Although many governments are now committed to reducing the number and rigidity of regulations that are thought to stifle economic innovation and competition, it is widely expected that the regulatory environment for agricultural producers and processors will become more complex in the coming years (OECD 1997). Income growth is fuelling demand for food safety and environmental amenities, and media coverage, such as reports on dioxin in European animal feed or on the effects of Bt corn on North American monarch butterfly populations, amplifies the political salience of this demand. On the ‘supply side’ of regulatory activity, officials who devise sanitary and phytosanitary (SPS) measures - regulations that sometimes restrict imports in order to reduce risks to animal, plant, and human health - face additional challenges. These officials are now bound by the multilateral legal obligations found in the Agreement on the Application of Sanitary and Phytosanitary Measures (SPS Agreement) of the World Trade Organization (WTO) which came into force in January 1995. By drawing attention to policies that were generally ignored (even by trade specialists) until the Uruguay Round, the SPS Agreement has had the intended effect of prompting widespread review of SPS measures by regulators and lawyers in both importing and exporting countries, and the unintended effect of begetting policy re-evaluation by others.

Economists in particular have started to scrutinise SPS policies in much the same way that they previously examined other risk-reducing measures, including asbestos removal or toxic waste cleanups. Taken together, these developments have substantially changed the parameters for regulating imports of agricultural products from the time when the maxim “when in doubt, keep it out” was viewed as an appropriate decision rule.

Specific Imaging Findings

The classic imaging appearance of a pilocytic astrocytoma (PA) is a well circumscribed cerebellar/fourth ventricle mass composed of a cyst and intensely enhancing mural nodule. This appearance is, however, absent in many cases of PA and is common with other neoplasms in this location and age group. The cysts are frequently absent, but the solid portions are characteristically hypodense on CT, T1 hypointense and of very high T2 signal. PAs typically exhibit avid contrast enhancement, which may be partial and patchy, and non-enhancing solid portions may also be present. The tumors ar3e bright on ADC maps, approaching the CSF signal. This very high diffusion is essentially pathognomonic for pediatric posterior fossa PA. Peritumoral edema is typically minimal or absent. PAs also occur in the brainstem, usually as well circumscribed, exophytic lesions. Supratentorial PAs commonly involve the optic pathways and hypothalamus. Perfusion imaging reveals relative cerebral blood volume to be only mildly elevated or similar to the normal brain. PAs are also found in adults and these tumors show the same imaging characteristics as in the pediatric population.

Specific Imaging Findings

On CT, ependymomas are usually iso- to hypodense compared with normal brain. Approximately 50% contain internal calcifications and hemorrhage can also be seen in approximately 10% of tumors. The tumor is heterogenous, T1 iso- to hypointense, and hyperintense on T2-weighted imaging. T1 hyperintense and T2 hypointense areas may be found, representing calcifications and sometimes blood products. The lesions are frequently heterogenous on ADC maps, the more solid portion of the tumor is generally slightly brighter than the normal brain; anaplastic (higher grade) ependymomas may contain dark areas of very low diffusion. Following contrast, ependymomas show some degree of usually heterogenous enhancement, although non-enhancing tumors can occasionally be seen, especially with recurrent disease. Perfusion studies demonstrate markedly elevated cerebral blood volume (but, unlike other glial neoplasms, poor return to baseline). Due to the propensity for leptomeningeal disease and drop metastases, imaging of the entire neural axis is required. A fourth ventricle mass that extends through the foramina of Luschka and Magendie into the cerebellopontine angle and cisterna magna is a characteristic appearance of the infra-tentorial ependymomas. Supratentorial ependymomas are commonly extraventricular, located along or near the ventricular margin within the cerebral hemispheres; they also tend to be larger and more heterogenous and are frequently anaplastic.

Specific Imaging Findings

Medulloblastomas typically arise in the midline of the posterior fossa, but may occur more laterally and sometimes extend through the fourth ventricle foramina. They are characteristically hyperdense on CT and with very low signal on ADC maps, typically darker than the normal brain. Cystic components are present in a majority of cases and the tumors are hypo- to iso-intense on T1WI. The appearance on post-contrast images is variable, ranging from marked and solid to only subtle marginal or linear enhancement. Calcification and hemorrhage may occasionally be observed, while surrounding edema is rarely prominent. Medulloblastomas have a high rate of early leptomeningeal disease and drop metastases, requiring MR imaging of the entire neural axis (head and spine). There are notable differences between the classic medulloblastoma (CMB) and some of the recently defined variants. CMB is T2 hyperintense, whereas desmoplastic/nodular (DMB) and medulloblastoma with extensive nodularity (MB-EN) are usually isointense; these two variants are also frequently located off-midline. MB-EN may show a characteristic gyriform pattern. In contrast to CMB, all medulloblastoma variants show marked contrast enhancement.

Pertinent Clinical Information

Patients with medulloblastoma are typically children, and increased intracranial pressure is responsible for common presentation with nausea, vomiting, and hydrocephalus. Due to the propensity for early leptomeningeal spread, the initial presentation may also be caused by metastatic disease, such as seizures or spinal cord compression. Extra-CNS spread may rarely occur, usually to the bone.

Specific Imaging Findings

Acute subdural hematoma (SDH) is usually a hyperdense mass located along the brain surface on CT. It may displace and compress the adjacent brain, but it does not extend into the cortical sulci. SDH has a characteristic concave interface with the underlying parenchyma, although it may also present with a lenticular shape, particularly in the hyperacute setting. Hyperacute SDH can also be of mixed density (due to active bleeding, unclotted blood and admixture of CSF), sometimes with a characteristic “swirling” pattern. SDH can spread along an entire hemisphere as it is located underneath the dura and not limited by dural attachments at the cranial sutures; however, it may not cross the falx and the tentorium. MRI reveals small SDHs that may be inconspicuous on CT. The evolution of MRI signal characteristics of SDHs differs from the intraparenchymal hematomas and they are most commonly bright on all sequences, including DWI, with a frequent relative hypointensity on T2* images. On post-contrast images, SDH shows peripheral enhancement of the dura, which may also be thickened, especially with chronic SDH. Chronic SDH may also show calcification of the thickened dura, best seen on CT.

Pertinent Clinical Information

vehicle collisions, falls, and non-accidental trauma; they may also occur spontaneously (as with coagulopathies). SDH may resolve or develop into chronic collections. Re-bleeding can occur within an existing SDH following only minor injury, often not even noticed by the patient. Surgical management of an acute SDH is based on the patient’s clinical status, size of the collection, and associated mass affect; rapid treatment is the goal as the chance of survival falls off steeply if elevated intracranial pressure is not relieved within the first 60 min, known as the “golden hour”.

Specific Imaging Findings

MRI is the imaging modality of choice for low-grade diffuse astrocytomas and the extent of the lesion is best defined on FLAIR images. They are typically homogenous intra-axial masses with the epicenter within the white matter and minimal to mild associated mass effect. They can be found in all parts of the supratentorial brain, more commonly in the insula. The tumors are usually well-delineated with high T2 signal and iso- to hypointense on T1WI, infiltrating and expanding the underlying brain; the margins are in some cases poorly defined. Diffuse low-grade astrocytomas are typically bright on ADC maps, consistent with increased diffusivity, and usually do not enhance with contrast. On perfusion studies they show relatively low cerebral blood volume, on average only slightly higher compared to the normal brain. MR spectroscopy usually shows decreased NAA levels with slightly elevated choline and without lactate. Low-grade diffuse astrocytomas are typically slightly hypodense to normal brain on CT.

Pertinent Clinical Information

The patients are typically young adults presenting with seizures without other symptoms, or even asymptomatic with the lesion found incidentally on imaging studies. These tumors undergo malignant transformation portended by clinical decline and radiographic progression. There is no consensus on the best management of adults with presumed low-grade glioma; it usually includes surgical resection, which may be followed by chemotherapy. Alternatively, the tumors can be followed with imaging after the initial biopsy, as patients may prefer to defer surgery until there is progression or a change in quality of life.

Imaging Findings

Diffuse brainstem gliomas infiltrate and occupy large portions of the brainstem, best seen as expansile hyperintense lesions on T2WI. They are hypodense on CT, of low T1 signal, and usually with minimal or no enhancement. Areas of necrosis, cystic change, hemorrhage, and focal enhancement may be present. Tumors most commonly arise in the pons and can infiltrate into the mesencephalon, medulla, or cerebellar peduncles. Exophytic growth with effacement of the basilar cisterns and engulfing of the basilar artery is frequently present. Appearance on diffusion imaging varies, usually from slightly brighter to slightly darker compared to the normal brain. MR spectroscopy shows nonspecific elevated choline and decreased NAA; increased lactate appears to be a poor prognostic sign. Focal brain stem tumors occupy less than 50% of the axial diameter of the brainstem, have well-defined margins and frequently an exophytic component.

Pertinent Clinical Information

The mean age at diagnosis is around 8 years, brainstem gliomas rarely occur in adults. Clinical presentation includes multiple cranial neuropathies, long tract signs, and ataxia. Diffuse pontine tumors are the most common and have the worst prognosis with a median survival of 9–18 months. A short duration of symptoms appears to be associated with worse prognosis. MRI is the main diagnostic modality, while biopsy is usually not performed because of the associated risks.

Differential Diagnosis

Pilocytic Astrocytoma (173)