Magnetic resonance imaging of neurodegenerative diseases

Accelerated MR parameter mapping with low-rank and sparsity constraints

PURPOSE

To enable accurate magnetic resonance (MR) parameter mapping with accelerated data acquisition, utilizing recent advances in constrained imaging with sparse sampling.

THEORY AND METHODS

A new constrained reconstruction method based on low-rank and sparsity constraints is proposed to accelerate MR parameter mapping. More specifically, the proposed method simultaneously imposes low-rank and joint sparse structures on contrast-weighted image sequences within a unified mathematical formulation. With a pre-estimated subspace, this formulation results in a convex optimization problem, which is solved using an efficient numerical algorithm based on the alternating direction method of multipliers.

RESULTS

To evaluate the performance of the proposed method, two application examples were considered: (i) T2 mapping of the human brain and (ii) T1 mapping of the rat brain. For each application, the proposed method was evaluated at both moderate and high acceleration levels. Additionally, the proposed method was compared with two state-of-the-art methods that only use a single low-rank or joint sparsity constraint. The results demonstrate that the proposed method can achieve accurate parameter estimation with both moderately and highly undersampled data. Although all methods performed fairly well with moderately undersampled data, the proposed method achieved much better performance (e.g., more accurate parameter values) than the other two methods with highly undersampled data.

CONCLUSIONS

Simultaneously imposing low-rank and sparsity constraints can effectively improve the accuracy of fast MR parameter mapping with sparse sampling.

A technique for rapid single-echo spin-echo T2 mapping

A rapid technique for mapping of T(2) relaxation times is presented. The method is based on the conventional single-echo spin echo approach but uses a much shorter pulse repetition time to accelerate data acquisition. The premise of the new method is the use of a constant difference between the echo time and pulse repetition time, which removes the conventional and restrictive requirement of pulse repetition time >> T(1). Theoretical and simulation investigations were performed to evaluate the criteria for accurate T(2) measurements. Measured T(2)s were shown to be within 1% error as long as the key criterion of pulse repetition time/T(2) > or =3 is met. Strictly, a second condition of echo time/T(1) << 1 is also required. However, violations of this condition were found to have minimal impact in most clinical scenarios. Validation was conducted in phantoms and in vivo T(2) mapping of healthy cartilage and brain. The proposed method offers all the advantages of single-echo spin echo imaging (e.g., immunity to stimulated echo effects, robustness to static field inhomogeneity, flexibility in the number and choice of echo times) in a considerably reduced amount of time and is readily implemented on any clinical scanner.

Brain magnetic resonance imaging (MRI) in parkinsonian disorders

Magnetic resonance imaging (MRI) is increasingly integrated into neurological diagnostics. In addition to functional MRI, a large number of sequences (T1W, T2W, PD, T2W gradient echo, diffusion-weighted imaging (DWI), and diffusion tensor imaging (DTI)), investigate CNS abnormalities. Objective quantification techniques (T1W voxel-based morphometry) can also discern subtle anatomical differences. Parkinsonian conditions such as Parkinson's disease, multiple system atrophy, progressive supranuclear palsy, corticobasal degeneration and manganese-induced parkinsonism can clinically overlap, yet have very different prognoses and treatments. Relatively little radiographic interest has focused on movement disorders. Nevertheless in the past decade, a variety of findings, often subtle and routinely overlooked, have emerged to help the clinician differentiate these conditions. This review will summarize and discuss MRI findings in parkinsonian conditions. Most data concern either structural abnormalities or the imaging sequelae of abnormal iron deposition, common in some parkinsonian conditions.

Clinical and neuroimaging features of good and poor seizure control patients with mesial temporal lobe epilepsy and hippocampal atrophy

PURPOSE

Hippocampal atrophy (HA) and signal changes, detected at magnetic resonance imaging, have been associated with intractable seizures. Such a relation has been established by tertiary centers, where the prevalence of more severe cases tends to be higher. We evaluated the clinical and imaging variables that may have relevance to seizure control in patients with mesial temporal lobe epilepsy (MTLE) and HA.

METHODS

MTLE patients from the outpatient clinic of University of São Paulo School of Medicine at Ribeirão Preto were evaluated with protocols for the temporal lobe. Patients were considered to have good seizure control (GC; n = 42) if they had three of fewer seizures per year. Patients with pharmacoresistance and who did not fit the criteria for GC were considered to have poor seizure control (PC; n = 44). We made group comparisons and correlations of clinical data and hippocampal volume (HV) with seizure frequency.

RESULTS

No statistical differences were observed between the GC and PC groups in the following parameters: age at the time of study, age at the time of the initial precipitating injury (IPI) or first epileptic seizure, epilepsy duration and follow-up, and family history of epilepsy. No differences were found in HV between GC (male, 2.04 +/- 0.60 cc; female, 2.00 +/- 0.70 cc) and PC (male, 2.26 +/- 0.47 cc; female, 2.15 +/- 0.48 cc) groups. Regression analysis indicated no correlation between seizure frequency and HV (p = 0.33).

CONCLUSIONS

These findings suggest that the intensity of HA does not have a direct correlation with seizure frequency in patients with MTLE with HA and that the detection of HA in MTLE patients does not mean an unequivocal indication of intractability.

Proton magnetic resonance imaging and spectroscopy identify metabolic changes in the striatum in the MPTP feline model of parkinsonism

We administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to adult, male cats to model Parkinson's disease (PD), and utilized proton magnetic resonance imaging (MRI) and spectroscopy (MRS) at a field strength of 1.5 T to identify metabolic degenerative changes in the striatum in vivo. Neurologic status and somatosensory-evoked potentials in vivo, as well as postmortem striatal histopathological and immunohistochemical parameters, were examined. Nine cats were equally divided into three groups and treated daily for 10 days as follows: saline, MPTP, and pargyline (a monoamine oxidase inhibitor) plus MPTP. The MPTP-treated cats displayed bradykinesia, head tremor, and reduced oculovestibular reflex activity. MRI showed a diffuse increase of the T2-weighted signal in the striatum of two MPTP-treated cats. Analysis of the MRS spectra indicated significantly lower N-acetylaspartate/creatine (CR) and glutamine-glutamate complex/CR ratios than the control baseline. Two MPTP-treated cats had low choline-containing compounds/CR ratio, whereas a lactate peak was present in all MPTP-treated cats. In the striatum of the MPTP-treated cats, there was a significant decline of tyrosine hydroxylase immunoreactivity and histological evidence for a diffuse cytotoxic reaction. Pretreatment with pargyline attenuated the MPTP-induced clinical signs, MRI and MRS changes, and the histopathological and immunoreactivity alterations. We conclude that proton MRI/MRS is a sensitive, noninvasive measure of neural toxicity and biochemical alteration of the striatum in a feline model of PD.

Documentation of electrode localization

In evaluating the success of deep brain stimulation (DBS), the benefit for the patient is the most important criteria. Nevertheless, correct placement of electrodes should also be determined in terms of their anatomic position. Therefore, we propose a suite of different imaging modalities and further processing, which leads to an exact anatomic and statistically comparable documentation of electrode localization. Forty-three consecutive patients with a total of 85 implanted DBS electrodes were evaluated with respect to postoperative imaging. T1-weighted magnetic resonance imaging (T1-MRI) was performed in all patients, 34 patients received T2-MRI, in 18 patients stereotactic X-ray of the scull was performed in anteroposterior and lateral projections, whereas 6 patients were additionally evaluated by pre- and postoperative MR-image fusion between T1-data sets and calculation of coordinates for electrode contacts. In T1-MRI, the artefacts of each electrode contact could be delineated in relation to anatomic reference structures, whereas T2-MRI allowed reproducibly for delineation of electrode artefacts within subthalamic nucleus or globus pallidus pars interna. By MR-image fusion it could be shown that the difference between planned target coordinates and coordinates of the active electrode contact ranged below 1 mm except for the z axis. The comparison with values obtained from stereotactic X-ray confirmed these results. The sequential and complementary use of the described imaging modalities and further image processing provide clinically reliable and statistically comparable results to prove the exact anatomic electrode positioning in DBS in addition to the objective and subjective improvements of the patients' symptoms.

Normal brain volume measurements using multispectral MRI segmentation

The performance of a supervised k-nearest neighbor (kNN) classifier and a semisupervised fuzzy c-means (SFCM) clustering segmentation method are evaluated for reproducible measurement of the volumes of normal brain tissues and cerebrospinal fluid. The stability of the two segmentation methods is evaluated for (a) operator selection of training data, (b) reproducibility during repeat imaging sessions to determine any variations in the sensor performance over time, (c) variations in the measured volumes between different subjects, and (d) variability with different imaging parameters. The variations were found to be dependent on the type of measured tissue and the operator performing the segmentations. The variability during repeat imaging sessions for the SFCM method was < 3%. The absolute volumes of the brain matter and cerebrospinal fluid between subjects varied quite large, ranging from 9% to 13%. The intraobserver and interobserver reproducibility for SFCM were < 4% for the soft tissues and 6% for cerebrospinal fluid. The corresponding results for the kNN segmentation method were higher compared to the SFCM method.