CT Scan Components

Sunday, March 5, 2017

Components of the CT Scanners

CT scanners are composed of many different connected parts, with many different components involved in the process of creating an image. More to the complexity, different CT scan manufacturers often modify the design of various components. To understand the basic function of each components, and some of the major variations in their design. From a broad perspective, all make and models of CT scanner are similar in that they consist of a scanning gantry, x-ray generator, computer system, operator’s console or the console panel and physician’s viewing console. Although hard copy filming has largely been replaced by workstation viewing and electronic archiving, most CT system still include a laser printer for transferring CT images to film.

Simplified 3 Segment of Image Processing

  • Data Acquisition – Get data
  • Image Reconstruction – Use Data
  • Image Display – Display Data

Data are acquired when the xray pass through a patient to strike a detector and are recorded. The major components that are involved in this phase of image creation are the gantry and the patient table.

ganrty
The gantry and patient table are major components of a CT image
system. (Courtesy of Siemens AG.)


Gantry


The gantry is the donut like or ring shaped part of the CT scanner. It houses many of the components necessary to produce and detect xrays. These components are mounted on a rotating scan frame.

components of gantry
The gantry houses many of the components
necessary to produce and detect x-rays. The gantry cover is
removed on this third-generation scanner configuration to
reveal the components necessary for data acquisition, including
the x-ray tube and detector array. Image courtesy of
Siemens AG.

Component of the gantry are mounted on a rotating scan frame. Gantries vary in total size as well as in the diameter of the opening or aperture. The range size of aperture is typically 70 to 90 cm. The gantry is designed to be tilted either forward or backward as needed to accommodate a variety of patients and examination protocols. The gantry can be tilted varies among systems, but more or less 15 degrees to 30 degrees is usual. The gantry also include a laser light that is used to position the patient within the scanner. Control panels located on either side of the gantry opening allow the radiologic technologist to control the alignment lights, gantry tilt, and movement of the table. In most scanners, these functions may also be controlled via the operator’s console. A microphone is installed in the gantry to allow communication between the patient and the radiologic technologist throughout the scanning procedure.

Slip Rings


Old model design CT scanner used recoiling system cables to rotate the gantry frame. This design limited the scan method to the step and shoot mode and considerably limited the gantry rotation times. Newer systems use electromechanical devices called slip rings. Slip rings use a brush like apparatus to provide continuous electrical power and electronic communication across a rotating surface. They permit the gantry frame to rotate continuously, eliminating the need to straighten twisted system cables. Slip rings allows the gantry frame to rotate continuously making helical scan modes possible.

Generator


High frequency generator is currently used in CT scanners. The generator are designed to be small enough so that it can be located within the gantry. Highly stable 3-phase generators have also been used, but because these are stand-alone units located near the gantry and require cables, they have become obsolete.

Generators produce high voltage and transmit it to the xray tube. The power capacity of the generator is listed in kilowatts (kW). The power capacity of the generator determines the range of exposure techniques like kV and mA settings, available on a particular system. CT generator produce high kV generally 120 – 140 kV to increase the intensity of the beam and thereby reduce patient dose. In addition, a higher kV setting will help to reduce the heat load on the xray tube by allowing a lower mA setting and reducing the heat load on the xray tube will extend the life of the tube.

Cooling Systems

 

Cooling mechanisms are included in the gantry. They can take different forms, such as blowers, filters, or devices that perform oil to air heat exchange. Cooling mechanisms are important because many components can be affected by temperature fluctuations.

X-ray Source – CT X-ray tube

X-ray tubes produce the xray photons that create the CT image. Their design is a modification of a standard rotating anode tube, such as the type used in angiography. Tungsten, with an atomic number of 74, is often used for the anode target material because it produces a higher intensity xray beam. This is because the intensity of xray production is approximately proportional to the atomic number of the target material. CT scan tubes often contain more than one size of focal spot; 0.5 and 1.0 mm are the common size of focal spot. Just like as in standard xray tubes, because of reduced penumbra small focal spot in Computed tomography tubes produce sharper images like better spatial resolution, but because they concentrate heat onto smaller area of the anode they cannot tolerate as much of the heat.

A very large amount of stress is places on the CT scan tube. Scanning protocols often require multiple long exposures performs on numerous patients per day. A CT scan tube must be designed to handle such stress. The way a tube dissipate the heat that is created during xray production is critical. All manufacturers list generator and tube cooling capabilities in their product specifications. These specification usually list the system generator’s maximum power in kW. Also listed is the anode hear capacity in million hear units MHU and the maximum anode heat dissipation rate in thousand heat units KHU. These specification can be help to differentiate the various CT Scan systems. It is important to remember that these values represent the highest limit of tube performance. It is also important to compare the length of protocols that the tube will allow and how quickly they can be repeated.

Filtration

Compensating filters are used to shape the xray beam. They reduce the radiation dose to the patient and help to minimize image artifact. As our teachers tough us that, radiation emitted by CT scan xray tube is polychromatic. Filtering the xray beam helps to reduce the range of xray energies that reach the patient by removing the long wavelength or soft xrays. These long-wavelength xrays are readily absorbed by the patient, therefore they do not contribute to the CT image but do contribute to the radiation dose to the patient. In addition, creating a more uniform beam intensity improves the CT image by reducing artifacts that result from beam hardening.

Filtering the xray beam helps to reduce the radiation dose taken by the patient and it also improves the image quality of the CT scanners.

Different filters are used when scanning the body then when scanning the head. Human body anatomy having distinctive quantities has a round cross section that is thicker in the middle than in the outer area. Hence, body scanning filters are used to reduce the beam intensity at the periphery of the beam, corresponding to the thinner areas of a patient’s anatomy. Because of their shape they are often referred to as bow tie areas.


bow tie filter
Filtering shapes the x-ray beam intensity. Removing low-energy x-rays minimizes
patient exposure and produces a more uniform beam.


Collimation

Collimation restrict the xray beam to a specific area, as a result it helps reduce scatter radiation. This scatter radiation reduces image quality and increase the radiation dose to the patient. Reducing the scatter radiation improves contrast resolution and decrease patient dose. Collimation control the slice thickness by narrowing or widening the xray beam.

The source collimator is located near the xray source and limits the amount of xray beam before it passes through the patient it is sometimes referred to as patient dose and determines how the dose is distributed across the slice thickness like the dose profile. The source collimation resembles small shutters with an opening that adjusts, dependent on the operator’s selection of slice thickness. In MDCT systems, slice thickness is also influenced by the detector element configuration. Scanner vary in the choices of slice thickness available. Choices range from 0.5 to 10 mm.

Some CT scan systems also use predictor collimation. This is located below the patient and above the detector array. Because this collimation shapes the beam after it has passed through the patient it is sometimes referred to as postpatient collimation. The primary functions of predetector collimations are to ensure the beam is the proper width as it enters the detector and to prevent scatter radiation from reaching the detector.

Detectors


The detectors is a component of CT scan machine which collect information regarding the degree to which each anatomic structure attenuated the beam. In Conventional radiography we used a film screen system to record the attenuated information. In CT, we use detectors to collect the information. The term detector refers to a single element or a single type of detector used in a CT system. The term detector array is used to describe the entire collection of detectors included in a CT scan system. Specifically the detector array comprises detector elements situated in an arc or a ring, each of which measures the intensity of transmitted xray radiation along a beam projected from the xray source to that particular detector element. Also, included in the array are elements referred to as reference detectors that help to calibrate data and reduce artifacts.

Detectors can be made from different substances, each with their own advantage and disadvantages.

Optimal Characteristics of a Detector

  • High detector efficiency – it is the ability of detector to capture transmitted photons and change them to electronic signals.
  • Low or no Afterglow – is a brief, persistent flash of scintillation that must be taken into account and subtracted before image reconstruction.
  • High Scatter Suppression 
  • High Stability – which allow a system to be used without the interruption of frequent calibration.

Overall detector efficiency is the product of a number of factors are as follows

  • Stopping power of the detector material
  • scintillator efficiency (in solid state types)
  • Change collection efficiency ( in Xenon types)
  • Geometric efficiency – it is the amount of space occupied by the detector collimator plated relative to the surface area of the detector
  • Scatter rejection

Other term to describe the aspect of a detector efficiency

  • Capture efficiency – refers to the ability with which the detector obtains photons the passed through the patient
  • Absorption efficiency - refers to the number of photons absorbed by the detector and dependent on the physical properties of the detector face like: thickness and material.
  • Response time – is the time required for the signal from the detector to return to zero after stimulation of the detector by xray radiation so that it is ready to detect another xray event.

The detector response is generally a function of the detector design. Dynamic range is the ratio of the maximum signal measured to the minimum signal the detectors can measure.

Types of Detectors

Xenon Gas Detectors

Pressurized xenon gas fills hollow chamber to produce detectors that absorb of approximately 60% to 87% of the photons that reach them. Xenon gas is used because of its ability to remain stable under pressure. Xenon gas are significantly less expensive compared with the solid – state variety, it is also somewhat easier to calibrate and are highly stable.

How Xenon Gas Detector works?

A xenon detector channel consists of 3 tungsten plates. When a photon enters the channel, it ionizes the xenon gas. These ions are accelerated and amplified by the electric field between the plates. The collection charge produces an electric current. This current is then processed as raw data. A disadvantage of xenon gas is that it must be kept under pressure in a certain extent. Loss of xray photons in the casing window and the space taken up by the plates are the major factors hampering detector efficiency.

xenon gas detector
Structure of a xenon gas detector array.


Solid State Crystal Detector

Solid state detectors are also called scintillation detectors because they use a crystal that fluoresces when struck by a xray photon. A photodiode is attached to the crystal and transforms the light energy into electrical (analog) energy. The individual detector elements are affixed to a circuit board.

solid state crystal detector
Structure of a solid-state detector array.

Solid state crystal detectors have been made from a variety of material, including cadmium tungstate, bismuth germinate, cesium iodide, and ceramic rare earth compounds such as gadolinium or yttrium. Because these solids have high atomic numbers and high density in comparison to gases, solid state detectors have higher absorption coefficients. They absorb nearly 100% of the photons that reach them.

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