Neuroradiology And Ultrasound

Child Skull Base Radiology

Fig. 2.5. a-b Severe headaches 2 weeks previously, with cerebellar signs. Subacute hematoma is shown with high signal on the T1- and T2-weighted images. a Sagittal T1. bTransverse T2.

FLAIR sequence (Fig. 2.6). Re-bleeding into an old collection produces characteristic appearances, with loculated areas of different signal and fluid-fluid levels.

The ability of MRI to stage different ages of subdural hematoma has an important role in the diagnosis of child abuse [13].

One recurring question concerning intracra-nial, and particularly parenchymal, hematomas is: "What is the underlying cause?". Paren-chymal hematomas occurring anywhere near the major intracranial arteries can be due to aneurysmal bleed. This is particularly seen with middle cerebral artery aneurysms, which can bleed into the adjacent temporal lobe, producing large hematomas and often surprisingly little SAH. Angiography should be considered for all patients without a clear cause of hemorrhage and who are surgical candidates, particularly young, normotensive patients [14]. Older, hypertensive patients with a hemorrhage in the basal ganglia, thalamus, cerebellum or brain-stem do not need to undergo angiography [15]. In a young adult who has negative investigations, drug abuse should then be suspected [5].

MRI is particularly helpful in the diagnosis of hemorrhagic tumor. Although there are no absolute criteria, features such as enhancement with gadolinium, marked heterogeneity of signal, and extent of edema are characteristic of tumor. Multiplicity of lesions is not always helpful as cavernous hemangiomas are multiple in 30% of cases (Fig. 2.7). Delayed scanning will be definitive with persisting edema, delayed and very heterogeneous evolution of the hemoglobin breakdown products being diagnostic of tumor. Hemorrhage tends to occur in the more aggressive tumors, such as glioblastoma multiforme, primitive neuroectodermal tumors, ependymomas, oligodendrogliomas and vascular metastases (most commonly from lung or renal primary tumors or malignant melanoma). An exception to these is pituitary adenomas, with hemorrhage occurring in up to 27% of cases, and many of these will have no clinical features of pituitary apoplexy [16].

Safety Issues of MRI

There are several potential problems with MRI and patient safety, namely the high magnetic field, the radio frequency pulses, the gradient coils and the size of the magnetic bore. Many studies have been performed to investigate the biological effects of MRI, but no biological risk has been found with MR scanners in clinical use. Even so, routine MRI is not performed during the first trimester of pregnancy, but if clinical urgency dictates, it would be used in preference to imaging modalities that use ionizing radiation. Acoustic noise produced by the gradient coils can be a problem, particularly with certain a

Fig. 2.6. a-c MRI of chronic subdural hematoma. a T1: low signal. b T2: high signal. c Proton density sequence: collection is high signal but note low signal of CSF.

c types of sequence, and routine use of earplugs is advisable.

Of most concern regarding patient safety is the presence of metallic implants, materials and foreign bodies. Listed in Table 2.4 are some of the more common contraindications to MRI.

All patients should complete a questionnaire to exclude these contraindications before entering the MR scan room. This may not be possible with the confused and unconscious patient, when it becomes the responsibility of the attending doctor.

The final problem is the size of the magnet bore, which results in significant claustrophobia and anxiety in 5% of patients. Some patients will require sedation, but once inside the scanner, direct observation is not possible and MRI-compatible monitoring equipment is required. Ferro-magnetic objects of certain types should not be brought into the MRI

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