What is the basic principle of CT scan?

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CT uses ionizing radiation, or x-rays, coupled with an electronic detector array to record a pattern of densities and create an image of a “slice” or “cut” of tissue. The x-ray beam rotates around the object within the scanner such that multiple x-ray projections pass through the object (Fig 1).

What is the physics behind CT scans?

During a CT scan, the patient lies on a bed that slowly moves through the gantry while the x-ray tube rotates around the patient, shooting narrow beams of x-rays through the body. Instead of film, CT scanners use special digital x-ray detectors, which are located directly opposite the x-ray source.

What are the 4 types of CT?

CT Scan Brain/ CT Scan Head. CT Scan Chest (CT Scan Lung) CT Scan Neck. CT Scan Pelvis.

What are three major components of a CT scanner?

CT scanners are composed of three important elements: an X-ray tube, a gantry with a ring of X-ray sensitive detectors, and a computer. In this method, images are created using the same physics principles as in conventional radiography.

How image is formed in CT scan?

CT images are two-dimensional pictures that represent three-dimensional physical objects. The images are made by converting electrical energy (moving electrons) into X-ray photons, passing the photons through an object, and then converting the measured photons back into electrons.

How is CT number calculated?

The CT numbers are calculated from the x-ray linear attenuation coefficient values for each individual tissue voxel. It is the attenuation coefficient that is first calculated by the reconstruction process and then used to calculate the CT number values.

What color is air on CT?

Air appears black. With CT scanning, several x-ray beams and electronic x-ray detectors rotate around you. These measure the amount of radiation being absorbed throughout your body. Sometimes, the exam table will move during the scan.

What is pixel and voxel in CT?

Voxel size is an important component of image quality. Voxel is the 3-D analog of a pixel. Voxel size is related to both the pixel size and slice thickness. Pixel size is dependent on both the field of view and the image matrix. The pixel size is equal to the field of view divided by the matrix size.

Why CT scan is used?

CT scans may be performed to help diagnose tumors, investigate internal bleeding, or check for other internal injuries or damage. CT can also be used for a tissue or fluid biopsy.

Are there 2 types of CT scans?

A head CT captures images of the brain and other areas of the head, such as the sinuses. Patients with ongoing sinus issues might benefit from a head CT to determine if there is ongoing inflammation in the area. CT scan chest/lungs: A CT scan of the chest can provide a doctor with detailed images of a person’s lungs.

Which ray is used in CT scan?

A computerized tomography (CT) scan combines a series of X-ray images taken from different angles around your body and uses computer processing to create cross-sectional images (slices) of the bones, blood vessels and soft tissues inside your body.

What is CT number in radiology?

Computed tomography (CT) number is a calculated value reflecting the X-ray attenuation coefficient in an image voxel, generally expressed in Hounsfield units (HU), where the CT number of water is 0 HU.

What is collimator in CT scan?

The collimator is located immediately in front of the detectors to protect them from scattered X-rays. Ideally, each detector in a CT scanner measures intensity of X-rays that reach the detector after traveling along a straight-line path from the X-ray source to the detector.

What is a gantry in CT?

Definition: gantry. gantry. A frame housing the x-ray tube, collimators, and detectors in a CT or radiation therapy machine, with a large opening into which the patient is inserted; a mechanical support for mounting a device to be moved in a circular path.

How do slip rings work in CT?

Slip rings are used to transfer the necessary electrical energy to the rotating gantry and to transmit the measured data from the rotating part to the computer system; the cables that were used traditionally in CT scanners and which limited scanning to single 360° turns (alternating in clockwise and counterclockwise …

What is the CT number of air?

The unit for CT numbers is Hounsfield units (HU); from the definition, water is zero HU and air is − 1000 HU .

What is contrast in CT scan?

CT scans may be done with or without “contrast.” Contrast refers to a substance taken by mouth or injected into an intravenous (IV) line that causes the particular organ or tissue under study to be seen more clearly. Contrast examinations may require you to fast for a certain period of time before the procedure.

What is CT measured in?

This calculation is always based on the X-ray attenuation characteristics of the tissues within the voxel. The CT number is given in Hounsfield units (HU). A voxel consisting of only water or air will have a CT number of 0 and −1000 Hounsfield units, respectively.

What is HU value?

The Hounsfield unit (HU) is a relative quantitative measurement of radio density used by radiologists in the interpretation of computed tomography (CT) images. The absorption/attenuation coefficient of radiation within a tissue is used during CT reconstruction to produce a grayscale image.

What is CT number accuracy?

accuracy: CT number consistency and uniformity. CT number consistency. dictates that if the same phantom is scanned with different slice thicknesses, at. different times, or in the presence of other objects, the CT numbers of the. reconstructed phantom should not be affected.

Why is blood bright on CT?

Tissues like air and water have little attenuation and are displayed as low densities (dark), whereas bone has high attenuation and is displayed as high density (bright) on CT. Among pathologic conditions, high density lesions are often seen with freshly clotted blood, hyperemia and with the use of contrast.

Why is CT blood white?

Acute haemorrhage absorbs X-rays and appears hyperdense (white) on CT scans. As the clot retracts it becomes more hyperdense over the first few hours up to 7 days; then isodense with brain over the following 1-4 weeks and finally hypodense compared with brain over the subsequent 4-6 weeks.

What are the black parts in a CT scan?

On CT or MRI scans, brain lesions appear as dark or light spots that don’t look like normal brain tissue. Usually, a brain lesion is an incidental finding unrelated to the condition or symptom that led to the imaging test in the first place.

What is CT pixel size?

Pixel: Two dimensional picture element that makes up the matrix. Each pixel represents a CT number and is the building block of the matrix and image. Pixel Size = DFOV/matrix. Prospective data: An image automatically reconstructed from the scan data.

What is matrix size in CT?

Image matrix size of 512 × 512 has been the standard for almost all clinical CT applications.

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