Computed Tomography

Computed Tomography GE Optima

About

Computed Tomography (CT), or Computed Axial Tomography (a CT scan, also known as a CAT scan), is a helical tomography (latest generation), which traditionally produces a 2D image of the structures in a thin section of the body. It uses X-rays. It has a greater ionizing radiation dose burden than projection radiography; repeated scans must be limited to avoid health effects. CT is based on the same principles as X-Ray projections but in this case, the patient is enclosed in a surrounding ring of detectors assigned with 500-1000 scintillation detectors (fourth-generation X-Ray CT scanner geometry). Previously in older generation scanners, the X-Ray beam was paired by a translating source and detector.

Older and less preferred terms that also refer to x-ray CT are computed axial tomography (CAT scan) and computer-assisted tomography. X-ray CT is a form of radiography, although the word "radiography" used alone usually refers, by wide convention, to non-tomographic radiography. CT produces a volume of data that can be manipulated in order to demonstrate various body parts based on their ability to block the x-ray beam. Although, historically, the images generated were in the axial or transverse plane, perpendicular to the long axis of the body, modern scanners allow this volume of data to be reformatted in various planes or even as volumetric (3D) representations of structures.

A CT image of a patients head.

Individuals responsible for performing CT exams are called radiologic technologists or radiographers[4] and are required to be licensed in most states of the USA. Usage of CT has increased dramatically over the last two decades in many countries. An estimated 72 million scans were performed in the United States in 2007. One study estimated that as many as 0.4% of current cancers in the United States are due to CTs performed in the past and that this may increase to as high as 1.5 to 2% with 2007 rates of CT usage; however, this estimate is disputed., as there is not a scientific consensus about the existence of damage from low-levels of radiation. Kidney problems following intravenous contrast agents may also be a concern in some types of studies.

Diagnostic use

Dual CT monitors. Left side is called scan. The right side is called display.

In a CAT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles called slices. For example, a 64 slice CT device is capable of producing up to 64 images per one rotation. The system moves the table platform along with the patient into the hole so the tube and detectors can scan the next slice. In this way, the machine records unprocessed X-ray image data across the body in a spiral motion called raw data. Once the machine raw data is captured it has to be processed within the reconstruction (recon) engine computer using fast Fourier transformation algorithms. Next, the computer processes the raw data into a human readable image. Finally, the computer puts together a 3-D image of the body for display on the technicians monitor.

Scan dose

The scientific unit of measurement for radiation dose, commonly referred to as effective dose, is the millisievert (mSv). Other radiation dose measurement units include rad, rem, roentgen, sievert, and gray.

Because different tissues and organs have varying sensitivity to radiation exposure, the actual radiation risk to different parts of the body from an x-ray procedure varies. The term effective dose is used when referring to the radiation risk averaged over the entire body. The effective dose accounts for the relative sensitivities of the different tissues exposed. More importantly, it allows for quantification of risk and comparison to more familiar sources of exposure that range from natural background radiation to radiographic medical procedures.

A CT device produces higher radiation dose than any conventional x-ray device.In October, 2009, the US Food and Drug Administration (FDA) initiated an investigation of brain perfusion CT (PCT) scans, based on overdoses of radiation caused by incorrect settings at one particular facility for this particular type of CT scan. Over 256 patients over an 18 month period were exposed, over 40% lost patches of hair, and prompted the editorial to call for increased CT quality assurance programs, while also noting that "while unnecessary radiation exposure should be avoided, a medically needed CT scan obtained with appropriate acquisition parameter has benefits that outweigh the radiation risks." Similar problems have been reported at other centers. These incidents are believed to be due to human error.

Components

Gantry

The gantry is the 'donut' shaped part of the CT scanner that houses the components necessary to produce and detect x-rays to create a CT image. The x-ray tube and detectors are positioned opposite each other and rotate around the gantry aperture. Continuous rotation in one direction without cable wrap around is possible due to the use of slip rings. The images below are of a Toshiba Aquilion 16 CT scanner with the external and internal components of the gantry, and the control panel labelled. This gantry weighs a total of 1750kg (3858lb).

CT gantry internal components.

1. gantry aperture (720mm diameter)
2. microphone
3. sagittal laser alignment light
4. patient guide lights
5. exposure indicator light
6. emergency stop button
7. gantry control panels
8. external laser alignment lights
9. patient couch or table
10. ECG gating monitor

Couch

A CT Patient couch or table

The patient lies on the couch (also known as a table) and is moved through the CT gantry aperture during the CT examination. Depending on the body part to be scanned and the protocol selected the patient may be positioned supine or prone and either head or feet first. A weight limit for the couch of approximately 205kg (450lb) is specified by the manufacturer beyond which the movement of the table is not guaranteed to be accurate and may even result in damage.

During conventional slice-by-slice scanning the couch is indexed (moved) between each scan depending on the slice thickness and slice incrementation (degree of overlap or separation) that has been selected for that examination. For spiral/helical CT, including multislice CT, the couch is translated through the gantry at a constant speed depending on the length of the area to be scanned, the total scan time, and the pitch that has been selected.

1. CT gantry
2. couch top
3. couch pedestal
4. couch control pedals (up/down & in/out)
5. velcro patient immobilisation strap

Slip Ring

CT slip rings

A slip ring is electromechanical technology that enables the transmission of power and electrical signals from a stationary to a rotating structure. This transmission of power/data is made possible through electrical connections made by stationary brushes pressing against rotating circular conductors.

CT Slip ring technology was introduced to enable helical (continuous rotating) scanning. Prior to the introduction of Slip Rings, only Axial scanning was possible (which had the need to stop / reverse direction of rotation, after no more than 700 degrees rotation due to the finite length of the attached cables.) Slip ring technology eliminated the need for cables and enabled the continuous rotation of the gantry components.

Slip rings used to bring power to x-ray tube on rotating gantry of a helical CT machine and, for some designs, to acquire information from the detector array. (a) The shiny metal strips carry electric signals that are swept off by special brushes. (b) The brushes are not in the form of bristles but rather of metal blocks (in this case a silver alloy). The five pairs of larger brushes provide the voltage required by the x-ray tube, and the three pairs of smaller ones transfer signals from the gantry controller.

Detectors

Multiple detector computed tomography or (MDCT) scanning is a array of crystals in the shape of a ring. A MDCT scan allows the radiologist to select the body part location, or abnormalities to be scanned. X-ray images are acquired by the detectors that pick up the x-ray emission which passes through the patients body. The x-ray images are then sent to the recon engine computer which reconstructs the images onto the screen for the technologist to view.

On multi–detector row CT scanners, each individual detector is segmented in the z-axis direction (Figure, part b). Although detector designs vary according to the manufacturer, all 16-channel multi–detector row CT scanners have a hybrid-array design in which the central 16 rows are narrow (0.5, 0.625, or 0.75 mm) and the outer rows, which number between eight and 24, are twice as wide (1, 1.25, or 1.5 mm, respectively). These dimensions are those measured at the isocenter because the actual dimension varies according to the gantry geometry. Thus, on 16-channel multi–detector row CT scanners, the total number of rows varies between 24 and 40.

Data Channels

CT detector useful in detector scintillation.

The concept of multi–detector row CT scanning is not new. An early first-generation CT scanner that had two separate detectors along the z-axis was introduced in the 1970s. However, this technologic innovation, which was designed to reduce the scanning time, was quickly eclipsed by second- and third-generation CT scanners that facilitated shorter scanning times.

Currently, multi–detector row CT scanners that have four to 16 channels are commercially available (2). Although at first glance it may appear that the number of sections per rotation equals the number of detector rows on a scanner, in actuality, the number of sections that are acquired depends on the number of z-axis data channels on the CT scanner. Thus, a four-channel multi–detector row CT scanner has four z-axis data channels and enables the acquisition of up to four sections per rotation, even though many more detector rows are present. Similarly, 16-channel multi–detector row CT scanners have 16 z-axis data channels and enable the acquisition of 16 or fewer sections per rotation, even though 24–40 detector rows may be present. The limited number of data channels is related primarily to the more complex reconstruction algorithms because the number of sections per rotation increases from four to eight to 16.

Detector Row Thickness

If the incident x-ray beam covers a greater number of detector rows than the number of z-axis data channels available on the CT scanner, then the signal from adjacent detectors will be combined and the detectors will function as a single unit. The effective detector row thickness is simply the sum of the widths of the contributing detector rows for each channel. In multi–detector row CT scan acquisitions, the effective detector row thickness of all channels must be identical. Thus, for example, on a 16-channel multi–detector row CT scanner with a 24–detector row hybrid-array design in which the detector has 16 0.75-mm-wide inner rows and eight 1.5-mm-wide outer rows, when a 24-mm-wide incident beam that covers all 24 rows is used, the effective detector thickness will be 1.5 mm where the central 16 rows have been paired to make up eight data channels and the eight outer rows will function as individual data channels. Similarly, on a four-channel multi–detector row CT scanner with a matrix-array design in which all 16 rows of the detector have a width of 1.25 mm, when a 20-mm-wide incident beam that covers all 16 rows is used, the effective detector thickness will be 5 mm where the 16 rows have been grouped in sets of four (4 · 1.25 mm = 5 mm) to yield the four data channels.

Effective detector row thickness is an important parameter because the reconstructed section thickness cannot be smaller than the effective detector row thickness. Thus, if diagnostic images are to be viewed on 2.5-mm-thick sections, then the effective detector row thickness must not be more than 2.5 mm. At a given effective detector row thickness, the specific options that can be used with a given thickness of reconstructed sections depend on the detector design and the reconstruction algorithm from the manufacturer.

Data Acquisition System

Data Acquisition System (DAS) measures the photons that pass through the patient and strike the detectors. DAS converts the analog signal from the detector into a digital signal. They are positioned near the detectors.

Scan Modes

Once the patient is on the table and the table is moved into the gantry bore, the technologist performs a preliminary scan called the CT radiograph. This image is also called the scout view, topogram, scanogram, or localizer; however, some of these terms are copyrighted to specific vendors. The CT radiograph is acquired with the CT x-ray tube and detector arrays stationary, the patient is translated through the gantry, and a digital radiographic image is generated from this line-scan data. CT systems can scan anterior-posterior (AP), posterior-anterior (PA), or lateral.

There are three common scan modes:

Axial

An Axial scan "Shoots, Stops, and Moves." "Shoots, Stops, and Moves."

1. Axial. The axial (also called sequential) CT scan is the basic step-and-shoot mode of a CT scanner.he gantry rotates at typical rotation speeds of 0.5 s or so, but the x-ray tube is not turned on all the time. The table is stationary during the axial data acquisition sequences. The system acquires 360 degrees of projection data with the x-ray tube activated, the tube is deactivated, the table is moved with the x-ray beam off, another scan is acquired, and so on.

This process is repeated until the entire anatomical area is covered. Because the table and patient are stationary during an axial scan, the x-ray tube trajectory defines a perfect circle around the patient. Due to the table’s start/stop sequence, axial CT requires more acquisition time than helical scanning. With the advent of MDCT, it is common to acquire contiguous CT images during an axial acquisition.

Helical

A helical scan moves at a constant speed while the gantry rotates around the patient

1. Helical. The With helical (also called spiral) scanning, the table moves at a constant speed while the gantry rotates around the patient. This geometry results in the x-ray source forming a helix around the patient.The advantage of helical scanning is speed—by eliminating the start/stop motion of the table as in axial CT, there are no inertial constraints to the procedure. Similar to the threads on a screw, the pitch describes the relative advancement of the CT table per rotation of the gantry.

The pitch of the helical scan is defined as where Ftable is the table feed distance per 360-degree rotation of the gantry and nT is the nominal collimated beam width. For example, for a 40-mm (nT) detector width in z and a 0.5-s rotation time for the gantry, a pitch of 1.0 would be obtained if the table moved 80 mm/s. For most CT scanning, the pitch can range between 0.75 and 1.5; however, some vendors give more flexibility in pitch selection than others. A pitch of 1.0 corresponds in principle to contiguous axial CT. A pitch lower than 1.0 (Fig. 10-35B) results in over scanning the patient and hence higher radiation dose to the patient than a pitch of 1.0, all other factors being equal. A pitch greater than 1.0 represents under scanning (Fig. 10-35C), and results in lower radiation dose to the patient.

Scout

1. Scout (also called Surview, Topogram, and Scanogram) the scanned projection radiograph, often acquired by the CT system to allow the user to prescribe the start and end locations of the scan range.

Regulatory standards

Refer to 21 CFR, SUBCHAPTER J, 1020.33 - Computed Tomography (CT) equipment

Links

• Kumpulan Materi Radiologi