COMPUTED TOMOGRAPHY (CT) Flashcards
Computed tomography (CT) uses ____ power supplies , capable of providing stable tube currents and voltages.
High-fequency power
Modern CT scanners make use of _____ in which high voltage is supplied to the tube through contact rings in the gantry.
slip ring technology
is a circular contact with sliding brushes that allows the gantry to rotate continually, untethered by wires.
slip ring technology
The use of slip-ring technology eliminated the _____
inertial limitations
The CT x-ray tube can be energized up to ___ continuously.
60s
Tube voltages range from ____
80 to 140 kV.
Tube currents can range up to ____
1,000 mA.
Time for a 360-degree rotation of the x-ray tube currently ranges between ____
0.3 and 2 seconds.
Power loading on CT x-ray tubes can be as high as ___
∼100 kW.
anode heat storage capacities are high
how much
exceed 4 MJ or 8 MHU or more.
Anode heat dissipation rates are ___
∼10 kW
is a principal cause of CT imaging system malfunction and is the principal limitation on sequential imaging frequency.
The anode-cooling rates of approximately 1
X-ray tube failure
anode-cooling rates of approximately ___
how long
1 MHU per minute
CT x-ray tubes are expected to last for at least _____
50,000 exposures
_______ that separate the individual xenon detectors can also be made quite thin, and this improves the geometric efficiency by reducing dead space between detectors.
Metal septa
is the fraction of primary x-rays exiting the patient that strike active detector elements.
geometric efficiency
was the crystal used in the earliest CT imaging systems
Sodium iodide (NaI)
Nal was quickly replaced by ___ and ___
bismuth germanate (Bi4Ge3O12 or BGO) and cesiumiodide(CsI), and calcium fluoride. Cadmium tungstate (CdWO4) and special ceramics are the current crystals of choice.
Detectors are separated from each other by a dead space of____, which reduces the geometric efficiency.
how thick
∼0.1 mm
Scintillator Detector common light detectors are ____ and ___
photomultiplier tubes and photodiodes.
Scintillators convert ___ of the absorbed x-ray energy into light energy (conversion efficiency).
∼10%
CT was first demonstrated successfully in (year) and place
1970; England
at the Central Research Laboratory of EMI, Ltd. _____, an engineer for EMI, and ______, Tufts University medical physicist from Johannesburg, South Africa,
Godfrey Hounsfield, Allan Macleod Cormack
After CT was shown to be a useful clinical imaging modality, the first full- scale commercial unit, referred to as a ______, was installed in_____ in 1971
what type of scanner/ place
brain tissue scanner; Atkin on Morley’ Hospital
The first CT scanners in the United States were installed in ____ at the Mayo Clinic, Rochester, Minn.
what month and year
June 1973
In 1974, Dr. ______of Georgetown University Medical Center, Washington, D.C., developed the first whole-body scanner,
Robert S. Ledley
first CT scanner, an ____, produced images with 80 X 80-pixel resolution (3-mm pixels), and each pair of slices required approximately 4.5 minutes of scan time and 1.5 minutes of reconstruction time
EMI Mark 1
first CT scanners were capable of producing only ___ and thus were called ________
axial images ; CAT (computed axial tomography)
first CT scanner used clinically was the ____ offered in ___(year)
mode/year
EMI Mark 1; 1972
This first scanner was a ___ only
head scanner
The system rotated __ degree, and then the x-ray tube/detector assembly translated in the other direction (Fig. 10-70). This scanning pattern continued until 180 degrees of data were acquired. This ____ geometry using the ____ is referred to as the first-generation CT. The scan required over____ and image reconstruction ran overnight.
1 ; rotate-translate ; pencil beam; 4 minutes
used the same rotate-translate motion as before; however, more detectors were added and so the initial pencil beam geometry became a ___. A detector array using ____ meant that more data could be acquired in a given, and so this led to a shorter scan time and a larger, 160 x 160 matrix, reconstructed image.
narrow fan beam; 30 detectors;
Third-generation scanners use a ____ technology and wide rotating ___ coupled with a large array of detectors (more than ___detectors).
rotate-rotate; fan beam; 800 detectors
The third-generation CT geometry leads to the potential for _____ production
ring artifact
Fourth-generation scanners used a ______ technology- use rotating tube and fixed ring of detectors (up to 4,800) in the gantry (rotate-fixed system).
rotate-stationary technology;
Fifth Generation scanners- used a Stationary/Stationary technology- ______ CT (EBCT) or ultrafast CT. CT scanner has been developed specifically for cardiac tomographic imaging. They are capable of ____scan times and can produce fast-frame-rate CT movies of the beating heart.
how long is the scan
Electron beam; 50-msec scan
Sixth Generation scanners- use _____ (also inaccurately called spiral CT) scanners acquire data while the table is moving; as a result, the x-ray source moves in a ____ pattern around the patient being scanned
Helical CT; helical pattern
Recent innovations in x-ray tube design include a rotating envelope vacuum vessel called ___. heat dissipation rate about >60kW
Straton tube
The Gantry includes:
xray tube, the detector array, high-voltage generator, patient support couch, and mechanical support
2 types of CT Dectectors include:
Scintillator Detectors/Solid State Detectors and Gas-filled Detectors
Detectors have high quantum efficiency, which is the percentage of incident x-ray photons that are absorbed.
- CT detectors have a quantum efficiency of ___ %.
> 90%
CT imaging, two collimators are used include:
Prepatient collimator and Predector collimator (postpatient collimator)
This collimator limits the area of the patient that interceptsthe useful beam and thereby determines the patient radiation dose.
Prepatient Collimator
This collimator restricts the x-ray beam viewed by the detector array. This collimator reduces the scatter radiation incident on the detector array and, when properly coupled with the prepatient collimator, defines the slice thickness, also called the_____.
Predectector collimator (postpatient collimator); sensitivity profile
The mathematical principles of CT were first developed by ___ in 1917. ___’s study proved that an image of an unknown object could be produced if one had an infinite number of projections through the object.
Johann Radon
The computer is a unique subsystem of the CT imaging system. At the heart of the computer used in CT are the ___ and ____
microprocessor and the primary memory.
The time between the end of imaging and the appearance of an image—.
the reconstruction time
Projection radiographs or CT radiographs are also known as ____
topographic or scout images.
CT is expressed in terms of ______ in image quality
spatial resolution, contrast resolution, noise, linearity, and uniformity.
For scan control: kVp is about ____ . maximum mA is usually ___The thickness of the tissue slice to be imaged also can be adjusted. Nominal thicknesses are ___ mm.
120 kVp; 400mA ; 0.5 to 5 mm.
The degree of blurring is a measure of the _____ of the system and is controlled by a number of factors.
______ is a function of pixel size: The smaller the pixel size, the better is the _____.
______ for a CT image is limited to the size of the pixel.
spatial resolution
The ability of the CT imaging system to reproduce with accuracy a high-contrast edge is expressed mathematically called as ___
edge response function (ERF).
The measured ERF can be transformed into another mathematical expression called the
modulation transfer function(MTF).
The spatial frequency for CT imaging systems is expressed often as
line pairs per centimeter (lp/cm)
is the ratio of output to input modulation (signal amplitude) in an imaging system at each spatial frequency.
MTF
The ability to distinguish one soft tissue from another without regard for size or shape is called
Contrast Resolution
The ability to image low-contrast objects with CT is limited by the __ and ___ of the object and by the noise of the system.
size and uniformity
Noise appears on the image as ___
graininess.
