Unit 6 (Computed Tomography)
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when was CT first introduced?
in the 1970s
who received the Nobel Peace Prize for inventing CT and when?
- G.N. Hounsfield and A.M. Cormack
_____________ demonstrated the first CT scanner in _______. He was using ______________'s mathematic formulas that were developed in __________.
- Godfrey Housfield
- the early 1970s
- Alan Cormack
- the 1960s
who first demonstrated that images could be created through mathematical projections and when?
the movable frame of the CT unit that contains the x-ray tube and detectors:
the part of the CT unit that delivers x-ray around the patient:
the part of the CT unit that receives the x-rays that are attenuated by the patient to acquire data to be sent to the CPU:
name seven main parts that make up the modern CT unit:
- x-ray tube
- display console
- image storage units
the part of the CT unit that processes data received at the detectors:
the part of the CT unit that shows post processed data:
the part of the CT units that can store massive amounts of data to be used to visualize images through various dimensions:
image storage units
what is the size range of the gantry aperture in CT?
describe the first generation CT unit:
- capable of scanning only the head
- a water bag was utilized to eliminate air interferences
- used a pencil slit x-ray beam
- had only two detectors
- used sodium iodide scintillation crystals and photomultiplier tubes
- capable of 180° rotation and 10-12 sections
- scans took 25-35 minutes to acquire
describe the second generation CT unit:
- scan time greatly reduced from 1st generation
- had a fan beam and 30 detectors
- but it increased the amount of scatter present in images
- (lead mask)
describe the third generation CT unit:
- had a wider fan beam and a curved array of 250-750 detectors
- capable of 360° rotation
- scan time became even faster (compared to 1st and 2nd gen)
- utilized dynamic imaging
- prone to ring artifacts (due to single detector array)
describe the fourth generation CT unit:
- had 600-2000 stationary detectors in a 360° array
- needed decreased calibration requirements
- AS&E funded by NIH
- was limited by its computer processing time
describe the fifth generation CT unit:
- utilized electron beam tomography (EBT) or cardiac cine (CT)
- used contrast injections synchronized to pt's ECG
- introduced a dedicated cardiac unit with a surrounding rotating electron beam with an electron gun and anode ring
- the electron gun shot down into the gantry
describe the sixth generation CT unit:
- called a helical or spiral CT, a type of 3D CT
- also called "Cone Beam Computed Tomography" (CBCT)
- used slip ring technology
- the patient was continually advanced through the bore
- it lowered the dose of contrast media required
- it could be acquired through a single breath hold
describe the seventh generation CT unit:
- a multiple section CT, with multiple detector arrays
- utilizes multiple bands of detectors as opposed to just one band
- allows for a reduction in section thickness
- improved resolution (determined by width of detector)
what generation CT unit does each image represent?
- A. 2nd generation
- B. 4th generation
- C. 6th generation
- D. 7th generation
- E. 1st generation
- F. 3rd generation
if you increase the pitch, what happens to the amount of tissue being scanned?
if you increase the pitch, what happens to the scan time?
if you increase the pitch, what happens to slice thickness?
it stays the same
smaller fields yield ___________ resolution.
give the typical scan field sizes for head & spine, small bodies, and large bodies:
- head and spine: typically 25 cm
- small bodies: typically 35 cm
- large bodies: typically 48 cm
what two things help determine the resolution on a helical multi section CT?
- the image matrix (in x and y planes)
- helical pitch
give all the versions of the name of the seventh generation CT unit:
- multi-section CT (best)
- multi-slice CT
- multi-detector CT
- multiple detector array scanners
what type of CT unit can be found at Holy Cross Hospital in Fort Lauderdale, Florida?
256 slice CT unit
what type of CT unit can be found at Le Bonheur in Memphis, Tennessee?
320 slice CT unit
what type of CT unit can be found at John Hopkins?
- 320 slice
- with viewing sizes less than 16 cm
- takes less than one second
describe the shape and composition of the CT table:
- flat or curved
- made of carbon graphite fiber (low attenuation, radioluscent)
- a motor drive provides exact section interval positioning
with the x-ray tube, rapid sequential loading requires a design with:
expanded heat capacity
give some technical aspects to the x-ray tube of a CT unit:
- rotating, with layered alloy and cylindrical anodes
- pulsed beam (120kVp, 1-5 msec pulses, 1000 mA)
- dual energy scanning (80kVp and 140kVp)
if taking a scan of a child or infant, would you raise or lower the kVp to reduce pt dose?
for a CT x-ray tube, ___________ exposures in a month is not uncommon. Nor is it unusual for tubes to fail after _______________.
double collimation controls:
the actual production of a CT image of a cross-sectional slice of anatomy begins with the ___________ and ends with a _______________ that represents the attenuation characteristics of the anatomy being imaged.
- x-ray tube
- matrix of CT numbers
between the x-ray tube and the matrix of CT numbers are the (6):
- data acquisition system
give some general characteristics of the CT x-ray tube:
- continuously pulsing
- lengthy exposures
- large anode that withstands large amounts of heat
- high frequency generator to supply electricity to tube
the high-frequency generator allows the production of an x-ray beam with ________________ energy levels, due to its ____________________.
- fairly consistant
- minimal voltage ripple
- (makes it more efficient)
describe the filter commonly used for a CT unit:
- typically a bow-tie filter shape
- used to filter low energy x-ray (to increase overall ave. energy)
name the two sets of collimators used in CT:
- prepatient collimator
- predetector collimator
where is the prepatient collimator located and what is its function?
- located between tube and patient
- limits the beam to a fan or cone shape
- limits patient exposure and scatter production
where is the predetector collimator located and what is its function?
- located before the detector array
- controls how much of the detector is exposed
- - in single slice CT, it controls the slice thickness
- - in MSCT, it controls how many rows of detectors being used
the _______________ is the physical component consisting of multiple detectors that efficiently absorbs the ______________ and accurately converts it to _______________.
- detector array
- transmitted radiation
- an electrical signal
name desirable traits in detectors:
- a fast response time without lag
- high stability
- unload signal quickly
- respond consistently
- be small
- have wide dynamic range
name the three elements of detector dose efficiency:
- capture efficiency
- absorption efficiency
- conversion efficiency
what are the detectors composed of?
- scintillation crystals
- photomultiplier tubes
- xenon ionization chambers
absorb the transmitted radiation and produce a proportional flash of light:
a solid-state device that converts the light to a proportional electrical signal:
A 64-slice CT Scanner has a detector array of ______ rows of _______ detectors.
the electrical signal produced by the detector-photodiode goes immediately to the ____________, which amplifies this weak signal, converts it to a ______________ (converts it from analog to digital data) and sends it to the _____________.
- Data Acquisition System (DAS)
- logarithmic data
the ______________ of the measured electrical signal is critical in CT.
logarithmic conversion produces attenuation information from the:
- original x-ray beam intensity
- intensity of the transmitted radiation
- thickness of the part
a measure of the probability that the x-ray beam will interact with the material it is in while traveling in a straight path:
linear attenuation coefficient
name two factors that can influence the linear attenuation coefficient:
- the type of material
- the energy of the x-ray
the logarithmic data goes through ___________ before reaching the computer for image reconstruction.
give two matrix sizes common to CT:
the __________________ consists of the computer analyzing raw data sent from the ________________ and reconstructing it into a digital array of ______________ based on the image matrix.
- reconstruction process
- CT numbers (image data)
the logarithmic data from the linear attenuation coefficient that is converted from analog to digital information by the ADC and sent to the computer:
help the massive amounts of data and calculations needed to reconstruct the image:
with a _____________________: using a computer or electronic filter, the computer analyzes all the data for one slice of the area being imaged to determine the _________________ which is then converted to a ____________.
- filtered back projection
- attenuation coefficient for each voxel
- CT number (hounsfield unit)
determined for each voxel when the computer analyzes all the data for one slice of area being imaged:
begins with the x-ray tube and ends with a matrix of CT numbers:
image data production
how is the Hounsfield Unit/CT number calculated?
by using a formula that relates the attenuation coefficient of the tissue to the attenuation coefficient of water
the CT number of water is set at:
the CT number of bone is:
approximately +1000 (appearing white)
the CT number of air is:
approximately -1000 (appearing black)
list items as they appear according to their CT number:
- A. air (-1000)
- B. water (0)
- C. bone (+1000)
- D. fat (-100)
- E. white matter (+25 to +30)
- F. grey matter (+35 to +45)
- G. congealed blood (+55 to +75)
name some of the imaging controls that the technologist must use in CT:
- multiple monitors
- power injector
name some responsibilities of the CT technologist:
- selecting the proper protocol for the requested exam
- using the power injector to deliver contrast
- manipulating the image to enhance visibility of key information
- storing the information on a temporary hard drive or long-term on large-capacity optical discs
- sending the information to PACS for interpretation by a radiologist
be familiar with the items that the imaging controls and data storage have protocols for:
- slice thickness
- image matrix size
- scan field of view (SFOV)
- display field of view (DFOV)
- scan time and image display
- RPM of tube
- energy supplied to tube
- pitch and distance table travels per second
- electronic reconstruction filters such as edge enhancement and smoothing
be familiar with the multiple options of post processing:
- selection of region of interest (ROI)
- using DFOV smaller than SFOV to magnify/magnification
- image smoothing or edge enhancement filters
- multiplanar reformation (MPR)
name a way to magnify in post processing:
use a DFOV (display field of view) smaller than the SFOV (scan field of view)
reformatting differs from reconstruction in that:
the computer uses the image data that was previously reconstructed to produce additional images.
Multiplanar reformation (MPR) is used to display:
- coronal planes
- sagittal planes
- oblique planes
windowing involves using WW and WL to adjust:
how many CT numbers are visible in the image
in CT there is a WW of _________ and a WL of ________.
in post processing, CT numbers range from:
50 to 450
in post processing CT numbers, pixels _____________ are black and pixels ____________ are white.
- 50 and lower
- 450 and higher
in CT post processing, decreasing WW will:
in CT post processing, increasing WL will:
decrease brightness (opposite of digital radiography)
name four basic things that affect image quality in CT:
- spatial resolution
- contrast resolution
an impediment of image quality that, in CT, typically results from statistical fluctuation recorded by the detectors
affects the low contrast resolving ability of the CT unit:
what type of relationship do noise and SNR have?
name four basic artifacts that affect image quality in CT:
- streak artifacts
- ring artifacts
- beam hardening artifacts
- partial-volume artifacts
be familiar with these artifact types:
- A. streak artifacts
- B. ring artifacts
- C. partial-volume artifacts
- D. beam hardening artifacts
name some routine quality control procedures:
- regular tube warm up procedures
- preventative maintenance
- testing to ensure accuracy and quality of image
- water phantoms used to calibrate CT number for water to "0"
- test objects used to monitor various areas
test objects are used in quality control to monitor:
- spatial resolution
- contrast resolution
- accuracy of table movement
- localization devices
today, CT accounts for approximately ________ of the U.S. population’s radiation exposure resulting from medical procedures.
to minimize exposure to patients the ___________ should be reduced while the _________ should be increased.
- reduce: mA and kVp
- increase: pitch
what type of shielding is available and for what specific parts is this shielding mentioned?
- thin in-plane bismuth shields
- breasts, thyroid, and eyes
name some technical amounts the pulsed beam CT unit uses:
- 120 kVp
- 1-5 msec pulses
- 1000 mA
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