CT Scan

Computed tomography (CT) or computerized axial tomography (CAT)

• The first clinical CT scanners were introduced in 1972 (A189).They drew on the work of two investigators who shared the Nobel prize for their work in this area in 1979 : –

1. Godfrey Hounsfield (Jharkhand 03, Nimhans 87) who worked at Electronic music industries (EMI) in London, England (sa’nhans 87) .Godfrey Hounsfield is considered as the inventer of CT/CAT ()harkhand 03).
2. Allen Cormack, who devised a means of displaying the varying attenuation coefficients found in the various portions of a section of body tissue as a gray scale image.

• Computed tomography is a special type of x-ray procedure that involves the indirect measurement of attenuation (weakening) of x-rays at numerous positions located around the patient being investigated.
• Basic principle of CT is that thin section (Slices) (PGI 99) of body are achieved to create axial sections of the body. Most CT slices (sections) are oriented vertical to the body’s axis. They are usually called transverse or axial section. For each section the x-ray tube rotates around the patient to otain a preselected section thickness. A circular array of detectors around the patient acquires data as the x-ray tube rotates around the patient. Sufficient data to image a single ‘slice’ of the body can be acquired in fractions of second, and multiple “slices” can be obtained.

Hounsfield’s number (CT number)

•  Hounsfield’s contribution is memorialized in the Hounsfield scale, which is used to measure the x-ray attenuation(AIIMS 09)
• 1ff E
• in CT scanning. Water is arbitrarily assigned a value of 0 Hounsfield units (HU) (AIIMS 04.
• The basic parameter in a CT image is CT number. The CT number is defined with respect to water X-ray absorption and is dependent on absorption / attenuation coefficient(A“ms 09)
• When an X-ray beam (used in CT) passes through the body, there is a logarithmic attenuation of its intensity over distance. A measurement of the intensity of the exiting beam is known as the linear attenuation coefficient when the attenuation thickness is 1 cm. The linear attenuation coefficient is related to the absorption in the material. This depends upon the energy of the X-ray beam and the type of material being irradiated. CT images are map of the relative linear attenuation coefficient of tissues. The relative attenuation coefficient is normally expressed in Hounsfield units (HU) or CT number(Anms 09)–

CT number / Hounsfield unit of a matter =_ x 100 1.1H20

[ix = Attenuation coefficient of matter.

 Types of CT

1. Conventional CT (non-helical) : – It obtains image data one slice at a time. The patient holds his or her breath, a slice is taken, the patient breaths, the table moves and the sequence is repeated. This requires at least 2 to 3 times the total scanning time of helical CT.
2. Helical (spiral) CT : – It uses the principle of volumetric acquisition with simultaneous patient motion and image acquisition. It is performed by moving the patient table at a constant speed through the CT gantry while scanning continously with an x-ray tube rotating around the patient. A continuously volume of image data is acquired during a single breath-hold. This technique dramatically improves the speed of image acquisition, enables scanning during optimal contrast opacification, and eliminates artifacts caused by misregisteration & variations in patient breathing.
3. Multislice CT : – The conventional CT scan has only a single row of detectors, thus only one tomographic slice or image was generated with each rotation of the x-ray tube around the patient. The multislice CT scanner has multiple contiguous rows of detectors that yield multiple tomographic slices with only one rotation of the x-ray tube around the patient. Hence, large volumes can be scanned in short period of time. This increased speed of volume coverage by the multislice CT is especially beneficial in CT angiography or dynamic CT. For example, in CT angiography or dynamic CT the multislice scanner can cover the entire abdominal aorta in 15 seconds.
4. High resolution CT (HRCT) : – HRCT acquiring 1-2 mm collimation images at 10-15 mm intervals and employing a high frequency reconstruction algorythm is the standard technique for evaluation of diffuse parenchymal lung diseases. It is the investigation of choice for Interstitial lung disease(A115, 03) and Bronchiectasis. Principles of HRCT are : ?