Fetal posterior fossa volume: assessment with MR imaging

PURPOSE: To retrospectively determine the relationship between posterior fossa volume (PFV) and estimated gestational age (EGA) and/or femur length (FL) during pregnancy for the purpose of developing a normal growth curve. MATERIALS AND METHODS: Advance institutional review board approval was obtained for this HIPAA-compliant study, and the need for parent informed consent was waived. A cross-sectional retrospective study was performed to measure PFV on in vivo magnetic resonance (MR) images obtained in 76 fetuses of 18-36 weeks gestation who had a morphologically normal CNS. Because this was a retrospective series, MR imaging techniques varied slightly, but all fetuses underwent imaging at contiguous 3-5-mm intervals in at least two orthogonal planes, with repetition time msec/echo time msec, 5-12/62-95; number of signals acquired, one; flip angle, 150 degrees -180 degrees; and matrix, 128-192 x 256. Posterior fossa areas were manually traced on half-Fourier rapid acquisition with relaxation enhancement in utero fetal MR images by one observer. PFVs were then calculated by manually summing areas from the contiguous sections and multiplying the total area by the section thickness. An average PFV (APFV) across orthogonal planes was calculated for each fetus, and the relationship between APFV and EGA was mathematically modeled. Coronal, transverse, and sagittal views were compared with correlations and Bland-Altman plots. Two additional observers repeated the measurements for a small subset of fetuses (n = 5). Paired t test analyses were also performed to determine significant differences between sagittal, transverse, and coronal measurements, as well as to determine preliminary intraobserver and interobserver variability of measurements in a subset of cases. RESULTS: The relationship between APFV (in cubic centimeters) and EGA (in weeks) was well described by a single exponential function [APFV = 0.689 exp(EGA/9.10)]. APFV doubling time was 6.31 weeks. Root-mean-square variation of values around the model line was 1.63 cm(3). There was no statistically significant intra- or interobserver variation (P > .16 for all fetuses) at preliminary analysis. No correlation between APFV and FL could be found. CONCLUSION: The normal fetal PFV growth curve generated in this study may have potential as a model for clinical application.     

MR imaging of the posterior fossa structures of human embryos and fetuses.

There have been few reports on MR imaging of the developing human fetal brain. The aim of this article is to establish a standard atlas of developing fetal brain, focusing in particular on posterior fossa structures. Eighty-eight formalin-fixed embryos and fetuses were examined using 1.5 Tesla MR units. Specimens ranged from Carnegie stage 17 to 28 gestational weeks. The morphologic changes in developing cerebellum, cerebellar fissures, pontine flexure, fourth ventricle, and cerebral aqueduct were observed in each developmental period. The height of the fourth ventricle and cerebral aqueduct and the thickness of the tectum and the tegmentum of the midbrain were measured. We obtained detailed MR images of the developmental changes in posterior fossa structures and produced an atlas of these images. Our study showed that the period of visualization of cerebellar structures and fissures was later on MR imaging than described in past anatomical and embryological studies. In addition, the sudden decrease in height of the fourth ventricle and the cerebral aqueduct found in our study might reflect the presence of communication between the fourth ventricle and subarachnoid space.     

MR findings of posterior fossa hemangioblastomas.

MR findings of five posterior fossa hemangioblastomas were reported. In the cystic type, a cyst was low intensity on T1WI and high intensity on T2WI. Mural nodule was clearly enhanced by Gd-DTPA. In the solid type, the tumor was shown as an isointense lesion on T1WI and as high intensity on T2WI, but it was markedly and homogeneously enhanced by Gd-DTPA. Multiple extremely low-intensity serpentine "flow void" signs, indicating afferent and efferent vessels, were observed within or around the tumor. These were characteristic pictures in this tumor.     

Ultrafast MR imaging of the normal posterior fossa in fetuses

OBJECTIVE: The purpose of our study was to determine if a standard imaging protocol using ultrafast MR sequences could adequately reveal normal posterior fossa anatomy in fetuses and, if so, to document a template on MR imaging for normal posterior fossa development. MATERIALS AND METHODS: A retrospective review found 66 MR imaging studies of 63 fetuses, 16-39 weeks' gestation age (mean, 25 weeks' gestation), who were referred between June 1996 and May 1999 for evaluation of non-central nervous system anomalies revealed on prenatal sonography. All fetuses had normal brains and spines on prenatal sonography. The standard MR imaging protocol included axial, sagittal, and coronal half-Fourier acquisition single-shot turbo spin echo (HASTE); sagittal and coronal two-dimensional fast low-angle shot (FLASH); and axial turbo T1-weighted FLASH images through the fetal brain. Structures that we analyzed were the fourth ventricle, the cisterna magna, the vermis, the cerebellar hemispheres, and the brainstem. Using the HASTE sequences, we documented gestational age-specific signal intensity changes in the cerebellar hemispheres and the brainstem. RESULTS: The posterior fossa anatomy was sufficiently well defined to exclude abnormalities of the fourth ventricle and cerebellar vermis in all cases. Because of high T2-weighting, good contrast enhancement, and good signal-to-noise ratios, HASTE images provided the best anatomic definition of the posterior fossa. CONCLUSION: Normal posterior fossa anatomy can be adequately shown on ultrafast MR images, which can be helpful when prenatal sonography is equivocal.     

MR of leptomeningeal spinal and posterior fossa amyloid.

We report an unusual cause of leptomeningeal MR enhancement, amyloid, along the surfaces of the spinal cord and brain stem and in the spinal subarachnoid space, with sacral intradural and epidural deposition. Type I familial amyloid polyneuropathy may cause amyloid deposition along the leptomeninges of the spinal cord and brain in addition to the visceral organs and the peripheral somatic and autonomic nerves.     

Posterior fossa hemangioblastomas: MR imaging

The magnetic resonance (MR) imaging findings of 18 surgically proved posterior fossa hemangioblastomas (15 patients) were retrospectively analyzed and correlated with computed tomographic (ten patients) and angiographic (eight patients) findings. Thirteen tumors were located in the cerebellar hemisphere, three in the vermis, and two in the medulla with associated syrinxes. Three patients had von Hippel-Lindau disease, two of whom had multiple cerebellar hemangioblastomas. Seven hemangioblastomas appeared as solid tumors, six as solid masses with central cysts, and five as cysts with mural nodules. Abnormal tumor vessels, with characteristic signal void, were demonstrated in 13 tumors. Associated hemorrhage was present in four tumors. Although angiography is usually required for the diagnosis and preoperative assessment of this tumor, MR imaging demonstration of a posterior fossa mass with abnormal vessels should suggest the diagnosis of hemangioblastoma. Moreover, the combination of a peripheral posterior fossa cyst with a mural nodule supplied by enlarged vessels may be pathognomonic.     

Applicability and advantages of flow artifact-insensitive fluid-attenuated inversion-recovery MR sequences for imaging the posterior fossa

We describe a new sequence, flow artifact-insensitive fluid-attenuated inversion recovery (FAIS-FLAIR), that capitalizes on the advantages of fluid-attenuated inversion recovery (FLAIR) while minimizing FLAIR-related artifacts such as those often encountered in the posterior fossa. Twenty-eight patients with posterior fossa disease underwent FAIS-FLAIR, conventional FLAIR, and spin-echo MR studies, and the findings yielded by the three techniques were compared. In this patient population, postcontrast FAIS-FLAIR imaging was obtained in 20 patients and compared with postcontrast T1-weighted images. The images were assessed for lesion conspicuity by three radiologists. FAIS-FLAIR markedly reduces the inflow artifacts from noninverted CSF on FLAIR images. It does so with and without contrast agent administration, and produces higher lesion conspicuity compared with T1- and T2-weighted spin-echo sequences and conventional FLAIR images of the posterior fossa.     

The pterygopalatine fossa: postoperative MR imaging appearance

BACKGROUND AND PURPOSE: The pterygopalatine fossa (PPF) is an important anatomic location of the deep portion of the face. It is essential to review this area on both pre- and posttreatment studies of head and neck malignancies to assess local extent of disease or recurrence and perineural tumor spread. The purpose of this study was to review the postoperative appearance of the PPF on MR images. METHODS: Imaging and clinical data of 10 patients who underwent surgical resection of tumor in which the PPF was violated at surgery were reviewed. Patients were included in the study if there was no imaging or clinical evidence of tumor in the PPF pre- or postoperatively. Postoperative MR studies were examined to assess the appearance of the PPF. RESULTS: The PPF is consistently and persistently abnormal after surgical violation. There is loss of the normal T1 signal hyperintensity and abnormal, increased contrast enhancement, as seen on fat-suppressed T1-weighted images. These postoperative changes are strikingly similar to those of tumor involvement. CONCLUSION: After surgical violation, the PPF will always appear abnormal on MR images, and the expected imaging findings must be recognized to avoid the misdiagnosis of tumor recurrence.     

Subacute and chronic cerebral infarctions: SPECT and gadolinium-DTPA enhanced MR imaging

Single photon emission computed tomography (SPECT) examinations of regional cerebral blood flow (rCBF) and regional cerebral blood volume (rCBV) were carried out in 23 patients with 19 subacute and 7 chronic cerebral infarctions. Corresponding magnetic resonance (MR) examinations (unenhanced and contrast enhanced scans) were done within a time interval of 2 days. Although all subacute lesions showed an increased rCBV, the rCBF could be either increased or decreased. Contrast enhancement in MR was associated with increased rCBV but not with increased rCBF. In all chronic lesions rCBV and rCBF were decreased and there was no contrast enhancement detectable in MR. The application of Gd-diethylenetriamine pentaacetic acid as contrast agent in MR demonstrates blood-brain barrier disruption as focal or gyral contrast enhancement in cerebral infarction. The SPECT examinations revealed an increased rCBV indicating a vasodilation or vasoparalysis. The rCBF was either increased or decreased depending on the collateral formation or recanalization of occluded vessels.     

Evaluation of CSF flow patterns of posterior fossa cystic malformations using CSF flow MR imaging

Introduction Differential radiologic diagnosis of cystic malformations of the posterior fossa is often difficult with conventional imaging techniques because of overlapping features of these entities. Posterior fossa cystic malformations occupy the cerebrospinal fluid (CSF) spaces. They may create secondary dynamic effects on the movements of CSF. The aim of this study was to investigate CSF flow alterations in posterior fossa cystic malformations with CSF flow MR imaging.Methods The study included 40 patients with cystic malformations of the posterior fossa. The patients underwent cardiac-gated phase-contrast cine MR imaging. CSF flow was qualitatively evaluated using an in-plane phase-contrast sequence in the midsagittal plane. The MR images were displayed in a closed-loop cine format.Results Twelve of the patients had communicating arachnoid cyst, seven had non-communicating arachnoid cyst, ten had mega cisterna magna, six had Dandy-Walker malformation, two had Dandy-Walker variant, and three had Blake’s pouch cyst. CSF flow MR imaging indicated the regions of no, slow or higher flow, direction of flow, and abnormal cystic fluid motion. Each malformation displayed a distinct CSF flow pattern.Conclusion Phase-contrast cine MR imaging for CSF flow evaluation may be a useful adjunct to routine MR imaging in the evaluation of the cystic malformations of the posterior fossa because it can improve the specificity in differentiating such malformations.Electronic Supplementary Material Supplementary material is available in the online version of this article at Part of this article was presented as a poster exhibition at the ESNR 28th Annual Congress and 12th Advanced Course, 11–14 September 2003, Istanbul.     

Cerebral infarctions: evaluation with single-axis versus trace diffusion-weighted MR imaging

OBJECTIVE: Our purpose was to determine the usefulness of single-axis diffusion-weighted imaging versus trace diffusion-weighted imaging in the evaluation of cerebral infarctions. SUBJECTS AND METHODS: Twenty-six patients harboring 34 infarctions were examined using single-axis and trace diffusion-weighted imaging within 48 hr of the onset of symptoms. Two neuroradiologists who were not aware of the clinical findings reviewed all images obtained with both techniques and noted the following: type of infarction (small [<15 mm] versus territorial), location of infarction, presence of infarction (seen only on single-axis images, seen only on trace images, seen on both), lesion conspicuity (better on single-axis images, better on trace images, or equal on both), and lesion size (larger on single-axis images, larger on trace images, or equal on both). Differences in opinion were resolved by consensus. RESULTS: Of the 18 small and 16 territorial infarctions, all were identified on both single-axis and trace imaging. Lesion conspicuity was judged to be slightly better on trace images for both types of infarctions. Lesion size was judged to be larger on single-axis images for territorial infarctions. CONCLUSION: Both single-axis and trace diffusion-weighted imaging showed all small and territorial cerebral infarctions. Both types of infarctions were slightly larger on single-axis images but this did not affect correct interpretation in any case. The single-axis technique provided sufficient information for the diagnosis of cerebral infarction in our clinical settings.     

Diffusion MR imaging: clinical applications.

Water self-diffusion, a recently discovered source of contrast on MR images, has already shown promise for some clinical applications. Most studies have been of the brain, essentially for technical reasons. Diffusion is useful in distinguishing the different components of brain tumors (cystic regions, edema, necrosis) from the tumor core itself. Recent studies have shown that diffusion is anisotropic in brain white matter (i.e., dependent on the fiber tract's orientation in space), offering new insights into myelin disorders. Diffusion is also dramatically altered in the minutes following ischemic injury in the cat brain, which may have tremendous impact for the diagnosis and management of hyperacute stroke. With ultrafast acquisition schemes, diffusion imaging has also been used outside the CNS, for instance, in the eye and kidney. Future applications include diffusion-localized spectroscopy and temperature imaging. This article reviews recent progress in this field and suggests potential applications.     

Sonography and MR imaging of posterior tibial tendinopathy.

OBJECTIVE: The purpose of the study is to describe the appearance of the posterior tibialis tendon on MR imaging and high-resolution sonography with color and power Doppler imaging and to determine whether sonography is as accurate for diagnosing tendinosis as MR imaging. SUBJECTS AND METHODS: Fifteen healthy volunteers and 31 patients (44 tendons) who were clinically suspected of having posterior tibial tendinopathy were prospectively evaluated with MR imaging and sonography. RESULTS: On MR imaging, the normal tendon was elliptic on cross section and showed low signal intensity on all sequences. Minimal peritendinous enhancement and fluid were seen. On sonography, the tendon showed homogeneous longitudinal echogenic fibers. No flow was seen in or around the tendon. Tendinopathy was characterized by enhancement of the tendon on MR imaging (19/44 tendons); intratendinous flow on color Doppler sonography (16/44 tendons); increase in the anteroposterior diameter causing a rounding of the tendon (18/44 tendons); and inhomogeneity of the tendon (16/44 tendons on MR imaging and 21/44 tendons on sonography). Peritendinosis was characterized by peritendinous enhancement on MR imaging (29/44 tendons); flow on color Doppler sonography (20/44 tendons); and increased soft tissue (20/44 tendons on MR imaging and 27/44 tendons on sonography). When compared with MR imaging, the sensitivity and specificity of sonography for diagnosing tendinopathy were 80% and 90%, respectively, and for diagnosing peritendinosis were 90% and 80%. Addition of abnormal size to the structural abnormality criteria did not improve diagnostic ability. CONCLUSION: Sonography can be useful as the initial imaging study in evaluating abnormalities caused by posterior tibial tendinopathy.     

The vascular territories in the cerebellum and brainstem: CT and MR study

More than 100 CT and 15 MR studies of infarcts in the cerebellum and brainstem were reviewed to define the most typical distribution of infarcts in the different vascular territories. Posterior inferior cerebellar artery and anterior inferior cerebellar artery territories are variable in size and are in a sort of equilibrium with each other. The posterior inferior cerebellar artery territory in transverse sections reveals a characteristic posterior crescent caused by its cranial posterior extension. The anterior inferior cerebellar artery territory may be limited to the lateral inferior pontine and floccular regions but usually extends over the whole petrosal surface of the cerebellum up to the lateral angle. Superior cerebellar artery territory is the most extensive territory and includes the largest part of the deep white matter. Infarcts in a single-branch distribution, vermian or hemispheric, have a characteristic sagittal or oblique orientation. Watershed cerebellar infarcts can also be recognized. In the brainstem, paramedian, lateral, and dorsal penetrating arteries have characteristic distributions at the medullary, pontine, and mesencephalic levels. With MR, lateral medullary infarcts can be demonstrated. Paramedian penetrating arteries are paired, and symmetric and small infarcts at medullary and pontine levels are sharply delimited on the midline. At the mesencephalic level, infarcts in this distribution usually involve all the arteries originating from the tip of the basilar artery and from the precommunicating segment of the posterior cerebral arteries, resulting in a central mesencephalic infarct with bilateral upward extension in the thalami. The different vascular territories in the cerebellum and in the brainstem are illustrated in schematic drawings in transverse, coronal, and sagittal planes. Knowledge of the vascular territories gained by the multiplanar capabilities of MR, and knowledge of the CT patterns of enhancement and evolution, will improve recognition and definition of infarcts.     

Fusion of MR coronary angiography and viability imaging: feasibility and clinical value for the assignment of myocardial infarctions

Purpose

To investigate the feasibility of image fusion of MR-coronary angiography (MRCA) and delayed gadolinium enhancement imaging (LGE) and to assign areas of myocardial infarction to the corresponding supplying coronary arteries.

Materials and methods

An interactive segmentation of the coronary arteries was performed in MRCA data sets (n = 25). The LGE slices were matched onto the vessel segmentation to perform a fused analysis of coronary artery anatomy and LGE. The results were compared to the segmental model recommended by the American Heart Association (AHA). Standard of reference was the identification of the culprit lesion in the invasive coronary angiography (CA) (n = 20).

Results

The fused analysis allowed the assignment of MI to the supplying coronary artery in 13/20 patients. The sensitivities/specificities for the assignment of MI to the three main vessels were: LAD 63%/100%, LCX 75%/100%, and RCA 56%/100%, respectively.Using the AHA segmental model the sensitivities/specificities for the correct assignment of MI to the three main vessels were: LAD 88%/58%, LCX 94%/75%, and RCA 77%/73%, respectively.

Conclusion

Fusion images of MRCA and LGE provides added diagnostic information in the effort to determine the epicardial vessels responsible for the postischemic myocardial injury and therefore might be helpful to establish appropriate future therapeutic steps.     

Pituitary fossa: chemical shift effect in MR imaging

The effect of chemical shift on magnetic resonance (MR) imaging of the pituitary fossa was studied. Healthy volunteers underwent conventional MR imaging of the pituitary fossa and then imaging with the frequency-encoding gradient reversed or with the phase- and frequency-encoding gradients interchanged. Comparison of the image pairs in each subject showed that the thin, black stripe evident at the water-fat interface within the pituitary fossa was altered when the gradients were changed. Therefore, the low-intensity signal within the pituitary fossa is a chemical shift misregistration effect.     

Revised classification of posterior fossa cysts and cystlike malformations based on the results of multiplanar MR imaging

MR and clinical data on 31 patients with posterior fossa CSF collections were analyzed. A clear separation of these patients into classical categories was not possible because of new information obtained from the MR images. We present a new classification of these disorders. The Dandy-Walker malformation, Dandy-Walker variant, and mega-cisterna magna seem to represent a continuum of developmental anomalies of the posterior fossa. A possible embryologic basis for this continuum is suggested. Discrete posterior fossa CSF collections that are clearly separate from the fourth ventricle and vallecula are classified as posterior fossa cysts. Posterior fossa CSF collections that communicate with the fourth ventricle and are associated with cerebellar atrophy are classified as prominent cisterna magna. Both the Dandy-Walker complex and posterior fossa cysts can cause enlargement of the posterior fossa and scalloping of the inner table of the occipital bone. The Dandy-Walker complex presents with seizures, developmental delay, and enlarging head size; it requires CSF diversion when associated with hydrocephalus. Posterior fossa cysts present with symptoms of a posterior fossa mass; they generally require surgical resection. Prominent cisterna magna is a result of degenerative disorders and requires no surgical therapy. This new classification facilitates both diagnosis and therapy of these disorders. MR revealed that disorders previously referred to as the Dandy-Walker malformation, the Dandy-Walker variant, and the mega-cisterna magna actually are not separate entities, but appear to represent steps on a continuum of developmental anomalies of the posterior fossa. Because of this, we suggest a new term, the Dandy-Walker complex, be used to describe this continuum.     

MR imaging of head and neck vascular malformations.

Between 1980 and 1990, 150 patients with cervicofacial vascular malformations were studied at the authors' institution with computed tomography, plain radiography, and angiography. Since 1989, 34 of these patients have also undergone magnetic resonance (MR) imaging. Capillary-venous hemangiomas seem to be the best indication for the adjunctive use of MR imaging. The venous pouches, characteristic of this type of lesion, cause elevated signal intensity, well seen on the T2-weighted images. Excellent fat and muscle differentiation with MR imaging allows appreciation of the depth of extension of these lesions and their delimitation from normal tissue. Arteriovenous malformations (AVMs) are characterized by serpentine signal voids, indicative of the high flow rate of these lesions. Delimitation of the AVM nidus in the midst of the afferent and efferent dilated vessels is often difficult. Study of immature angiomas with MR imaging should be restricted to lesions in specific locations (eg, orbital, laryngeal). Lymphatic malformations showed either tissular or cystic signal intensity changes. MR imaging does not replace other studies but represents an important complementary study for the delineation and diagnosis of deep extensions of vascular malformations, allowing better planning of therapy.     

Normal and degenerative posterior spinal structures: MR imaging

A retrospective study of the magnetic resonance (MR) images of the lumbar spines of 13 healthy subjects and 30 patients with degenerative changes was done. In the healthy subjects, the vertebral facets, thickness of the cartilage and ligamentum flavum, signal characteristics of the bone marrow, and size of the spinal canal were studied. In the patients with degenerative changes in one of these structures, MR images in the sagittal plane were useful in demonstrating hypertrophy of the ligamentum flavum or the vertebral facets, in grading the degree of foraminal stenosis, and in measuring the sagittal diameter of the spinal cord. MR images in the axial plane facilitated detailed analysis of the facet joint and more accurate measurements of the thickness of the ligamentum flavum and spinal canal diameter. MR images were compared with computed tomography scans in 12 patients.     

MR imaging of posterior mediastinal thymus

The diagnosis of posterior mediastinal extension of thymus was made by magnetic resonance imaging, which showed (a) continuity of the normally placed thymus with a posterior mediastinal mass, (b) identical signal characteristics between the anterior and posterior components, (c) homogeneous signal intensity greater than muscle and less than fat on T1- and moderately T2-weighted images which is characteristic of thymic tissue, and (d) characteristic posterior extension of the thymus between the superior vena cava and trachea.