Neuroradiology And Ultrasound

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Nuclear Medicine

Nuclear medicine is used to look at function, physiology and metabolism, and the images are generally of a low spatial resolution. Radioactive elements such as technetium (Tc), which decay to produce gamma-rays, are used. They are labeled to pharmaceutical compounds (e.g. hexamethyl propylene amine oxime, HMPAO), enabling them to enter appropriate body compartments. The radiopharmaceutical can be introduced into the body and the gamma-rays (photons) can be externally recorded to produce an emission image. The recorded image can be a planar single-photon image or it can be produced using single-photon emission computed tomography (SPECT) or positron emission tomography (PET).

PET and SPECT are used to create cross-sectional functional images of the brain. SPECT, which is available in all nuclear medicine departments, can be used to study changes in regional blood flow in the brain. It has been used in the investigation of dementia, where changes in regional blood flow can differentiate between multi-infarction dementia and Alzheimer's disease. It is helpful when considering surgery for intractable complex partial seizure disorder, giving further weight to structural information concerning mesial temporal sclerosis seen on MRI.

PET radiopharmaceuticals are very short lived, requiring close proximity to a cyclotron. Installation and maintenance costs are very high, severely restricting their availability in many countries. PET can produce unique information concerning regional utilization of glucose metabolism as well as oxygen, or even fatty acid, metabolism or neurotransmitter receptor densities. It is mainly a research tool for neurophysiology and neuropharmacology. In a clinical context, it is used to help differentiate recurrent tumor from radiation necrosis and in assessing tumor response to therapy, but has a low accuracy. Applications in the study of psychiatric disease, movement disorders and epilepsy, to name but a few, are reported [22]. To some extent, the rapid developments in MRI have overtaken these applications of a very expensive technique.

Applications of conventional nuclear brain scans, such as diagnosis of cerebral infarction or subdural hematoma, are now historical.

CSF rhinorrhea or otorrhea can be investigated with nuclear medicine but CT cisternography (Fig. 2.9), or even MR cisternography [23], are now advocated by many neuroradiologists.

Ultrasound

Ultrasound is a relatively cheap, portable and safe technology. Sound-wave reflection takes place at tissue interfaces within the body, and the depth of reflection is determined by the time taken for the echo to return to the crystal. Tissues with a very high acoustic impedance, such as bone, reflect almost all of the sound waves, producing an acoustic shadow and effectively no useful image in the distal field. This is clearly of paramount importance in neuro-surgery when imaging the brain and spine.

Some of the most important applications of ultrasound within neurosurgery are described below.

Transcranial Ultrasound in the Neonate

The first real-time images of the neonatal brain were obtained through the anterior fontanelle of a newborn infant in 1979. The rate of subsequent development has been exponential, with high-quality machines and high-frequency small footprint transducers now able to produce extremely high resolution images of both the normal and abnormal neonatal brain. Doppler studies of the cerebral vasculature allow physiological monitoring of both the normal and abnormal brain with analysis of birth asphyxia, pre-term brain injury and hydrocephalus.

Vascular lesions are common in the brains of immature neonates - so frequently seen on special-care baby units. Subependymal germinal matrix hemorrhage can be reliably demonstrated, as can any subsequent intraventricular or parenchymal extension. The ischemic lesions of periventricular leukomalacia may also be recognized and are important in the differential diagnosis of brain injury. In the mature infant, extra- and intracerebral hemorrhage may result from both traumatic and other pathology and may again be diagnosed with ultrasound.

Ultrasound is the first investigation of choice in a neonate with an enlarging head and will reliably diagnose ventriculomegaly and, frequently,

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