Computed tomography

Computed tomography:

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.

History of CT scan:

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.

Generations of CT scan:

First generation CT scanner of EMI (electronic and musical industry) scanner: The first generation CT scanner was developed only for the study of brain and named after the company which produced it. In this CT scanner the head of the patient was enclosed in a water bathe because the detectors used in first generation CT scan were not able to record the large radiation flux produced at the different areas of brain and therefore to produce a phantom effect.

Pencil beam geometry was used in first generation CT scan and single electron along with a reference detector was used. The tube and detector moved synchronously. Along with the x-ray tube the connected table was also spooling. The first generation CT used translate-rotate motion. The scintillation material used in the detector was Sodium iodide, it used single slice in 360° rotation.

Advantage:

Since the first generation CT used pencil beam geometry, scattered radiation and so the patient dose due to scattered radiation was also less.

Disadvantage:

The scan acquisition time was very high and a single slice would take 5-6 minutes. A complete brain study would take 55-60 minutes. Due to long scan the overall radiation dose to the patient was high.
Since the cable was moving along x-ray tube, cable spooling was a problem and this lead to cable wear and tear.
The scintillation material used in first generation CT was hygroscopic material and therefore it had to be kept in hermetically sealed envelope which causes large size of detectors. Because of large size we could not use more number of detectors. Further large sized photo multiplayer tube was used inside the detectors to convert the light signals into electrical signals which increased the size of the detector.
First generation CT was only for study of brain and not for the other body parts.
Sodium iodide was used as the scintillation material had low absorption conversion efficiency.

Second generation CT scan: It used fan beam geometry with fan angle of 10° and the number of detectors were increased up to 30 therefore to complete a semicircle of 180° the x-ray tube has to rotate only six times resulting in improvement of scan acquisition time and overall reduced radiation dose to the patient. The detectors were fitted in an arch which moved synchronously with the x-ray tube. The scintillation material was replaced with cesium iodide which had better conversion efficiency than sodium iodide.

Advantages:

Overall scan time was drastically improved and complete study of the brain was over in 5-6 minutes.
Decreased radiation dose to the patient due to less scanning time.
Improved efficiency of detectors.

Disadvantages:

Fan beam geometry caused more scattered radiation.
Increased number of detectors increases overall cost of CT unit.
Second generation CT also used translate rotate motion and cable spooling was still a problem which caused cable wear and tear.
The scanning time was still very high for emergency or cardiovascular imaging.
There was a possibility of ring artifact due to the miscallibration or defecting detector in the detector array in this generation.

Third generation CT scan: In order to enhance overall scanning time third generation CT scan used a wider fan angle of 30° so that the slice can be taken at short scanning time. Further the number of detectors was increased up to 800. They were tightly fixed in an arch due to the use of silicon diode, the size of detector was reduced and so the detectors can be packed compactly in small arch. Third generation CT used rotate-rotate motion after every 360° slice acquisition. Slice acquisition the movement of the x-ray tube would have been stopped and then the tube restarted and moves in anti-clock wise direction to take another slice of a new area.

Advantages:

Reduced scanning time due to increased number of detectors and therefore the overall radiation dose to the patient was less.
Imposed contrast resolution due to advance imaging features.

Disadvantage:

Increases scattered radiation due to wider fan angle.
Increased cost of the CT unit due to increased number of CT detectors.
Problems of cable spooling remain the same.
Ring artifact could not eliminate in this generation.

Fourth generation CT scan: It was rotate only generation. In this generation revolutionary slip ring technology was introduced which help the x-ray tube rotate continuously inside the track without interruption which emitting finely collimated x-ray beam. Fourth generation CT scan used a complete circle of fixed detectors closely packed. There were around 8000 detectors with small sized silicon photo diode for conversion of light photons into electrical signals. About 7-8 slip ring bushes were installed inside the rotating track at regular intervals. This slip ring provide necessary energy to the x-ray tube to move forward and continuously emit the x-ray. While the tube was continuously rotating, the CT couch would move ahead at a pre-determined motion rate so that every time a new surface area of the patient was exposed to take a new slice. Further the fan angle was increased up to 50° and the scintillation material used was cadmium sulphate with much better absorption and conversion efficiency. Since cables were replaced with slip ring technology, the problem cable spooling was over because the detectors ring was fixed. There were very rare chance of detector miscalibration or defeat and ring artifact was eliminated in this generation.

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.

Components of CT scan:

CT unit has two types of components:

Hardware components
Software components

Hardware components of CT include:

CT Gantry: The CT gantry is the most important component of the CT unit which accommodates x-ray tubes, slip rings, detectors system and positioning light in it. An ideal gantry has central aperture having 90cm-100cm diameter. It gives passage for the movement of CT couch during scanning. The gantry is fitted with positioning light (both ventricle and horizontal) for the patient positioning. It also has intercom facility for communication and delivery of instruction between the patient and the operator. Based on requirement CT gantry can move forward or reclined backward by 20° for parallel passing of the x-ray beam. It is also having various switches and buttons at its surface for different operations.

X-ray tubes: The x-ray tube in the CT scanner is a heavy output machine as in CT scan high KV_p180-200 and high mAs may be required to emit continuously. Because of high heat generation at the focal spot, the heat bearing capacity is 5 mega heat units to avoid the focal spot cracking; the focal spot is made up of tungsten, rhenium and molybdenum.

Detector acquisition system: In modern CT unit the detectors are closely packed in a detector ring they are fitted with very small size silicon photo diode and have highly efficient scintillation material to absorb the radiation and convert them into light photons. There is a set of pre detector collimators for every individual detector to absorb any scattered radiation before it reaches the detector system. The detectors absorb the radiation energy and produce CT signals according to attenuation of x-ray beam through the path.

CT couch: The CT couch is the examination table on which the patient lies during the scanning. It is about 2 meter long and 3 feet in width. An ideal CT couch has the following features:

It should be steady as the weight of the patient coming for examination may be above 150 kilogram. The normal weight bearing capacity of CT couch is 200 kilogram.
It should be light in weight so that it can easily move during scanning.
It should be radio apparent so that radiation can easily pass through it.
It should have low atomic number and should not interfere with the x-ray beam.
Carbon fibers with all these features fulfill the requirement and mostly used in making CT couch. The CT couch is electronically can move with a speed of 17-20mm/sec. it has the features of forward, backward as well as upward and downward motion.

Fourier transformer or ADC (analog to digital converter): It is a high power converts which extracts the electrical signals from the DAS and converts it into digital signals to be displayed on the monitor.

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.

Patient preparation for CT scan:

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.

Procedure of CT scan:

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.

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