Image Creation with Computed Tomography and Magnetic Resonance Imaging

Computed tomography (CT) scanners employ X-ray tubes, which are rotated around the patient's body while the amount of radiation actually penetrating the patient is measured by detectors. This data is analyzed mathematically (filtered back projection technique), and the density and position of different tissues are calculated and displayed. Density is represented in Hounsfield units (HU) relative to that of water (0 HU by definition). Materials that are denser than water or "hyperdense" (e.g., bone) have positive HU, while less dense or "hypodense" tissues (e.g., fat) have negative HU.

Magnetic resonance imaging (MRI) relies on the behavior of hydrogen nuclei (protons) within a magnetic field. These nuclei have random orientations in the absence of an applied magnetic field. When a patient is placed into a magnetic field created by an MR scanner, the vast majority of his or her protons are aligned along the direction of the field parallel to the bore of the magnet, creating a net equlibrium magnetization (M0). The orientation of these protons can be temporarily changed by "exciting" them with radiofrequency (RF) pulses, thereby altering the net magnetization parallel and perpendicular (transverse) to the original M0. When these protons naturally return or "relax" towards their equilibrium state—they emit measurable radio waves, the specific frequencies of which depend on their physical location and their local magnetic and molecular environment. Mathematical analysis (Fourier transformation) of these signals enables the creation of T1- and T2-weighted (WI) MRI images. T1 signal intensity (SI) depends on the time it takes for magnetization parallel to the magnet bore to return to the original value of M0. T2 SI is determined by the time required for the initial transverse magnetization that existed immediately after RF excitation to

Pediatric Neurosurgery, edited by David Frim and Nalin Gupta. ©2006 Landes Bioscience.

Table 1. Preferred initial imaging modalities classified by clinical indications

Clinical Indication

Preferred Imaging Modality

Acute neurological nhange Acute head or spinal trauma Acute stroke

Follow-up evaluation for hydrocephalus Craniosynostosis and craniofacial anomalies

Brain tumors

Developmental delay

Initial evaluation of hydrocephalus

Epilepsy

Cranial nerve palsies

Arteriovenous malformations Dural arteriovenous fistulas Vasculitis or other vasculopathies Aneurysms

Spinal fractures

Myelopathy

Suspected spinal infection Congenital spinal malformations Scoliosis

MRI, MR angiography and cerebral angiography

Unstable (untransportable) neonates

Head ultrasound dissipate. Repetition time (TR) is the time between RF excitations (or "imaging cycles"), and echo time (TE) is the point after excitation at which measurements are obtained.

Specific parameters can be selected to allow optimal visualization of stuctures and pathology in the developing nervous system. A number of these CT and MRI protocols are summarized in Tables 2-4.

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