CT SCAN (Computed tomography)

Thursday, March 12, 2015

Computed Tomography CT Imaging System is Innovative

CT scanning has no usual image receptor, such as film or an image intensifier tube. This machines are well collimated x-ray beam and directed to the patient. Attenuated image forming radiation is usually measured by a detector then transmitted to a computer for analysis and construction of the images. After the computer analyzes the signal from the detector, the computer reforms the image and displays the image on a screen. The computer reconstruction of the cross-sectional structure is accomplished with mathematical equations (algorithms) and revised for computer processing.To do CT imaging well, the Radiologic technologist needs to know a little of the development of CT, the system components, and the basic features of the CT images.
ct scan
Allesandro Vallebona

What is CT SCAN?

Is a method of acquiring and reconstructing the image of a thin cross section on the measurement of attenuation. Ct is also called as the following name; Computed Tomography, Computed Axial Tomography and Body Section Roentgenography. It is came from the greek word TOMOS which means a slice or section, and GRAPHIA means describing. A binary system is use to construct a diagnostic images of the ct scan provided by the computer.
CT scan is an x-ray source and has detectors across from each other located in about 180 degrees from tube to end, it move 360 degrees around the patient, continuously and sends information about attenuation of x-rays as x-ray pass through the body. Very thin x-ray beams are utilized, which minimizes the degree of scatter radiation and results in optimizing the images unlike conventional radiographs. In Computed Tomography a computer act upon and calculate the gathered data and assigns numerical values based on the subtle differences in x-ray attenuation. Based on these values, a gray scale axial image is generated and can be identify between objects with even small differences in density. 

BIT / Binary Digit Convertion

Can code for 2 values or 2 shades of gray which correspond to white and black.
8 bits = 1 Bytes ; 2 Bytes = 1 word (16 Bits)
A total of 256 shades of gray 2 can be coded for by 1 byte (8 bits)
A total of 4096 shades of gray 2 can be coded for by 12 bits.
kilobytes, megabytes, gigabytes or terabytes

Brief History of the Ct Scan

1930's  - Allesandro Vallebona proposed a method to represent a single slice of the body on the radiographic fim; ''TOMOGRAPHY''.
ct scan
Godfrey Nerbold Hounsfield
1972 - Invented by Godfrey Newbold Hounsfield of EMI laboraties, England and Allan Mcleod Cormark of tuffs university; Cormark Developed the mathematics used to reconstruct ct images.
1974 - 1976  - First clinical ct scanners were installed
1979 - Both Hounsfield and Cormark shared the nobble prize in medicine
1980 - CT scan machine became widely available and its the first Ct scan system that can produced a ct images of any part of the body without requiring a water tank for scanning.

Geometrical Design of the CT Machine

1st Generation CT scan

The First generation of ct is a Rotate - Translate type of scanner. It has a Pencil beam and  has only one detector, it is slow scanning and took around 5 minutes to complete one scan (a 180° rotation).

2nd Generation CT scan

The 2nd generation ct scan is Rotate - Translate scanners. It has a narrow fan beam with multiple detector installed ranges from 5 to 30 detectors. The disadvantage of the is scanners is that, it has a drawback effect of increase scatter radiation.

3rd Generation CT scan

The 3rd generation is a Rotate - Rotate scanners, it has a wide fan beam and installed with a curvilinear array detector. The 3rd generation contains many detectors and has a drawback of Ring artifacts. 3rd Generation ct scanner is designed to have a thin tungsten SEPTA placed between each detector in an array and focus to the xray source, this advancement is to reject the scatter radiation by the detectors.

4th Generation CT scan

The 4th generation of ct is a Rotate - stationary scanners. Similar to 3rd generation scanners but this has detector rings. Disadvantage of this ct scanner is a drawback patient dose, as the detectors are not coupled to the xray source and hence it cannot focus and make use of SEPTA to reject the scattered radiation. However, the detectors are program to a self-calibrating system, the detectors are calibrating twice during each rotation. While the 3rd generation scanner are automatically callibrating once only in every fer hours.

5th Generation CT scan

The 5th generation of the ct machine is called the ultrafast ct scanner or the electron beam ct. It was introduced in the early 1980's by medical physicist Andrew Castagnini.
Castagnini use a different scanning technology than other CT scanners, this were x-ray tube rotation. xray tube rotation is a mechanical and have no moving parts, which makes scanning fast as possible. It has also a feature called the Electron Gun that produce a focused electron beam and generate a rotating x-ray fan beam, this fan beam are produced after being steered along tungsten target ring.

6th Generation CT Scan

The 6th generation ct is a helical or spiral CT scan or sometimes called the Volumetric Scanners. It was introduced in the early 1990's, with much development of these developers Will Kalender and Kazuhiro Katada. The x-ray source and detectors in 3rd generation is design to freely moves and attach to the rotating gantry,  The scanning in the 6th generation, table with the patient moves smoothly through the scanner, this derives its name from the helical path traced out by the x ray beam. It is also uses a slip ring technology with a multiplanar reformation excels in  3D.

What is Slip Rings?

Slip Ring is an electromechanical device that conducts electricity or electric signals through rings and brushes across a rotating surface onto a fixed surface.

7th Generation CT scan 

The 7th generation of ct scan machine is sweep or translation (refers to the movement of xray tube source and detectors) type of scanners, it has a multiple detector array or commonly called the 64 slice ct machine.
Multiple detector array (64 slice ct).

Attenuation Coefficient in CT Scanning

The tissue contained within each image unit called pixel take up a certain amount of the x-rays that pass through it like bone receive more x-ray and air receives almost nothing. This ability to cast x-rays as they pass through a substance is as attenuation. For a given body tissue, the amount of attenuation is relatively constant and is known as the tissue’s attenuation coefficient. In CT, these attenuation coefficients are mapped to an arbitrary scale between -1000 hounsfield units (HU) the air, and +1000 hounsfield (HU) the bone.

There are 4 components in the CT Scanner the following are:

  1. Gantry
  2. Patient Table
  3. The Computer
  4. Operating / Control Console

The Gantry of CT Scan

The gantry is the largest component of the CT installation, containing the x-ray tube, collimators detector arrat, DAS ( Data Acquisition System), other control electronics, and the mechanical components required for the scanning motions.

The X- ray Tube / X-ray Source of CT Scan

  • The device that generates the x - ray beam, all CT scan are rotating anode brehmsstralung x-ray
  • tubes except one - 5th Generation system which uses a unique scanned electron beam and a strip anode.
  • Power requirements - Typically 120 kV at 200 to 500 mA
  • Anode Heating Capacity -  1 MHU - 4 MHU
  • High Speed Rotors for heat Dissipation
  • Focal Spot Size, small for spatial resolution
NOTE: All CT scanners use bremsstralung x-ray tubes as the source of radiation.

Focal Spot

  • The anode area from which x-rays are emitted, projected along the direction of the beam.

Collimation / Collimation Assembly

  • Restriction of the useful x-ray beam to reduce patient dose and improve image contrast.
  • It is used to control the width of the fan beam between 1.0 and 10mm, which in turn controls the width of the imaged slice.

CT Scan Machine : Type of Collimation

1. Pre- Patient Collimator

  • Is mounted on the tube housing or adjucent to it; this collimator limits the area of the patient that intercepts the useful beam and there by determines the slice thickness and patient dose.

2. Post patient Collimator / Pre Patient Collimator

  • Restrict the x-ray field of view by the detector array and reduce scatter radiation incident.
  • Post patient collimator does not contribute to patient dose.

Detector / X - ray Detector of a CT Scan

The Detector of CT is a device that absorbs radiation and converts some or all of the absorbed energy onto a small electrical signal.

Criteria for a CT Scan System X- ray Detectors:

  • Have a high overall efficientcy to minimize the patient radiation dose, have a large dynamic range.
  • Be very stable with time.
  • Be insensitive to temperature variations within the gantry.

Two General Classification of CT scan Detectors

  • Solid State Detectors / Scintillation Detector
  • Required power supply coupled with PMT / PHOTODIODE; Generally have very high quantum and conversion efficiencies and a large dynamic range.

Scintillation Crystals

The Scintillation Crystals produces light when x-ray photons are absorbed.

Sodium Iodide (Na10)

Sodium Iodide was the first crystal used in the earliest scanner, and was replaced by: Solid State Crystals
  1.  Bismuth Germinate (BGO)
  2. Cadium Iodide (CDWO4)
  3. Cesium Iodide (CSI)

The Gas Detector of CT Scan

  • Gas detector are constructed of a large metallic chamber with baffle space at approximately 1mm interval. A high voltage is applied to tungsten septa between chambers to collections produced by the radiation.
  • Gas detectors have excellent stability and a large dynamic range, however they generally have a lower quantum efficiency than solid state detectors.

Three Important Factors Contributing to Detector Efficiency

1.Geometric Efficiency

  • Refers to the area of the detectors sensitive to radiation as a fraction of the total exposed area.

2. Quantum Efficiency

  • Refers to the fraction of incident x-rays on the detector that are absorbed and contribute to the measured signal

3. Convertion Efficiency

  • Refers to the ability to accurately convert the absorbed x-ray signal into an electrical signal.

Overall Efficiency of CT scan Machine

head ct scan image
Brain Ct Scan
  • Is the product of the three detector efficiency and it generally lies between 0.45 and 0.85 a value of less than 1 indicates a non-ideal detector system and results in a required increased in patient radiation dose if imaged quality is to be maintained. The term dose efficiency sometimes has been used to indicate overall efficiency.

Data Acquisition System

  • Interfaces the x-ray detectors to the systems computer and may consist of a pre-amplifier, integrator, multiplexer, logarithmic, amplifier and analog to digital converter.

Generator of CT scan

  • It operates in 3 phases or high frequency power.

CT scan Support Couch / Patient Couch

The Support Couch or sometimes called the Patient Couch is located at the rear of the CT gantry to assure more room for Computed Tomography operators at the front of the gantry. The patient table works in a cooperation with a trolley means having a slight inclined table top holding a stretcher carrying the patient; low atomic number material like Carbon fiber.

NOTE: Indexing - term refers to the movement of the patient couch.

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