Specific Brain Imaging Methods Computed Tomography

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History and Physical Principles

X-rays were discovered by Wilhelm Conrad Rontgen in 1899. For this he was awarded the Nobel Prize in 1901. The high energy of X-rays makes them penetrate most biological tissues as a function of the electron density or the chemical composition of the tissue. Some tissues with a high calcium content (e.g. bone) have a high X-ray absorbing capacity (attenuation); soft tissues (e.g. brain) produce little attenuation. Accordingly, the conventional skull X-ray gives a distinct image of the bone structure surrounding the brain but is virtually useless with regard to information concerning brain structure. In 1918, Dandy developed the principle that X-rays are absorbed by air to a much lower extent than brain tissue. He injected air into the ventricular system and partly filled the cerebrospinal fluid (CSF) space with air, which allowed visualization of the ventricular system [1]. This principle, called pneumoencephalography, was used for several decades to examine brain structure in neurology. It was also used to describe ventricular enlargements in many patients with schizophrenia [2]. The painful side effects of this procedure precluded its use when other methods became available.

In commercial X-ray procedures, the X-ray tube and the detector (or film) are fixed in position and a single shot of X-rays is allowed to pass through the structure to be imaged. In 1973, Allan McCormac and Godfrey Houns-field developed methods based upon mathematical linear integrals developed by Radon in 1919 for calculation of projections (transforms) and used the information from a number of X-ray exposures of the tissue from different angles [3]. They developed algorithms to reconstruct the tomo-graphic distribution of the X-ray absorbtion in cross-sectional planes throughout the brain. The term "tomography" comes from tomos, the Greek word for cut. McCormac and Hounsfield were awarded the Nobel Prize in Physiology or Medicine in 1979 for developing the principle of computerized axial tomography (now called computed tomography, CT).

The CT technique is accordingly based on the principle of X-ray absorb-tion (attenuation) in tissues. The information from a number of X-ray exposures of the brain from different angles is used to reconstruct tomographic image planes through the brain (Figure 3.1). This is accomplished by moving the X-ray tube and the detector in a circular or spiral process around the head. In every position the attenuation profile of the patient's brain is measured. This profile is called a projection. For each turn around the head, thousands of projections are measured. Each projection is transferred to the computer for image reconstruction. This procedure markedly increases the sensitivity at low absorption or attenuation levels. Recent CT cameras use scintillation crystals that produce light when hit by an X-ray and semiconductor technique. The light signal is converted to an electric signal which is fed into the computer. Modern X-ray cameras have several hundred detector elements to record the projection. Using this technique, the absorption in many brain sections can be recorded. Various soft tissues in the brain can be imaged with reasonably good contrast. In CT scan images, various shades of grey, black or white can then be assigned to a density number corresponding to each volume element (voxel). Thus, CSF, which has a low attenuation similar to that of water, appears almost black, whereas bone appears white. As shown in Figure 3.2, the best available CT scanners produce images of brain sections where details of the tissue structures can be visualized. The outline of the ventricular system is easily distinguished from the brain tissue proper. Although it is difficult to distinguish grey from white matter, several brain nuclei such as the basal ganglia and the thalamus can often be outlined. Using various contrast media that absorb X-rays, blood flow distribution can also be visualized (Figure 3.2).

Currently available CT scanners have a resolution of approximately 1 mm. The recent elaboration of computer techniques allows the imaging of the brain of living patients through several geometrical planes. Conventional high-resolution CT scanners show morphological features of about 2 to 5-mm thick sections throughout the brain.

A major advantage of CT is that it is quite patient-friendly in comparison with MRI and PET. It takes only 5-10 minutes to make a brain investigation. It is also quite cost-effective; for routine use the cost is in the order of US$ 500. It is also available in most medical institutions.

The major disadvantage is the radiation exposure, which is of the same order as that of an ordinary skull or chest X-ray. This precludes carrying out many repeated investigations. Another disadvantage is that it can only be used to examine structural features of the brain. Compared to MRI it gives rather low resolution and poor tissue differentiation, e.g. with regard to grey and white matter.

Aplicaciones Del Fosforo

Figure 3.1 Computed tomography (CT) acquisition of data. The X-ray tube (X) and the detector (D) rotate stepwise (S) around the patient. In each position of the detector system (1, 2, etc.), the attenuation profile of the patient's head is measured. The profile is called a projection (P1, P2, etc.). For each turn thousands of projections can be created. Each projection is sent to the computer for image reconstruction Modified after Siemens-Elema AB. Copyright Per-Ake Pahlstorp

Figure 3.1 Computed tomography (CT) acquisition of data. The X-ray tube (X) and the detector (D) rotate stepwise (S) around the patient. In each position of the detector system (1, 2, etc.), the attenuation profile of the patient's head is measured. The profile is called a projection (P1, P2, etc.). For each turn thousands of projections can be created. Each projection is sent to the computer for image reconstruction Modified after Siemens-Elema AB. Copyright Per-Ake Pahlstorp

Precontrast CT Flow Image

Figure 3.2 CT images of the brain of a 75-year-old woman with acute ischaemic stroke. A Precontrast CT (note visualization of the ventricular system and bone structure; note also poor contrast between grey and white matter). B CT image after injection of X-ray absorbing contrast medium reflecting blood flow. See colour plates Modified after Siemens-Elema AB. Copyright Per-Ake Pahlstorp

Precontrast CT Flow Image

Figure 3.2 CT images of the brain of a 75-year-old woman with acute ischaemic stroke. A Precontrast CT (note visualization of the ventricular system and bone structure; note also poor contrast between grey and white matter). B CT image after injection of X-ray absorbing contrast medium reflecting blood flow. See colour plates Modified after Siemens-Elema AB. Copyright Per-Ake Pahlstorp

Research Findings

The first systematic application of the CT technique in psychiatric research was made by Johnstone et al. [4], These authors demonstrated unequivocally the occurrence of wide ventricles, wide cortical sulci and reduced size of cortical gyri in patients with schizophrenia as compared to healthy control subjects. Since then many studies have replicated these findings, and recent meta-analyses of studies in schizophrenic patients demonstrate a high consistency in this regard [5]. Alterations of the attenuation property of brain tissue and a reduced volume of specific neocortical and cerebellar regions in schizophrenia have also been reported in studies using the CT technique [6-9]. However, similar changes are often observed in pre-senile and senile dementia as well as in many cases of affective disorders, chronic alcoholism and drug abuse. Brain morphology as examined by CT also varies considerably with regard to the age of the patient and other individual factors. For this reason, all CT scanning measures in all studies have demonstrated a considerable overlap of data between psychiatrically specific diagnostic groups and control subjects. There is also evidence from several studies that subjects with apparently normal mental activity may have signs of marked cerebral changes on a CT scan. Thus, the specificity of changes reported from CT studies in various psychiatric diagnostic groups as well as their functional significance are still matters for further analysis and exploration.

CT scanning as hitherto applied by psychiatric research cannot be of diagnostic value as a single technique in psychiatry, even in dementia. In the future more refined analyses of brain morphology and the establishment of standardized quantitative reference values for various brain structures will be necessary before brain morphological dimensions as measured by CT can be introduced in psychiatric diagnostics. For this to be accomplished, the development of standardized databases for various features of human brain morphology will have to be developed. Comprehensive methods of assessing routine CT scans have recently been developed, allowing the pooling of data obtained by different research groups [10].

During recent years, developments in MRI and PET technology have to a large extent led to their gaining over the use of the CT technique in psychiatric research, because of the radiation hazards. However, it is evident that CT was the first brain imaging technique successfully used to examine biological changes in psychiatric disorders. Before the development of CT scanning, the occurrence of morphological brain alterations in such disorders besides dementias was generally not accepted by the scientific community.

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