpose of this study was to compare the CSF flow artifacts of 2D-FLAIR and 3D-FLAIR sequences at 3 T in eight healthy volunteers. The grade of CSF-related artifacts were scored through observing the perimedullary cistern, cerebellopontine angle cisterns, fourth ventricule, prepontine cistern, suprasellar cistern, ambient cisterns, sylvian fissures, third ventricle and lateral ventricles. Grading was performed on either axial or sagittal images. The CSF in-flow artifact scores were significantly higher on axial 2D-FLAIR than on axial 3D-FLAIR MPR images in all areas except the bilateral sylvian fissures, and higher on sagittal 2D-FLAIR than on sagittal 3D-FLAIR MPR images in perimedullary, bilateral CP angle and suprasellar cisterns. The CSF-related flow artifacts were significantly reduced by 3D-FLAIR, while structures in the cistern were depicted more clearly, even at 3 T. Further study is necessary to compare the clinical efficacy between 2D-FLAIR and 3D-FLAIR in depicting subtle abnormalities.

FASCINATE: A pulse sequence for simultaneous acquisition ofT2-weighted and fluid-attenuated images

A pulse sequence that enables simultaneous acquisition of T2-weighted and fluid-attenuated images is presented. This sequence is referred to as FASCINATE (Fluid-Attenuated Scan Combined with Interleaved Non-ATtEnuation). In this new technique, the inversion pulse of conventional fast fluid-attenuated inversion recovery (FLAIR) is replaced with a fast spin echo (FSE) acquisition that has an additional 180(y)-90(x) pulse train for driven inversion. By using appropriate scan parameters, the first part of the sequence provides T2-weighted images and the second part provides fluid-attenuated images, thus allowing simultaneous acquisition in a single scan time comparable to that of fast FLAIR. FASCINATE was compared with conventional scanning techniques using a normal volunteer and a patient. A signal simulation was also conducted. In the human study, both T2-weighted and fluid-attenuated images from FASCINATE showed the same image quality as conventional images, suggesting the potential for this technique to replace the combination of fast FLAIR and T2-weighted FSE for scan time reduction.

Effective blood signal suppression using double inversion-recovery and slice reordering for multislice fast spin-echo MRI and its application in simultaneous proton density and T2 weighted imaging

PURPOSE

To design a multislice double inversion-recovery fast spin-echo (FSE) sequence, with k-space reordered by inversion time at slice position (KRISP) technique, to produce black-blood vessel wall magnetic resonance imaging (MRI).

MATERIALS AND METHODS

In this sequence, central k-space sampling for each slice is required at inversion time (TI) of the blood signal. To fill the entire k-space, the peripheral lines are obtained less or greater the TI and using a rotating slice order. Blood flow signal suppression was first evaluated using a phantom. Simulation studies were used to investigate FSE image quality. The final sequence was then applied to the rabbit abdominal aorta MRI at 4.7 T.

RESULTS

In the flow phantom study, artifacts from slow-flowing water were substantially reduced by the KRISP technique; residual water spins were dephased by the strong phase-encoding gradient required for peripheral k-space. These dephased spins flowed into the slice plane where the center of k-space was being acquired at the TI of the flowing water signal. Multislice black-blood MR images were successfully obtained in the rabbit abdomen using the sequence with the k-trajectory optimized by the simulation study.

CONCLUSION

The KRISP technique was effective both in multislice double inversion-recovery FSE and in blood signal suppression.

High-speed Imaging at 3 Tesla: A Technical and Clinical Review with an Emphasis on Whole-brain 3D Imaging

Improvements to the inherently high specific-absorption rate (SAR) of high-speed imaging at 3T are necessary in order to render this method clinically feasible. Various efforts have been undertaken to improve the associated hardware and software. In this review, we focus on whole-brain isotropic 3D imaging with a turbo spin-echo sequence with variable flip-angle echo trains (3D-TSE-VFL) and present its technical and clinical features. This sequence can be used to acquire images of various contrasts including T2-weighted, fat-suppressed T2-weighted, fluid-attenuated inversion recovery (FLAIR), fat-suppressed FLAIR, and STIR (short tau inversion recovery). Various aspects of 3D-TSE-VFL are discussed, including CSF (cerebrospinal fluid) and metal artifacts, STIR contrast, small-part visualization other than brain, and the possibility of serial subtraction. Some images from clinical cases are presented.

Homogeneous preparation encoding (HoPE) in multislice imaging

Fast magnetization preparation techniques acquire a series of echoes after a single magnetization preparation. If these echoes are acquired from different slices using a multislice technique the change in the preparation state of the echoes due to relaxation effects leads to different contrast modification for each slice. Encoding different preparation states along the phase-encoding direction of each slice instead of acquiring each slice in a different preparation state is introduced as a general concept to obtain images of identical contrast and point-spread function. This can be realized either by cycling the slice excitation order several times over the total number of repetitions or by moving the point of time at which the preparation is applied within each repetition. One possible application of this method is chemical shift selective fat saturation imaging. A homogeneous fat suppression across a multislice volume could be achieved using a FLASH sequence at a repetition time of TR = 145 ms, including a single fat saturation preparation. Conventional fat saturated spin-echo imaging at any TR can be accelerated significantly by reducing the number of applied preparations per repetition. A further application of the homogeneous preparation encoding (HoPE) method is described that encodes the spatial self-saturation of the multislice excitation order homogeneously in all slices. Only a reduced number of slices of the total volume are excited in each repetition and the slice excitation order is continuously moved along the imaging volume. This method is applied for time of flight (TOF) imaging. Using a TONE-like series of flip angles for the slice excitations of each repetition homogeneous TOF images can be obtained on the basis of a multislice acquisition.