CT Scan (Computed tomography)

      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 tissue inside your body. CT 

scan images provide more detailed information than plain X-rays do.

Tomography is derived from Greek word “tomos” meaning ‘slice’ and “graphen” meaning ‘to write’.

     CT scan is one of the best imaging method for analyzing the chest, brain and abdomen.

 It is often used for the diagnosing various cancers like lung, liver and pancreatic cancers. 

The image reveals to a physician to confirm the presence of a tumour and to measure its size,

 location and the extent of damage for the near by tissue.

    It uses special X-ray equipment to obtain a set of image data at different angles 

around the human body. The set of data processed in a computer to show a cross-section of

human body tissues and organs.

    By using CT scan we can produce clear 2-D or 3-D cross sectional images of deep internal organs.

    CAT scanner block diagram

       In CT scan X-ray slice data is generated using an X-ray source that rotates around the 

subject. X-ray sensors are placed on the opposite side of the circle from the X-ray source. 

  The sensors are scintillation detectors based on photodiodes are used.

   Measurements are taken by passing x-rays through out the body. Many data scans are taken progressively from the body and they are combined together by a mathematical procedures know as tomographic reconstruction.

   The mathematical procedure is called back projection reconstruction. By using reconstruction methods we can reconstruct the image.

X-ray source: 

     The x-ray tube generates the x-rya and direct towards the subject. The x-ray tube is fitted on a circumference of a gantry so that the image can be obtained in all 360 degrees. The person will be placed inside the gantry. A set of x-ray detectors is placed exactly opposite to the x-ray tube.

 The radiation dose for a particular study depends on many factors such as volume scanned, number and type of scan sequences, the desired resolution of image and the image quality. The intensity of x-ray can be regulated by controlling the anode voltage and beam current. The timing, anode voltage (in KV) and beam current (in MA) are controlled by a computer through a control bus.

X-ray Detector  

      The x-ray detectors are placed in ring shaped apparatus which rotate around the patient. The detectors sense the intensity of x-ray as a function of absorption property of the internal structure of body.

   

       Detection should be done by scintillation system based on photo detectors. Detection which utilizes 8, 16 or 64 detectors during continuous motion of patient through the radiation beam to obtain much clearer images with resolution and image quality.

Reconstruction of tomogram

       The CT produces a group of data which can be manipulated and processes to demonstrate various bodily structures based on their ability to block the x-ray beam. It is called windowing technique. The reconstruction of tomogram is done using a suitable computational algorithm using a computer.

      By using a computer the image ca be produced in a television screen. This is called tomogram and it can provide a very accurate cross sectional view of any area of the body. To reconstruct the image a number of mathematical operations has to be done and for this we use different computational tools.

      Back projection is a simplest tomographic reconstruction method. Modern software allows reconstruction of the tomograms in many planes so that any plane can be selected to display an anatomical structure. This may be useful for visualizing the structure of extremely small elements of body such as bronchi.

Merits and Demerits of CT scan 

  Merits

 

        * CT scan offers detailed views of tissues like lungs, bones, soft tissues and blood vessels.

        * CT scan is quick and simple. In trauma cases, it can reveal internal injuries and bleeding quickly so as to save lives. 

        * CT scan is painless, noninvasive and accurate.

        * Diagnosis with the help of CT scan has the potential to eliminate the need for invasive exploratory surgery and surgical biopsy.

         * CT scan can distinguish between normal and abnormal structures. It is a useful tool to guide radiotherapy, needle biopsies and all minimally invasive procedures.

        

  Demerits

        * CT scan involves exposure to X-ray radiation. The radiation dose from this procedure is equal to a dose that the average person receives from background radiation in three years, but the benefit of an accurate diagnosis far outweighs the risk.

       * Pregnant women can't undergo CT scan.

       * Lactating mothers can't breast feed for 24 hours after contrast injection.

       * The contrast material injected may lead to allergic reaction.

       * The gives images of only transverse section of the body.

Limitation of CT scan

     Very fine soft-tissue details in areas such as the knee or shoulder may not be revealed using CT scan. It can be more readily and clearly seen with the magnetic resonance imaging (MRI). The CT scan is not generally suitable for pregnant women.