Current Protocols in Magnetic Resonance Imaging最新文献

Magnetic Resonance Imaging (MRI) of the orbits and optic nerves has developed into the “gold standard” of imaging modalities for the evaluation of many soft tissue abnormalities of the orbit. Computed tomography (CT) remains the modality of choice for evaluation of the bony structures of the orbit. MRI is also more flexible, allowing for multiplanar imaging that is not possible with CT. This unit presents a basic protocol for imaging of the orbit. An alternate protocol is presented for the case of dedicated high-resolution surface coil orbital study.

Cranial nerves III, IV, V, and VI are small structures that travel in a reproducible manner from the midbrain and pons to the cavernous sinus and then to the orbit. While there are branches that course through other foramina of the skull, the emphasis in MRI is to evaluate the brainstem, the cavernous sinus, and the pericavernous regions for pathology. This unit present a basic protocol for imaging cranial nerves III to VI. Because the nerves run from a posterior to an anterior position, coronal scanning is ideal for visualizing the nerves in cross-section. Thin sections and contrast enhancement are required to best visualize the diseases that affect these nerves. An alternate protocol is also discussed for the case when demyelinating etiologies for the cranial nerve deficits are considered.

Cardiac MRI plays a pivotal role in both anatomical and functional assessment of shunts, admixture lesions, transpositions, and the surgical correction of these lesions. This unit presents basic techniques for the evaluation of congenital heart disease. While sequence parameters described are meant to be as generic as possible, parameters are most appropriate for the Siemens 1.5 T Vision or Symphony, and may need to be altered for magnets of different field strengths and manufacturers.

This unit presents the basic protocol for imaging cranial nerve I. The olfactory bulbs and tracts mediate the sense of smell from the nasal cavity to the brain. Unfortunately they are located in a precarious position for MR imaging, above the air-filled nasal cavity and ethmoid sinuses at a bone-air-soft tissue interface. This creates problems with susceptibility artifact. This issue, plus the very small size of the structures to be studied and the superimposed eye motion artifact makes imaging of the olfactory system a technical challenge.

This unit discusses the four primary sources of signal variation in a standard MRI sequence. T₁ and T₂/T₂* relaxation reveal the time dependence of the longitudinal and transverse component of the magnetization, respectively. When imaging is performed, the effective spin density is considered as the number of spins per voxel. The variation of signal with voxel size is discussed. Modification of Bloch equations that govern the motion of the magnetization is discussed for all four cases.

Cranial nerves IX to XII are rarely affected by pathology compared with cranial nerves III, V, VII, and VIII. Nonetheless, their evaluation is challenging, since lesions of these nerves span the gamut from intracranial to extracranial sites. Imaging of these cranial nerves requires a focused approach based on clinical symptomatology and signs. This unit presents the basic protocol for imaging cranial nerves IX to XII. An alternate protocol is presented for cases where non-neoplastic lesions are considered.

Extra-axial tumors most commonly arise from the meninges, calvarium, or skull base. Localization of a lesion as extra-axial or extra-cerebral in origin has significant clinical importance in terms of treatment planning and predicting prognosis. High-resolution fat-suppressed imaging is crucial for specific evaluation of the skull base, and correlation with computed tomography (CT) is often helpful in these cases. This unit presents basic MRI protocols for imaging extra-axial tumors in specific locations; sequence modifications are discussed where necessary. The sequences described in this unit are based on a 1.5 T scanner (GE Medical Systems), but can be expected to be equally applicable to other field strengths and scanners from other manufacturers.

The role of MRI imaging in the metastatic work-up is to detect the spread of tumor to the brain parenchyma, and define the location. Intravenous contrast (gadolinium-DTPA) provides the greatest sensitivity for detecting brain lesions and is almost always indicated except when there is no intravenous access. This unit presents a for imaging intra-axial brain metastases and specific modifications are discussed for the case of metastatic brain tumors demonstrating evidence of hemorrhage. The sequences described in this unit are based on 1.5 T scanner (Echospeed GE Medical Systems, Milwaukee, Wisconsin), but can be expected to be equally applicable to other field strengths and scanners from other manufacturers.

Magnetic Resonance Imaging (MRI) may be indicated in patients who have poor acoustic windows or in whom lesions are incompletely characterized by echocardiography. This unit presents a basic technique for evaluating cardiac masses, with optional contrast-enhanced sequences for specific indications. The parameters are based on experience on a Siemens 1.5 T Vision or Symphony and should be altered accordingly for different field strengths and machines from different manufacturers.