About CT Scan

What is CT Scan ?

Computed tomography is more commonly known as ‘CT’. It was also known as ‘CAT’ which stands for Computed Axial Tomography.

CT is a way of using X-rays to take detailed pictures or images in very fine slices through the part of the body that the doctor has asked to be investigated - similar to looking at one slice of bread within the whole loaf.

Tomography is actually originated from the Greek words which are ‘tomos’ means slice, and the Greek word ‘graphein’ means write.

These multiple slices of images which are two-dimensional (2D) X-ray images that are generated during a CT scan can be reformatted in multiple planes, and reconstructed into three‐dimensional (3D) images - in other words, many pictures of the same area are taken from many angles and then placed together to produce a 3D image.

2D images of CT Scan of heart

3D images for CT Scan of heart

When CT scanners were first invented, they took one slice at a time and were quite slow when compared to today’s machines. Most modern scanners now take more than one slice at a time.

This may range from 4 to 64 slices and up to 320 slices for the most recent machines. This is referred to as “multislice” or “multidetector” technology and may be abbreviated as MSCT or MDCT.

What CT Scan is used for?

CT images of internal organs, bones, soft tissue and blood vessels typically provide greater detail than traditional x‐rays, particularly of soft tissues and blood vessels.

Using specialized equipment and expertise to create and interpret CT scans of the body, radiologists can more easily diagnose problems such as cancer, cardiovascular disease, infectious disease, appendicitis, trauma and musculoskeletal disorders.

 CT can be a life-saving tool for diagnosing illness and injury in both children and adults.

It is also often used to look inside the body before another procedure takes place, such as radiotherapy treatment or a biopsy (where a small sample of tissue is taken so that it can be examined under a microscope).

A radiographer is performing a CT Scan procedure for her patient

How does it work?

CT Scanner produces more detailed image rather than X-ray machine as CT Scanner emits a series of narrow beam compare to diagnostic X-ray machine only emit one way radiation beam.

The X-rays from the CT Scanner will be received by a detector that is located on the opposite side of your body which can see hundreds of different levels of density and also tissues inside a solid organ.

Then, images of the scan will be produced by a computer and can be viewed on a computer monitor, printed on film or transferred to a CD or DVD.

Sometimes a contrast dye is used because it shows up much more clearly on the screen. 

If a 3D image of the abdomen is required the patient may have to drink a barium meal. The barium appears white on the scan as it travels through the digestive system. If images lower down the body are required, such as the rectum, the patient may be given a barium enema. If blood vessel images are the target, the barium will be injected.

Bottles of barium that patients need to drink before undergoes CT procedure

Who does Computed Tomography?

Radiographers or radiation technologists will perform the CT Scan procedure for the patient. The CT scans are interpreted by radiologists who will examine the images taken by the radiographers in great detail to know any pathological conditions of the patient.

Videos

About how CT Scan works

About CT Scan procedure

Components of CT Scan

Ever seen this giant machine before?

CT Scan Components  and Functions

CT imaging system is made up of two main components located within the simulator which are :

1) CT Scanner
2) Laser System

Scanner Gantry

• The gantry includes the x-ray tube, the detector array, the high-voltage generator, the patient support couch, and the mechanical support for each.
•  The scanner acquire the image data that will be constructed by the virtual  simulation application into  3D virtual model of the patient.

A)    X-ray Tube

•  The x-ray tube use in multislice helical CT imaging have special requirements. The x-ray tube  can be energized up to 60s continuously.
•  Some x-ray tube operate at relatively low tube current, however  for  many, the instantaneous power capacity must be high.

B)     Scanner Bore

•  The traditional 70 cm scanner aperture with 50 cm  maximum field of view is too limiting for many radiotherapy technique. Therefore, the manufacturer  now produce ‘large bore’ versions providing apertures up to 85 cm which enable most therapy patient to be scanned in an optional position without gantry restriction.
• However, the use of wide-bore scanner will results in low contrast resolution and will increase image noise compared standard scanner.

C)     High-speed Rotor

• High-speed rotors are used in most for the best heat dissipation. Experience has shown that x-ray tube failure is a principal cause of CT imaging system malfunction and is the principal limitation on sequential imaging frequency.

D)    Focal Spot Size

•  CT imaging systems designed for high spatial resolution imaging incorporate x-ray tubes with a small focal spot.

E)     Detector Array

•  Multislice helical CT imaging systems have multiple detectors in an array up to tens of thousands. Previously, gas-filled detectors were used, but now, all are scintillation, solid state detectors.
• Scintillation detectors have high detection efficiency. Approximately 90% of the x-rays incident on the detectors are absorbed.

F)      Scanner Couch

•  A flat, stable couch is essential for radiotherapy  planning.
• Usually made up of carbon fibre which can be be adapted to fit any scanner cradle.
•  The couch must be level and orthogonal in its movement through the scanner aperture with maximum deflection of 2mm along its range.

Figure of basic components of CT Scanner

Laser System

 There are 3 types of lasers used in the CT Scan which are :

•  Internal  laser
• Wall-mounted laser
• Overhead saggital laser 

When lasers are position at zero setting, their intersection point is coincident with the center of the scan plane.

 A) Internal laser

• All scanners contain an internal laser to identify the scan plane.
• This internal laser is mounted at the scanner bore of the CT scan machine.
• This type of  laser use to mark the patient during the simulation process as well as treatment process.
• Other than that, it is also used during the QA test to determine the alignment of the laser with the center of the water phantom.

B) Wall-mounted Laser

• These types of laser usually situated at right and left of the room .
• These laser is used to mark a reference within the image data from which beam isocenter coordinate are defined.
• Other than that, it is also used to aligned the patient during  simulation and treatment process as well as aligned the couch.
• If non-moving laser is provided, these are mounted at a set height in relation to the scan aperture, requiring vertical movement to be affected  through adjustment of couch height.

C) Overhead Saggital Laser

• These laser also have the same function as wall-mounted laser which is used to mark (on the patient) coordinates defined during simulation and that may represent isocenter, field corners or markers.
• It is projecting at the same fixed distance  as lateral laser bot orthogonal to the scan plane.
• These lasers are always capable  of lateral movement because the scanner couch may not.

Treatment Couch for CT Scanner in Diagnostic Imaging

Treatment Couch for CT Scanner in Radiotherapy

Flash CT (open) rotating at full speed! 0.28sec!

Evolution of CT Scan

The History

In 1960's, after so many research, the world's first CT scanner was invented. The inventor was Godfrey Newbold Hounsfield, who was born in England 1919. He and Alan Cormack, a medical physicist, together developed and placed the first brain scanner into operation in 1971 for a company called EMI Ltd. Hounsfield conceived his idea in 1967 In 1979, they were awarded the Nobel Prize in medicine and physiology. The first EMI-Scanner was installed in Atkinson Morley Hospital in Wimbledon, England, and the first patient brain-scan was done on 1 October 1971. It was publicly announced in 1972.

 

Left; Godfrey Newbold Hounsfield

Right; Alan McLeod Cormack

The original 1971 prototype took 160 parallel readings through 180 angles, each 1° apart, with each scan taking a little over 5 minutes. The images from these scans took 2.5 hours to be processed by algebraic reconstruction techniques on a large computer. The scanner had a single photomultiplier detector, and operated on the Translate/Rotate principle.

The first production X-ray CT machine (in fact called the "EMI-Scanner") was limited to making tomographic sections of the brain, but acquired the image data in about 4 minutes (scanning two adjacent slices), and the computation time (using a Data General Nova minicomputer) was about 7 minutes per picture. This scanner required the use of a water-filled Perspex tank with a pre-shaped rubber "head-cap" at the front, which enclosed the patient's head. The water-tank was used to reduce the dynamic range of the radiation reaching the detectors (between scanning outside the head compared with scanning through the bone of the skull). The images were relatively low resolution, being composed of a matrix of only 80 × 80 pixels.

In the U.S., the first installation was at the Mayo Clinic. As a tribute to the impact of this system on medical imaging the Mayo Clinic has an EMI scanner on display in the Radiology Department. Allan McLeod Cormack of Tufts University in Massachusetts independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.

The first CT system that could make images of any part of the body and did not require the "water tank" was the ACTA (Automatic Computerized Transverse Axial) scanner designed by Robert S. Ledley, DDS, at Georgetown University. This machine had 30 photomultiplier tubes as detectors and completed a scan in only nine translate/rotate cycles, much faster than the EMI-Scanner. It used a DEC PDP11/34 minicomputer both to operate the servo-mechanisms and to acquire and process the images. The Pfizer drug company acquired the prototype from the university, along with rights to manufacture it. Pfizer then began making copies of the prototype, calling it the "200FS" (FS meaning Fast Scan), which were selling as fast as they could make them. This unit produced images in a 256×256 matrix, with much better definition than the EMI-Scanner's 80×80.

Since the first CT scanner, CT technology has vastly improved. Improvements in speed, slice count, and image quality have been the major focus primarily for cardiac imaging. Scanners now produce images much faster and with higher resolution enabling doctors to diagnose patients more accurately and perform medical procedures with greater precision. In the late 1990s CT scanners broke into two major groups, "Fixed CT" and "Portable CT". "Fixed CT Scanners" are large, require a dedicated power supply, electrical closet, HVAC system, a separate workstation room, and a large lead lined room. "Fixed CT Scanners" can also be mounted inside large tractor trailers and driven from site to site and are known as "Mobile CT Scanners". "Portable CT Scanners" are lightweight, small, and mounted on wheels. These scanners often have built-in lead shielding and run off of batteries or standard wall power.

In 2008 Siemens introduced a new generation of scanner that was able to take an image in less than 1 second, fast enough to produce clear images of beating hearts and coronary arteries.

Generations of CT Scanner

There are four generation of CT Scanner. In the first and second generation designs, the X-ray beam was not wide enough to cover the entire width of the 'slice' of interest. A mechanical arrangement was required to move the X-ray source and detector horizontally across the field of view. After a sweep, the source/detector assembly would be rotated a few degrees, and another sweep performed. This process would be repeated until 360 degrees (or 180 degrees) had been covered. The complex motion placed a limit on the minimum scan time at approximately 20 seconds per image. In the 3rd and 4th generation designs, the X-ray beam is able to cover the entire field of view of the scanner. This avoids the need for any horizontal motion; an entire 'line' can be captured in an instant. This allowed simplification of the motion to rotation of the X-ray source.

The a, b, c and d images are the x-ray beam distribution of 1st, 2nd, 3rd and 4th generation of CT scanner respectively. First generation  detectors: one type of beam: pencil-like X-ray beam tube-detector movements: translate-rotate duration of scan (average): 25-30 mins Second generation  detectors: multiple (up to 30) type of beam: fan shaped x-ray beam tube-detector movements: translate-rotate duration of scan (average): less than 90 s Third generation  detectors: multiple, originally 288; newer ones used over 700 arranged in an arc type of beam: fan shaped x-ray beam tube-detector movements: rotate-rotate duration of scan (average): approximately 5s Fourth generation detectors: multiple (more than 2000) arranged in an outer ring which is fixed type of beam: fan shaped x-ray beam tube-detector movements: rotate-fixed  duration of scan (average): few seconds