Bilateral Lateral Ventricles Research Articles

Geodesic distance on a Grassmannian for monitoring the progression of Alzheimer's disease

We propose a geodesic distance on a Grassmannian manifold that can be used to quantify the shape progression patterns of the bilateral hippocampi, amygdalas, and lateral ventricles in healthy control (HC), mild cognitive impairment (MCI), and Alzheimer's disease (AD). Longitudinal magnetic resonance imaging (MRI) scans of 754 subjects (3092 scans in total) were used in this study. Longitudinally, the geodesic distance was found to be proportional to the elapsed time separating the two scans in question. Cross-sectionally, utilizing a linear mixed-effects statistical model, we found that each structure’s annualized rate of change in the geodesic distance followed the order of AD>MCI>HC, with statistical significance being reached in every case. In addition, for each of the six structures of interest, within the same time interval (e.g., from baseline to the 6th month), we observed significant correlations between the geodesic distance and the cognitive deterioration as quantified by the ADAS-cog increase and the MMSE decrease. Furthermore, as the disease progresses over time, this linkage between the inter-shape geodesic distance and the cognitive decline becomes considerably stronger and more significant.

Recurrent Giant Cell Glioblastoma Across Bilateral Lateral Ventricles

Giant cell glioblastoma is an extraordinary subtype of glioblastoma that merits mention as a distinct variant of glioblastoma in World Health Organization classification of gliomas. It represents for about 1% of brain tumors and about 5% of glioblastomas in general. It has no preferential region and may occur in various locations of the central nervous system, most commonly in the temporal lobe. To our knowledge, there have been a few cases involved in the unilateral lateral ventricle reported in the literature, but there have been no reports about the recurrent cases involved in bilateral lateral ventricles. We herein present a very rare case of recurrent giant cell glioblastoma that occurred across bilateral lateral ventricles.

Abnormal asymmetries in subcortical brain volume in schizophrenia.

Subcortical structures, which include the basal ganglia and parts of the limbic system, have key roles in learning, motor control and emotion, but also contribute to higher-order executive functions. Prior studies have reported volumetric alterations in subcortical regions in schizophrenia. Reported results have sometimes been heterogeneous, and few large-scale investigations have been conducted. Moreover, few large-scale studies have assessed asymmetries of subcortical volumes in schizophrenia. Here, as a work completely independent of a study performed by the ENIGMA consortium, we conducted a large-scale multisite study of subcortical volumetric differences between patients with schizophrenia and controls. We also explored the laterality of subcortical regions to identify characteristic similarities and differences between them. T1-weighted images from 1680 healthy individuals and 884 patients with schizophrenia, obtained with 15 imaging protocols at 11 sites, were processed with FreeSurfer. Group differences were calculated for each protocol and meta-analyzed. Compared with controls, patients with schizophrenia demonstrated smaller bilateral hippocampus, amygdala, thalamus and accumbens volumes as well as intracranial volume, but larger bilateral caudate, putamen, pallidum and lateral ventricle volumes. We replicated the rank order of effect sizes for subcortical volumetric changes in schizophrenia reported by the ENIGMA consortium. Further, we revealed leftward asymmetry for thalamus, lateral ventricle, caudate and putamen volumes, and rightward asymmetry for amygdala and hippocampal volumes in both controls and patients with schizophrenia. Also, we demonstrated a schizophrenia-specific leftward asymmetry for pallidum volume. These findings suggest the possibility of aberrant laterality in neural pathways and connectivity patterns related to the pallidum in schizophrenia.

Noncommunicating Hydrocephalus Presenting With Pseudoseizures: A Case Report.

To the Editor: Hydrocephalus is one of the most common neurosurgical disorders observed in both adult and pediatric populations. On the basis of pathology, hydrocephalus is classified as either communicating (nonobstructive) or noncommunicating (obstructive) types. Etiology can be congenital or acquired. Another variant that does not fit into the above categories is normal pressure hydrocephalus1 Case report. A 16-year-old boy in the 10th grade presented to a psychiatry outpatient department with episodes of sudden onset of mutism along with hyperventilation lasting for 10–15 minutes for the past month. The episodes were not associated with loss of consciousness, incontinence, tongue biting, or any abnormal jerky limb movements. During a detailed workup, the patient disclosed that he was particularly distressed about his upcoming school examinations. His mother also added that he had complaints of headache and declining academic performance for the past year. A general physical examination revealed a thin-built boy with stable vital signs. A mental status examination revealed ill-sustained eye-to-eye contact, anxious affect, intact judgment, and higher mental functions. There was no past or family history of any chronic psychiatric or medical disorder or substance abuse. During his stay in the hospital, the patient had episodes of abnormal behavior varying in nature from mutism along with hyperventilation to sudden onset of screaming, fearfulness, and impaired consciousness. The episodes were dissociative in nature. An electroencephalograph showed a normal awake study. He was started on tablet etizolam 0.5 mg twice a day and supportive psychotherapy sessions, which led to resolution of the dissociative episodes. However, the patient complained of persistent headache. A funduscopic examination was normal. Radiologic imaging in the form of noncontrast head computed tomography revealed dilated lateral and third ventricles. Subsequently, brain magnetic resonance imaging confirmed the diagnosis of noncommunicating hydrocephalus with dilated bilateral lateral and third ventricles with normal fourth ventricle with no periventricular edema (Figure 1). Neurosurgical opinion was sought and conservative management in the form of tablet acetazolamide 250 mg twice a day was started. This regimen led to a reduction in the frequency and severity of his headaches. At follow-up 3 months later, the patient was back in school and adjusting well in his new class. Figure 1. Magnetic Resonance Image Showing Dilated Third and Lateral Ventricles Patients mostly present with headache, nausea, vomiting, blurring of vision, diplopia, poor coordination, incontinence, and behavioral manifestations such as lethargy, drowsiness, irritability, cognitive decline, and personality changes. In the pediatric population, declining school performance with intermittent headache is often an important clue for diagnosis.2–4 Psychiatric manifestations are more commonly encountered in the normal pressure hydrocephalus variant.5 This is the first report of noncommunicating hydrocephalus presenting with pseudoseizures. A high index of suspicion and thorough evaluation even in cases of pseudoseizure can help in early diagnosis and timely intervention.

Enlarged lateral ventricles inversely correlate with reduced corpus callosum central volume in first episode schizophrenia: association with functional measures.

The lateral and third ventricles, as well as the corpus callosum (CC), are known to be affected in schizophrenia. Here we investigate whether abnormalities in the lateral ventricles (LVs), third ventricle, and corpus callosum are related to one another in first episode schizophrenia (FESZ), and whether such abnormalities show progression over time. Nineteen FESZ and 19 age- and handedness-matched controls were included in the study. MR images were acquired on a 3-Tesla MRI at baseline and ~1.2years later. FreeSurfer v.5.3 was employed for segmentation. Two-way or univariate ANCOVAs were used for statistical analysis, where the covariate was intracranial volume. Group and gender were included as between-subjects factors. Percent volume changes between baseline and follow-up were used to determine volume changes at follow-up. Bilateral LV and third ventricle volumes were significantly increased, while central CC volume was significantly decreased in patients compared to controls at baseline and at follow-up. In FESZ, the bilateral LV volume was also inversely correlated with volume of the central CC. This inverse correlation was not present in controls. In FESZ, an inverse correlation was found between percent volume increase from baseline to follow-up for bilateral LVs and lesser improvement in the Global Assessment of Functioning score. Significant correlations were observed for abnormalities of central CC, LVs and third ventricle volumes in FESZ, suggesting a common neurodevelopmental origin in schizophrenia. Enlargement of ventricles was associated with less improvement in global functioning over time.

Impact of Common Variations in PLD3 on Neuroimaging Phenotypes in Non-demented Elders.

Rare variants of phospholipase D3 (PLD3) have been identified as Alzheimer's disease (AD) susceptibility loci, whereas little is known about the potential role of common variants in the progression of AD. To examine the impact of genetic variations in PLD3 on neuroimaging phenotypes in a large non-demented population. A total of 261 normal cognition (NC) and 456 mild cognitive impairment (MCI) individuals from the Alzheimer's Disease Neuroimaging Initiative (ADNI) database are included in our analysis. Multiple linear regression models were applied to examine the association between four single-nucleotide polymorphisms (SNPs; rs7249146, rs4490097, rs12151243, and rs10407447) with the florbetapir retention on florbetapir 18F amyloid positron emission tomography (AV45-PET), the cerebral metabolic rate for glucose (CMRgl) on 18F-fluorodeoxyglucose PET (FDG-PET), and regional volume on magnetic resonance imaging (MRI) at baseline and in the cohort study. We did not detect any significant associations of PLD3 SNPs with florbetapir retention on AV45-PET. In the analysis of FDG-PET, rs10407447 was associated with the CMRgl in the left angular gyrus and bilateral posterior cingulate cortex in the MCI group. Regarding the MRI analysis, rs10407447 was also associated with bilateral inferior lateral ventricle and lateral ventricle volume in MCI group. The main findings of our study provide evidence that support the possible role of PLD3 common variants in influencing AD-related neuroimaging phenotypes. Nevertheless, further work is necessary to explain the functional mechanisms of differences and confirm the causal variants.

Ruptured Intracranial Teratoma: A Case Report and Literature Review

Teratomas are germinal cell tumors originating from all three germ cell layers. When intracranial in location, they typically occur in the pineal or suprasellar regions. Ruptured intracranial teratomas are exceedingly rare. The authors report a case of a 34-year-old woman with a ruptured fourth ventricular mature teratoma that was treated with open surgery. The patient initially presented complaints of headache, nausea, and dizziness. CT scan demonstrated a mixed density lesion in the fourth ventricle and hypo dense lesions in bilateral lateral ventricles. MRI imaging revealed a complex fourth ventricular lesion and high T1 signal bilaterally in the lateral ventricles. A ruptured fourth ventricular teratoma was suspected and the patient was taken for surgery. Histopathology confirmed the diagnosis. Due to CSF leaking from her wound she ultimately underwent a ventriculoperitoneal shunt procedure several days later for persistent pseudomeningocele and hydrocephalus. A review of the literature is performed. Spontaneous rupture of intracranial mature teratomas is seldom encountered. The later than typical presentation may be attributed to the tumor’s unusual location, precluding the more typical visual or endocrine abnormalities which may have otherwise resulted in an earlier diagnosis.

Treatment strategies for huge central neurocytomas.

Central neurocytomas (CNs), initially asymptomatic, sometimes become huge before detection. We described and analyzed the clinical, radiological, operational and outcome data of 13 cases of huge intraventricular CNs, and discussed the treatment strategies in this study. All huge CNs (n=13) in our study were located in bilateral lateral ventricle with diameter ≥5.0 cm and had a broad-based attachment to at least one side of the ventricle wall. All patients received craniotomy to remove the tumor through transcallosal or transcortical approach and CNs were of typical histologic and immunohistochemical features. Adjuvant therapies including conventional radiation therapy (RT) or gamma knife radiosurgery (GKRS) were also performed postoperatively. Transcallosal and transcortical approaches were used in 8 and 5 patients, respectively. Two patients died within one month after operation and 3 patients with gross total resection (GTR) were additionally given a decompressive craniectomy (DC) and/or ventriculoperitoneal shunt (VPS) as the salvage therapy. Six patients received GTR(+RT) and 7 patients received subtotal resection (STR)(+GKRS). Eight patients suffered serious complications such as hydrocephalus, paralysis and seizure after operation, and patients who underwent GTR showed worse functional outcome [less Karnofsky performance scale (KPS) scores] than those having STR(+GKRS) during the follow-up period. The clinical outcome of huge CNs seemed not to be favorable as that described in previous reports. Surgical resection for huge CNs should be meticulously considered to guarantee the maximum safety. Better results were achieved in STR(+GKRS) compared with GTR(+RT) for huge CNs, suggesting that STR(+GKRS) may be a better treatment choice. The recurrent or residual tumor can be treated with GKRS effectively.

Phenotypic Expansion of Congenital Disorder of Glycosylation Due to SRD5A3 Null Mutation.

We present a boy, admitted at 4 months, with facial dysmorphism, hypertrichosis, loose skin, bilateral inguinal hernia, severe hypotonia, psychomotor disability, seizures with hypsarrhythmia (West syndrome), hepatosplenomegaly, increased serum transaminases, iris coloboma, glaucoma, corneal clouding and bilateral dilated lateral ventricles, and extra-axial post-cerebellar space. Serum transferrin isoelectrofocusing (IEF) showed a type 1 pattern. Whole-exome genotyping showed a previously reported homozygous nonsense mutation c.320G>A; p.Trp107X in SRD5A3. Epilepsy and glaucoma have been reported only once in the 19 described SRD5A3-congenital glycosylation defect patients, and corneal clouding not at all.

Multivariate longitudinal shape analysis of human lateral ventricles during the first twenty-four months of life.

BackgroundLittle is known about the temporospatial shape characteristics of human lateral ventricles (LVs) during the first two years of life. This study aimed to delineate the morphological growth characteristics of LVs during early infancy using longitudinally acquired MR images in normal healthy infants.Methods24 healthy infants were MR imaged starting from 2 weeks old every 3 months during the first and every 6 months during the second year. Bilateral LVs were segmented and longitudinal morphological and shape analysis were conducted using longitudinal mixed effect models.ResultsA significant bilateral ventricular volume increase (p<0.0001) is observed in year one (Left: 126±51% and Right: 145±62%), followed by a significant reduction (p<0.02) during the second year of life (Left: −24±27% and Right: −20±18%) despite the continuing increase of intracranial volume. Morphological analysis reveals that the ventricular growth is spatially non-uniform, and that the most significant growth occurs during the first 6 months. The first 3 months of life exhibit a significant (p<0.01) bilateral lengthening of the anterior lateral ventricle and a significant increase of radius (p<0.01) and area (p<0.01) at the posterior portion of the ventricle. Shape analysis shows that the horns exhibit a faster growth rate than the mid-body. Finally, bilateral significant age effects (p<0.01) are observed for the growth of LVs whereas gender effects are more subtle and significant effects (p<0.01) only present at the left anterior and posterior horns. More importantly, both the age and gender effects are growth directionally dependent.ConclusionsWe have demonstrated the temporospatial shape growth characteristics of human LVs during the first two years of life using a unique longitudinal MR data set. A temporally and spatially non-uniform growth pattern was reported. These normative results could provide invaluable information to discern abnormal growth patterns in patients with neurodevelopmental disorders.

A I-segment aneurysms: management protocol based on a new classification.

Aneurysms of proximal (AI)-segment of anterior cerebral artery (ACA) constitute <1% of all intracranial aneurysms. Management dilemmas of A1-segment aneurysms were studied utilizing a new classification based upon their location on the longitudinal and circumferential axis of A1-segment. Tertiary care referral center. This is a retrospective analysis of 14 patients (0.98%; mean age: 38.02 ± 15.74 years) with AI-segment aneurysms. The data collected included clinical features, computed tomography (CT) scan and CT-angiography (CTA)/digital subtraction angiography (DSA) findings, modified Hunt and Hess (H and H) grade, surgical steps and difficulties encountered. The modified Hunt and Hess (H and H) grades in the 14 patients were: grade I in two, grade II in two, grade III in four, grade IV in five and grade V in 1. The mean ictus-admission duration was 5.07 ± 2.30 days (range: 1-10 days). Multiple aneurysms were two. Thirteen patients underwent clipping and one, wrapping. Bilateral lateral ventricle hemorrhage occurred in 8 (66%) patients and frontal intracerebral hematoma in 2 (16.66%) patients. In one patient, the aneurysm could only be detected following the third angiogram. AI-aneurysms were classified as proximal (n = 6), distal (n = 7), and mid-segment (n = 1); and, anterior (n = 2), posterior-inferior (n = 7) and posterior-superior (n = 5). Follow-up (range: 6 months-10 years, mean: 2.9 years) recovery (assessed using Modified Rankin's score or mRS) correlated with preoperative status. The preoperative H and H grade and follow-up mRS status were as follows: H and H I (n = 2): mRS 0 (asymptomatic, n = 2); H and H II (n = 2): mRS 1 (minor symptoms without disability, n = 2); H and H III (n = 4):mRS 1 (n = 2) and mRS 2 (slight disability but performing unassisted activities of daily living, n = 1); H and H IV (n = 5): mRS 3 (moderate disability, requiring help for daily living but unassisted walking, n = 2) and mRS 4 (moderately severe disability, requiring help for daily living and walking, n = 2). One patient each from H and H grade III, IV and V died (mRS 6) during treatment due to severe vasospasm, pneumonitis and septicemia. AI-segment aneurysms have unique properties: rupturing of small-sized aneurysms; multiplicity; undetectable on initial imaging; frontal lobar/intraventricular bleeding; origin from main trunk and not bifurcating points; neck obscuration by AI-trunk; close proximity to perforators; and, associated AI-segment and ACA anomalies. A new classification identifies surgical difficulties inherent in different sites of origin of A1-aneurysms.

Congenital Porencephaly in a New Born Child

A 2-day-old male child presented with history of enlarged head and seizures since birth, born by caesarean section. Head circumference was 56 cm (dilated) with widely open anterior and posterior fontanelle. Routine investigations were within normal limits. CT head revealed a large non-enhancing fluid attenuating cystic lesion in posterior parietal and occipital region with communicating to dilated bilateral lateral ventricles and subarachnoid space.

Donepezil-Induced Confusional State in a Patient With Autopsy-Proven Behavioral-Variant Frontotemporal Dementia

Donepezil-Induced Confusional State in a Patient With Autopsy-Proven Behavioral-Variant Frontotemporal Dementia

Trigeminal Neuralgia Due to Dandy-Walker Syndrome

Trigeminal neuralgia (TN) is a common pain in the orofacial region. Dandy-Walker syndrome (DWS) is a congenital malformation of the cerebellar and the fourth ventricle foramina atresia. Dandy-Walker syndrome is rarely found in patients with TN. This article presents a 36-year-old man with the symptoms of typical TN. His physical examination was entirely normal. An enhanced magnetic resonance imaging was taken. Magnetic resonance imaging revealed the bilateral lateral ventricle, the fourth and third ventricle significantly enlarged with severe obstructive hydrocephalus, a huge posterior fossa cyst connected with the fourth ventricle, and hypoplastic vermis. The pain was controlled by Tegretol. The reported case suggests that DWS is an unusual cause of TN.

Clinical and imaging features of central neurocytomas

To describe the clinical and imaging characteristics of patients with central neurocytoma (CN), we reviewed data on 27 patients who had histologically confirmed CN and were treated in our institution between 1999 and 2010. Neuro-imaging findings on CT scan (n=18) and MRI (n=25) were retrospectively evaluated. There were 15 males and 12 females with a mean age of 29years (range, 11–46years). The most frequent presentations included headache (n=21) and vomiting (n=6). Tumor sites included bilateral lateral ventricles (n=10), right lateral ventricle (n=7), left lateral ventricle (n=7) and fourth ventricle (n=3). On MRI, the T1-weighted signal was hypointense in 12 patients and isointense in 13, and the T2-weighted signal was isointense in 8 patients and hyperintense in 15. CT scans/MRI revealed a cystic component in 18 patients. Tumors showed a mild to marked enhancement in 26 patients. Flow voids from tumor vessels on MRI were present in 14 patients, and calcification was noted in six of 18 patients with CT scans. All lateral ventricle tumors were resected through a transcortical or transcallosal approach. Gross total resection was achieved in 19 patients, near total in two and subtotal in six. One patient died of cerebral infarction in the perioperative period. At the last follow up, there were three known clinical recurrences in this series. However, no recurrence was noted in 17 patients who underwent gross total resection with no adjuvant therapy.

Chiari Type 1 Malformation and Periventricular Nodular Heterotopia in a 6-Year-Old Boy With Congenital Hemiplegia

A 6-year-old boy presented with weakness of the right upper and lower limbs, noted since infancy. Magnetic resonance imaging of the brain revealed periventricular nodular heterotopia lining the trigone and occipital horns of bilateral lateral ventricles along with herniation of the cerebellar tonsils below the foramen magnum suggestive of Chiari type 1 malformation. The association of periventricular nodular heterotopia with Chiari type 1 malformation has not been described earlier in children.

Clinical features and surgical management of four peculiar cases of intracranial metastases from renal cell carcinoma

Intracranial metastasis from renal cell carcinoma is rare. In this study, four patients (age range 56-72 years) were treated for intracranial metastases from renal cell carcinoma. The metastasis was solitary in all four patients and was located in the temporoparietal region in two patients, cerebellum in one patient, and bilateral lateral ventricles in one patient. Preoperative magnetic resonance imaging showed hemorrhage and necrosis in the tumor in all four patients, and one patient had edema in the region of the tumor. All patients were treated with craniotomy for tumor resection, and histopathologic examination showed clear cell carcinoma. Marked bleeding occurred in all patients during the operation, but preoperative direct injection of ethanol into epidural metastases (2 patients) was effective in decreasing intraoperative blood loss. Prognosis was poor with limited survival time (range 2 weeks-45 months), but prognosis was best in the two patients who were treated with postoperative radiation therapy and chemotherapy (survival times 26 and 45 months). To our knowledge, this is the largest report of four uncommon intracranial metastases from RCC in Chinese patients. In summary, intracranial metastasis from renal cell carcinoma should be considered in the differential diagnosis of intracranial tumor because of the high risk of bleeding and poor prognosis.

Late Onset of Psychotic Symptoms in a Patient With Cavum Septum Pellucidum and Cavum Vergae

Late Onset of Psychotic Symptoms in a Patient With Cavum Septum Pellucidum and Cavum Vergae

Rosette‐forming glioneuronal tumor of the septum pellucidum with extension to the supratentorial ventricles: Rare case with genetic analysis

In the original description, rosette-forming glioneuronal tumors (RGNTs) were restricted to the fourth ventricle and/or posterior fossa. Here, we first report an unusual case of RGNT centered in the septum pellucidum and associated with multiple masses occupying the wall of the bilateral lateral ventricles and the third ventricle. No mass was found in the fourth ventricle. Histological and immunohistochemical examination revealed that the tumor presented biphasic differentiation characterized by predominantly neurocytic rosettes and pilocytic astrocytoma-like components with obvious microvascular proliferation. Chromosome 1p/19q deletions and isocitrate dehydrogenase 1 and 2 (IDH1/2) mutations were not identified. Because this case exhibited a worrisome growth pattern, further studies and long-term follow-up are needed to determine the true nature of these tumors.

MR diffusion tensor imaging voxel-based analysis of whole brain white matter in patients with amnestic-type mild cognitive impairment and mild Alzheimer disease

Objective To evaluate the microstructural integrity of white matter (WM) in patients with amnestic mild cognitive impairment (aMCI) and mild Alzheimer's disease (AD) using voxel-based analysis (VBA), and investigate the relationship between WM abnormalities and gray matter(GM) atrophy.Methods Thirty-three cases with aMCI, 32 cases with mild AD and 31 normal aging volunteers as control subjects were scanned on a 3.0 T MR system using diffusion tensor imaging (DTI) and three-dimensional spoiled gradient-recalled(3DSPGR) sequences. Fractional anisotropy (FA) maps and morphological images were preprocessed by SPM5 and voxel-based comparisons between the 2 patient groups and the control group were performed by t test. Results Relative to the control group, patients with aMCI showed significantly reduced FA value in bilateral frontal, temporal and left occipital WM, left anterior part of cingulum, left inferior parietal lobule, and the W M adjacent to the triangular part of the right lateral ventricle(k≥20 voxels).In mild AD,significantly reduced FA value was found in bilateral hippocampal,inferior parietal lobular,frontal,temporal,and occipital WM,bilateral corpus callosum,anterior part of cingulums,the WM adjacent to the triaangular part of the bilateral lateral ventricles,left temporal stem,left thalamus,right precuneus(k≥20 voxels).Significantly reduced GM volume was found in left hippocampus,parahippocampal gyrus,lingual gyrus and superior temporal gyrus,bilateral insulae and middle temporal gyri in aMCl group whencompared with control group(k≥50 voxels).In mild AD,significantly reduced GM volume was found in bilateral hippoeampi,parahippocampal gyri,amygdalae,thalami,temporal,parietal,frontal,occipital cortex(k≥50 voxels).The pattern of areas with reduced FA differs;from that of the GM volumetric reduction.No areas with significantlv reduced FA was detected in aMCl compared with mild AD. There was no significant correlation between FA value of WM in patient groups and Mini-Mental State Examination(MMSE)scores.Conclusions Voxel-based MRI DTI analysis of whole brain white matter can objectively reveal widespread white matter abnormalities in early-stage AD.The difierence between WM FA reduction pattern and GM volumetric reduction pattern indicates that the pathological WM changes in earlyslage AD were caused by multiple mechanisms. FA did not vary significantly in patients pr0gressing from aMCI to mild AD and can hardly reflect the severitv of cognitive function damage in these patients. Key words: Cognition disorders; Alzheimer disease; Magnetic resonance imaging