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marlasherman
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What is Tomograghy
- A radiographic technique that employs motion to demonstrate anatomy lying in a plane of tissue while blurring or eliminating structures above and below the plane of interest
- Method to overcome superimposition
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Tomography
3 Elements
- ¤The x-ray tube
- ¤The patient (object/structure)
- ¤The image receptor
¨Based on the synchronous movement of two elements in a tomographic system
- ¤In opposite directions during exposure
- ¤Exposure produced from motion in 1 direction
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Fulcrum
- The object of interest is placed at the level of the fulcrum
- Can be adjusted to different heights
- Object at fulcrum will not be blurred
- it hasn’t moved in relationship to tube and image receptor
- The arrow remains at same position on image receptor
- Circle and square streaked across entire length of image receptor
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Tomographic Amplitude (ARC)
- Also called tomographic arc or angle
- ¤the total distance the tube travels
- The exposure does not need to be the entire tomographic amplitude
- Always ≥ exposure amplitude
- Inverse relationship between tomographic amplitude (ARC) & section thicknessIncrease Tomo amplitude: Decrease section thickness
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Exposure Amplitude
- Total distance the tube travels during the exposure
- Always ≤ tomographic angle
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Focal Plane
- Also called the section
- Area where the image has good detail
- Controlled by fulcrum level
- Plane is parallel to IR
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Section Thickness
- Width of the focal plane
- Controlled by the exposure amplitude & tomographic amplitude
- Increased amplitude = thinner cuts
- Decrease amplitude = thicker cuts
- inversely proportional
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Blur
- Streaking of objects outside the focal plane Objects outside the focal plane do not have good recorded detail
- More blur
- ¤object has less density
- ¤object is more transparent
- objects in the focal plane are visualized better
- The larger the tomographic angle, greater the blur
- ¤Direct relationship to number degrees the tube travels
- The further from the fulcrum the greater the blur
- ¤Direct relationship The greater the distance from the IR the greater the blur
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Blur Affected by:
- Tomographic Amplitude
- Distance from Fulcrum
- Distance from Image receptor
- Orientation of tube motion
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Tomographic amplitude has
direct linear relationship to blur
- Objects close to the image receptor have less blur than objects farther
- ¤same distance from the focal plane
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Tube Motion
Objects that are perpendicular to the tube motion have increased blurring
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Phantoms
- Images that do not correspond to existing structures
- Produced during complex tube movements
- Exposure amplitude = phantom images
- ¤Increase amplitude, decrease phantom images
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Where did the term CAT scan originate from and is accurate for most modern CT units?
Computerized axial tomography (No)
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What were 1st generation CT scanners designed for
Head scans only
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List 3 advantages of helical scanning CT units.
- Scanning of entire body without stopping tube
- Shorter scan times
- Reduced possibility of motion artifacts
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What are the CT advantages to the patient?
- Less contrast media
- Examination can be done on one breath hold
- Pt throughput
- Less repeats
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What is pitch?
Extension of contraction of the helix is the ratio of the distance the table moves (feed) to total beam collimation
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How is pitch calculated
- P=1/b Pitch =table increment /360°
- Beam width in mm
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What is a major concern with a pitch that is greater than 1?
That some data may/is being missed
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What is the advantage to a greater pitch value?
Permits a greater field of view in a shorter time
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Why are CT tube failures fairly common?
Because CT examination produce massive amounts of heat
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Section thickness using an MSCT scanner is determined by the width of the detector and not the width of the collimator?
TRUE or FALSE
True
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How are MSCT units rated
According to the # of detector rows that can be exposed from a single beam collimation.
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What are the 3 parameters that determine detectors dose efficiency?
- Capture efficiency
- Absorption efficiency
- Conversion efficiency
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List the 3 types of detectors that have been used in various generations of CT scanners
- Scintillation Crystals & Photomultiplier Tube
- Xenon Ionization Chambers
- Solid State
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Of the 3 types of detectors which one was typically used in earlier models of CT scanners
Scintillation
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What is the major disadvantage of this type of detector
- Lag
- Size & interspace material (they could be placed close together, less effective)
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Referring to Solid-State detectors what is responsible for (pick-up) receiving the emitted photons?
Scinillator
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What does the photodiode do
Take light from the scintillator and converts it into an electrical signal
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What does the scintillator do?
It received x-ray photon and emits light
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Why must there be a small gap between adjacent detectors?
Because of cross-talk between detectors
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Within the MSCT helically scanned beam where is the highest resolution captured?
In the centre of helically scanned beam
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Describe both pre and post patient collimators
- Pre (before patient near the tube port)
- Post after patient just before detectors
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What is one of the main concerns if a detector is even slightly misaligned
Is an artefact “Ring Artifact
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What is the primary mathematical method used in the creation of computerized medical images
Fourier Transform
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What is targeting and when would it be used? (What type of exam)
- A form of magnification technique
- An area is selected for reformatting
- Smaller pixels with magnification and less distortion (i.e. Inner ear)
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Is noise always present with a digital image
Yes
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Noise on a CT image is directly related to the
Amount of data collected by the detector
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What is partial volume effect and how can it be compensated for
- When a small object (smaller than the voxel width) or section thickness may be obscured.
- It can be compensated for by slight over sampling of that area
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What is a bow tie filter
A special beam filter used in CT that matched the beam divergence & shape of the patients body in transverse section.
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What are the 3 steps involved in the production of a CT image?
- Image acquisition
- Image reconstruction
- Display image
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What happens to section thickness as the tomographic amplitude increases
Thinner slice/cut
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What happens to the amount of object blur as the tomographic amplitude increases
Greater the blur
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Where is the best detail recorded during a linear tomographic exposure
Focal plane
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What are the main components of a typical CT imaging system
The gantry ,patient couch(table) , operators console , the computer
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What is the gantry
The moveavble frame work on the CT unit mount that it surrounds the patient in a vertical plane
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What imaging components are located in the CT gantry
- X ray tube
- high tension generator
- collimators
- detectors
- detector elements
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What are some features of xray tubes currently used in CT scanners
- rotating anode x ray tube with small focal spot sizes
- Have heat loading and heat dissipation(cooling) rates
- anode disk made of layered alloys
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who is generally given credit for the development of CT
- Godfrey N. Hounsfield and Alan M Cormack
- In 1979
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