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Computed Tomography (CT)

(Synonym: Cat Scan, Computed Axial Tomography)
Godfiery N. Hounsfield (1971), while working with central research laboratories of EMI (electromusical instruments) limited Hayes, England first described an elaborate technique in which x – ray transmission readings were taken through the head at a multitude of angles. He got a nobel prize of medicine in 1979 for describing this first effective scanning system, which was called EMI scanner, the Generation I CT machine.
Such instruments have been designed along series of generations, since their introduction by Sir Godfrey Hounsfield.
  1. 1st GEN: Translate rotate: Pencil beam, Single detection.
  2. 2nd GEN: Translate – Rotate: Fan beam, Single row array of detectors.
  3. 3rd GEN: Rotate- Rotate: Wide Fan beam, wide single row detectors to eliminate translation.
  4. 4th GEN: Rotate – Fixed: Wide fan beam, complete circular single row of detectors to eliminate one rotation component.
  5. Helical scanning: Continuous 360 degree rotation and data acquisition possible with slip rings.
  6. Multidetector CT: Cone beam, multiple row array of detectors, with slip rings.
  7. Electron Beam Scanner: Staionary source, stationary detectors, electron gun and focusers used.
  1. CT number/HU value:
    1. Basic principle of CT is linear attenuation of X-rays.
    2. Incident X-rays are linearly attenuated by their interaction with orbital electrons of tissues. Measurement of attenuation of emerging / detected beam→ gives density of intervening tissues and this density forms basis of signal intensity variation obtained in X-ray tomograms.
    3. For unenhanced CT, there is an essentially linear relationship between voxel signal intensity (image brightness) and the X-ray linear attenuation coefficient, which is scaled relative to air and water and converted to an integer.
    4. This is expressed in Hounsfield units (HU), which range from –1000 to +4000. Q
Where  and  are respectively the linear attenuation coefficients of water and air.
  1. Air have a value of –1000 HU                              
  2. Fat of -70 to -200 HU
  3. Water of 0 HUQ                                      
  4. Soft tissues of 20–60 HU
  5. Blood of 50-60 HU                                
  6. Bone of + 1000 HU
  1. Advantages of CT:
    1. Less time of acquisition             
    2. High spatial and temporal resolution      
    3. Best to detect calcification
  2. Disadvantages of CT:
    1. Modality that gives highest radiation dose (Ionizing radiation hazard)
    2. Contrast related toxicity.
    3. Due to ‘beam hardening’ artifact not good for posterior fossa tumors.
  1. Multidetector row scanners (MDCT) have more than one row of x-ray detectors, which allow them to acquire more than one image slice at the same time. Multidetector row scanners are much faster than single detector row scanners. Radiologists have used this technology to acquire ever-greater numbers of thinner and thinner slices, which has led to the emergence of many new CT applications, including Q
    1. CT angiography                                            
    2. Complex joints
    3. Facial bones                                                  
    4. ‘Virtual endoscopy
    5. Spiral pneumocolon                                    
    6. ​CT urography.
  • HRCT
    Three factors significantly improve the spatial resolution of CT such that it can be described as High Resolution Computed Tomography/HRCT and be used for studying the greater details of lung parenchyma/petrous temporal bone. These 3 principle factors include:
  1. Narrow beam collimation
  2. High spatial reconstruction or edge enhancing sharp bone algorithm for image reconstruction
  3. Small field of view
  1. HRCT clearly depicts distribution and higher definition of appearances of pulmonary parenchymal disease.
  2. HRCT is used for detection of bronchiectasis when surgery is undertaken without preoperative bronchography. Severity and extent of bronchiectasis is demonstrated.
  3. HRCT is excellent of detection and evaluation of emphysema also.
  4. HRCT can identify regions most suitable for biopsy at a time when the chest radiograph is normal.
  5. HRCT is superior to the plain chest x-ray for early detection and confirmation of suspected ILD.
Ct scan is best in the evaluation of
  1. calcification
  2. calculus
  3. acute hematoma
  4. air
Radiation Dose In a CT Scan
CTDI (CT Dose Index): CT dose index (CTDI) is a standardized measure of radiation dose output of a CT scanner which allows the user to compare radiation output of different CT scanners.
CTDI represents the average absorbed dose, along the z-axis, from a series of contiguous irradiations. It is measured from one axial CT scan (one rotation of the x-ray tube), and is calculated by dividing the integrated absorbed dose by the nominal total beam collimation. The CTDI is always measured in the axial scan mode for a single rotation of the x-ray source, and theoretically estimates the average dose within the central region of a scan volume consisting of multiple, contiguous CT scans [Multiple Scan Average Dose (MSAD)] for the case where the scan length is sufficient for the central dose to approach its asymptotic upper limit.
Dose length Product(DLP):
DLP (mGy-cm) = CTDIvol (mGy) x scan length (cm)
The DLP reflects the total energy absorbed (and thus the potential biological effect) attributable to the complete scan acquisition. Thus, an abdomen-only CT exam might have the same CTDIvol as an abdomen/pelvis CT exam, but the latter exam would have a greater DLP, proportional to the greater z-extent of the scan volume.
Radiation dose depends on tube current (amperage), slice scan time, and tube peak kilovoltage.

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