What is CT (Computed tomography) scan?
Tomo means slices and graphy means study.
Computed tomography is also known as the CAT (computer axial tomography) scanning. Literal meaning of computed tomography is to record the image of internal anatomical structure in the form of slices. A CT image display of internal anatomical structures in the form of thin slices obtained with the help of multiple absorption measurement made around the body periphery of the patient with the help of high output x-ray machine. The x-ray tube continuously rotates around the body periphery of the patient emitting finely collimated x-ray beam.
In each projection there is certain attenuation of the x-rays which is measured by the corresponding detectors. The detectors have scintillation material which absorbs the energy of the x-ray photons reaching the detector and convert them into light photons. These light photons are then converted into electrical signals with the help of the silicon photo diode, digital signals using ADC (analog to digital converter) or Fourier transformer. These digital signals are then displayed in the form of thin slices over the monitor.
CT scan was combining developed by Sir Godfrey Hounsfield of EMI from UK and Sir Allacorma RC of USA in 1979 that provides complex mathematical formulae for image reconstruction.
Non contrast computed tomography (NCCT): Plain CT scan without the contrast media, it takes approx. 5 minutes to do the scan. It is normally done in routine cases.
Contrast enhanced computed tomography (CECT): In CECT contrast media is injected, that contrast media enhance the internal anatomical structures; it is easy to diagnose the pathology like mass lesion, calculi, tumor etc.
High resolution computed tomography (HRCT): in high resolution CT scan the thickness of slice of image is too small it is mainly done in the case of diagnosis of chest pathology. Its thickness is about 0.1mm that is very small by the normal image slice.
Basic principle of computed tomography:
The images of the internal parts are taken in the form of thin slices by linear and rotatory movement of x-ray tube.
In earlier CT scan two out of three components i.e. patient, x-ray tube and detector were moving. In modern CT scanners patient and detectors are fixed whereas x-ray tube and CT couch are moving. CT scan gives very good contrast and special resolution and presents a 3D image of anatomical structures for better diagnostic information.
Since its invention in 1956 CT scan has been evolved with many new features and rapid scan acquisition time. Based on this CT scan is categorized into generations from 1-4 generations.
Fourth generation CT scanner drastically improved scanning time and a complete brain study was over 10-12sec. further introduction of the mathematical filters improved the signal to noise ratio for better contrast resolution. Fourth generation CT scan also opened up a new horizon in cardio vascular imaging because of its short scanning time and negligible possibility of motion artifact.
CT unit has two types of components:
Hardware components of CT include:
Software components of CT scan:
The software components of CT includes a series of computers with sophisticated interlink with gantry. Also laser film printers, monitors, keyboards, electronic pressure injectors etc. are included in software component of a CT unit. The CT computer has picture archiving system and can store a large number patient information and slice images for reference purpose however it has a memory capacity and very old examination data are replaced with new data. There is a dedicated work station in CT department with advance computer system for information and 3D re-construction and printing.
Explain the procedure.
Apply 10 day rule. If there is any chance of pregnancy or pregnant patient she will not allowed in the radiation zone.
Ask for any previous allergic history.
While examine for a female patient a female co-worker should be present and for male patient a male co-worker should be present.
Patient should be fasting for 4-6 hours before the procedure.
Ensure normal KFT & LFT values.
Sensitivity test for ensure allergic reaction.
Contrast enhanced computed tomography (CECT) abdomen require filling the abdomen with contrast medium fluid to visualize them better.
Bladder should be filled with urine for better visualization in CT KUB.
Prior abdominal scan laxatives and purgatives are prescribed for bowel evacuation.
Take the consent from the patient.
IV cannula inserted should be of suitable size [<20G].
Patient should be lying comfortably on the CT couch.
Position the part that will be examined.
Tell patient when to hold the breath and when to release in chest and abdominal scan.
In CECT scan first take plain scan after that inject appropriate amount of contrast media by pressure injector or manually take another scan.