Chordomas of the skull base: MR features.

PURPOSE: To characterize the MR features of skull base chordomas with regard to signal intensity, size, position, extension, and Gd-DTPA enhancement. PATIENTS AND METHODS: The MR imaging studies of 28 patients with surgically proven chordomas of the skull base were retrospectively reviewed. Twenty-two of these patients received intravenous administration of Gd-DTPA. RESULTS: On short TR/short TE images, chordomas generally had low to intermediate signal. On long TR/long TE images, chordomas generally had very high signal that was heterogeneous in 79%. After Gd-DTPA administration, all chordomas demonstrated some degree of contrast enhancement. In most cases, enhancement was demonstrated throughout most of each tumor in a heterogeneous pattern. Chordomas were associated with MR findings of displacement and encasement of vessels, and frequent extension into adjacent structures such as the cavernous sinus, sella, nasopharynx, and hypoglossal canal. CONCLUSION: The MR characterization of the position and extent of these neoplasms played an important role in determining the optimal surgical approaches for gross total tumor resection.     

Neoplastic diseases affecting the central skull base: CT and MR imaging.

Modern imaging techniques play a vital role in the diagnostic evaluation and follow-up of patients with neoplastic disease affecting the skull base. Many of these lesions have a high rate of recurrence if surgical removal is not complete. Newer, more aggressive surgical approaches can have a maximal effect only if the full extent of a lesion is known preoperatively. CT and MR imaging provide the surgeon with detailed information about every site of tumor involvement. Although these lesions are not common in clinical radiologic practice, the radiologist must be prepared to offer a reasonable differential diagnosis and a full evaluation of the extent of disease. This review presents some of the more commonly encountered tumors that can affect the skull base and describes their radiologic features, with emphasis on CT and MR imaging.     

CT and MR imaging of the central skull base. Part 2. Pathologic spectrum

The radiologist must have a thorough knowledge of the normal anatomy and the pathologic spectrum of the skull base to determine the extent of abnormality and to help plan the surgical approach. The authors describe and present examples of congenital, benign, and malignant lesions that affect this region, including cephalocele, fracture, fistula, juvenile angiofibroma, meningioma, chordoma, pituitary adenoma, chondrosarcoma, nasopharyngeal carcinoma, and rhabdomyosarcoma. Metastatic, infectious, and other miscellaneous processes are also discussed. Imaging strategies with computed tomography and magnetic resonance imaging to aid in the diagnosis are suggested.     

Radio-frequency coil selection for MR imaging of the brain and skull base

Radio-frequency coils play a crucial role in the quest for optimal magnetic resonance (MR) image resolution. Given the growing variety of specialized coils available for neuroradiologic imaging applications, it is critical that radiologists use a coherent strategy for successfully matching these coils to specific imaging situations. First, fundamental concepts of coil design are reviewed. Subsequently, a coil-selection algorithm for neuroimaging applications is described. The algorithm uses the patient's clinical history to derive a region of interest, a desired spatial resolution, and a desired contrast resolution. These factors are then used to impose anatomic coverage and imaging protocol constraints on the set of available coils. Finally, coil selection is further refined according to patient tolerance factors. The following coils are considered for use with a 1.5-T superconducting MR imager; namely, quadrature birdcage head, neurovascular phased-array, and dual single-circular-element coils, as well as investigational coils that have not yet been approved by the U.S. Food and Drug Administration: reduced-volume birdcage end-cap, temporal lobe phased-array, carotid artery phased-array, coils. Rationales are discussed regarding appropriate coil selection for screening whole brain and imaging brainstem, cranial nerves, orbits, cerebral cortex, mesial temporal lobes, and internal auditory canal, and for MR angiography.     

Orbit, skull base, and pharynx: contrast-enhanced fat suppression MR imaging

The high signal intensity of fat on T1-weighted magnetic resonance images has limited the utility of gadopentetate dimeglumine in imaging of the extracranial head and neck. Enhancing lesions may be obscured either by proximity to fat or by chemical misregistration artifact. The authors evaluated the role of a gadolinium-enhanced fat suppression imaging technique in the detection of extracranial head and neck abnormalities in 29 patients. These studies were directly compared with conventional pre- and postcontrast T1- and T2-weighted SE sequences. In detecting and defining the extent of abnormalities, fat-suppressed images were superior to non-fat-suppressed gadolinium-enhanced T1-weighted images in the majority of cases (22 of 27 [81%]). Fat-suppressed images were particularly beneficial in the detection of perineural spread of tumor as well as in defining lesions situated within or adjacent to fat-containing areas such as the base of the skull. These findings demonstrate that fat suppression techniques in combination with gadolinium enhancement are of value in extracranial head and neck imaging and should replace conventional postcontrast T1-weighted SE imaging.     

Registration of MR and CT images for skull base surgery using point-like anatomical features.

We have developed a registration technique for combining magnetic resonance imaging (MRI) and computed tomography (CT) images of the skull base for use in surgical planning. The technique is based on user identification of point-like landmarks visible in both modalities. The combination of images involves a small amount of expert interaction, is relatively quick and preliminary evaluation indicates that it is accurate to within 1.5 mm. Registered or fused images can be viewed either on an image processing workstation, or fused images can be printed onto conventional film for convenience in clinical use. We present one patient in order to demonstrate the technique's indications and advantages.     

兔VX2鼻咽癌侵犯颅底的MR检查与病理对照研究

目的 探讨建立兔VX2鼻咽癌颅底侵犯模型的可行性,并借助动物模型研究MR诊断鼻咽癌颅底侵犯的病理基础.方法 采用25只新西兰兔建立鼻咽癌颅底侵犯动物模型,对模型进行鼻咽颅底瘤灶MR平扫检查和病理检查,将MRI所见与病理检查结果进行对照分析.结果 成功建立新西兰兔鼻咽癌颅底侵犯动物模型18具,建模成功率72%.18具模型肿瘤最大径的MRI测量值和病理(大体标本)测量值均数分别为(3.5±0.8) cm和(3.4±0.7) cm.18具MRI显示颅底骨信号异常的模型中,病理检查示颅底骨小梁结构和骨髓破坏16例、局部骨皮质和骨髓变性伴纤维组织增生2例.2具模型MRI和病理检查均显示颅底骨结构无异常.模型肿瘤最大径的MRI测量值和病理测量值均数无统计学差异(P＞0.05);MR对兔VX2鼻咽癌颅底侵犯的诊断敏感性和特异性分别为100%和50%.结论 兔VX2鼻咽癌颅底侵犯模型为最接近理想状态的动物模型;MR平扫检查可准确显示颅底骨病变的存在,尚不能明确判断颅底骨病变的病理性质.     

Arrested pneumatization of the skull base: imaging characteristics

OBJECTIVE: Arrested skull base pneumatization is a benign developmental variant that can be confused with significant skull base disease processes. This study reviews the imaging findings in 30 suspected cases of arrested skull base pneumatization. CONCLUSION: When encountering a nonexpansile lesion with osteosclerotic borders, internal fat, and curvilinear calcifications in the basisphenoid bone or adjacent skull base, radiologists should strongly consider the diagnosis of arrested pneumatization.     

Magnetic resonance imaging of the skull base

Magnetic resonance imaging (MRI) is the principal imaging procedure for assessing skull-base lesions. Superior contrast resolution, increased lesion conspicuity, better definition of the margin of the lesions and multiplanar capability are its main advantages. Computed tomography is currently used to assess skull-base lesions when superior bone detail is required or when MRI cannot be carried out.     

CT and MR imaging of the central skull base. Part 1: Techniques, embryologic development, and anatomy

Recent advances in surgical techniques have enabled surgeons to approach previously inoperable deep-seated lesions of the skull base. The radiologist requires a thorough knowledge of the normal anatomy and the pathologic spectrum of this region and an understanding of imaging modalities in order to determine the extent of pathologic conditions and help plan the surgical approach. The embryologic development of the central skull base, normal gross anatomy, and anatomy as seen on computed tomographic and magnetic resonance images are presented.     

MR imaging features of medulloblastomas.

The preoperative MR studies of 25 patients with surgically proved medulloblastomas were retrospectively reviewed in order to characterize these neoplasms with regard to their MR signal intensity, size, location, and appearance after contrast enhancement. Gadopentetate dimeglumine--enhanced MR images were available in 11 patients. On short TR/short TE images, medulloblastomas generally had low to intermediate signal, and were predominantly slightly hyperintense relative to muscle and hypointense relative to white matter. On long TR/long TE images, medulloblastomas generally had intermediate to moderately high signal, predominantly hyperintense relative to muscle and white matter. Tumor signal relative to gray matter varied considerably on both short TR and long TR images. Signal heterogeneity on long TR/long TE images was observed in 91% of the lesions and resulted from intratumoral cystic zones, small blood vessels, and/or calcifications. In the patients who received gadopentetate dimeglumine, the fraction of tumor volume showing enhancement was found to be less than one third in two cases, between one third and two thirds in four cases, and greater than two thirds in five cases. The mean tumor size was 3.6 x 4.0 x 3.5 cm. The most frequent location of medulloblastoma was the mid and inferior vermis. We conclude that the unenhanced and enhanced MR characteristics of medulloblastomas are somewhat variable. Medulloblastomas should be included in the differential diagnosis when the MR findings described are present in the appropriate patient population.     

Three-dimensional MR reconstruction images of skull base tumors

Forty-eight patients with skull base tumors were evaluated prospectively with T1-weighted spin-echo two-dimensional (2D) magnetic resonance (MR) sequences, a three-dimensional (3D) MR TurboFLASH (fast low-angle shot) sequence, and a 3D reconstruction window technique. All patients underwent surgery with histopathologic correlation, and the three MR imaging techniques were compared to assess representation of tumor margins and the topographic relationship of tumor to surrounding tissue and adjacent vasculature. The best results were obtained with standard 2D spin-echo sequences after administration of the paramagnetic contrast agent gadopentetate dimeglumine. The 2D MR sequences gave the highest contrast-to-noise ratios, with decreasing values for 3D sequences and 3D reconstructions, respectively. Nevertheless, 3D MR imaging, by virtue of its good representation of adjacent structures, aided surgeons in planning surgical intervention. This study presents the technical features of 3D imaging of the skull base, the choices involved in its implementation, and its potential clinical applications.     

MR imaging evaluation of endoscopic cranial base reconstruction with pedicled nasoseptal flap following endoscopic endonasal skull base surgery

Introduction

Postoperative CSF leak is the most common complication of endoscopic endonasal approach (EEA) to skull base lesions. Endoscopic multilayer closure of skull base defect using pedicled nasoseptal flaps (NSF) based off the sphenopalatine artery reduces CSF leaks. EEA robustly expands in surgical arena, yet postoperative imaging evaluation remains poorly studied. This work illustrates normal MR imaging appearance of skull base reconstruction utilizing NSFs during immediate postoperative period.

Methods

We retrospectively identified patients who had skull base reconstructions utilizing NSFs following EEAs and immediate postoperative-enhanced brain MRI. NSFs and free grafts were evaluated for signal intensity, thickness, configuration, enhancement, vascular pedicle of NSF, relationship in multilayer reconstruction, and defect coverage. Imaging findings were correlated with surgical technique and CSF leaks.

Results

Twenty-eight patients had 26 multilayer reconstructions and 34 NSFs. Twenty-nine NSFs showed enhancing C-shaped arc at the skull base. Of those, 26 flaps (90%) were confidently identified by visualization of their vascular pedicles, 3 were not distinguishable from adjacent mucosa and pedicles were not identified. Five NSFs showed no enhancement (1 CSF leak). Twenty-seven enhancing NSFs approximated defects with close abutment to free grafts. One flap was displaced; one incompletely covered the defect (2 CSF leaks). Fisher exact test demonstrated an association between incomplete defect coverage and displacement of NSFs with CSF leak (P = 0.05).

Conclusion

Endoscopic skull base reconstruction utilizing NSF has characteristic MR imaging appearance. Non-enhancing mucosal gap or displacement of NSF may indicate incomplete defect coverage, identifying patients at risk for CSF leak.     

MR, CT, and plain film imaging of the developing skull base in fetal specimens

BACKGROUND AND PURPOSE: The developing fetal skull base has previously been studied via dissection and low-resolution CT. Most of the central skull base develops from endochondral ossification through an intermediary chondrocranium. We traced the development of the normal fetal skull base by using plain radiography, MR imaging, and CT. METHODS: Twenty-nine formalin-fixed fetal specimens ranging from 9 to 24 weeks' gestational age were examined with mammographic plain radiography, CT, and MR imaging. Skull base development and ossification were assessed. RESULTS: The postsphenoid cartilages enclose the pituitary and fuse to form the basisphenoid, from which the sella turcica and the posterior body of the sphenoid bone originate. The presphenoid cartilages will form the anterior body of the sphenoid bone. Portions of the presphenoid cartilage give rise to the mesethmoid cartilage, which forms the central portion of the anterior skull base. Ossification begins in the occipital bone (12 weeks) and progresses anteriorly. The postsphenoid (14 weeks) and then the presphenoid portion (17 weeks) of the sphenoid bone ossify. Ossification is seen laterally (16 weeks) in the orbitosphenoid, which contributes to the lesser wing of the sphenoid, and the alisphenoid (15 weeks), which forms the greater wing. CONCLUSION: MR imaging can show early progressive ossification of the cartilaginous skull base and its relation to intracranial structures. The study of fetal developmental anatomy may lead to a better understanding of abnormalities of the skull base.     

Deltoid contracture: MR imaging features

AIM: To describe the magnetic resonance imaging (MRI) features of deltoid contracture and compare these findings with the operative and histological findings and to determine the utility of MRI for diagnosis and treatment planning. PATIENTS AND METHODS: Retrospective review of clinical and imaging in six patients with deltoid contracture, as well as the operative and histological findings of four operated patients. RESULTS: Magnetic resonance imaging clearly demonstrated the intramuscular fibrous bands of the deltoid as a homogeneously hypointense area with distinctive margins on T1-, T2- and T2*-weighted images. Operative findings were exactly consistent with the findings observed by MRI. CONCLUSION: Deltoid contracture is best evaluated with MRI which facilitates visualization of the intramuscular fibrous bands being pathognomonic of this entity and may provide information useful in treatment planning.     

Hemifacial spasm: MR imaging features

MR imaging was used to evaluate the relationship of the root exit zone of the seventh cranial nerve to surrounding vascular structures in 13 patients with clinically documented hemifacial spasm and in 70 asymptomatic patients. MR imaging clearly demonstrated the course of the seventh nerve from the root exit zone of the brainstem to the internal auditory canal and its relationship to the surrounding vertebrobasilar system. The presence of a vascular structure at the root exit zone of the seventh nerve was identified in all 13 patients with hemifacial spasm. In the 70 asymptomatic patients, examination of 140 seventh nerves revealed that 21% had contact by a vascular structure at the root exit zone of the seventh nerve. Our results indicate that although neurovascular contact may be asymptomatic, MR demonstration of a vascular structure at the root exit zone of the seventh cranial nerve in a patient with hemifacial spasm may implicate neurovascular compression as the cause of symptomatology. This finding may alter therapeutic management. Because of the inherent limitations of CT in the visualization of posterior fossa structures, MR imaging should be considered the initial screening procedure in the assessment of patients with hemifacial spasm.     

Trigeminal neuralgia: MR imaging features

Magnetic resonance (MR) imaging was used to evaluate the relationship of the cisternal portion of the fifth cranial nerve to surrounding vascular structures in six patients with documented trigeminal neuralgia and in 85 asymptomatic patients. MR imaging clearly demonstrated the course of the fifth nerve from its root entry zone (REZ) to the Meckel cave and its relationship to the surrounding vertebrobasilar system. In the six patients with trigeminal neuralgia, the presence of a vascular structure at the REZ of the fifth nerve was identified. In the 85 asymptomatic patients, examination of 170 trigeminal nerves revealed that 30% had contact between a vascular structure and the fifth nerve at the REZ, but only 2% had actual deformity. These results indicate that although neurovascular contact may be asymptomatic, MR demonstration of a vascular structure at the REZ of the fifth cranial nerve in a patient with trigeminal neuralgia may implicate this as the cause of symptoms, which may influence the treatment of choice. Because of the inherent limitations of computed tomography in the visualization of posterior fossa structures, MR imaging should be considered the initial screening procedure in the assessment of patients with trigeminal neuralgia.