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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
- ¤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
- 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
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
- Total distance the tube travels during the exposure
- Always ≤ tomographic angle
- Also called the section
- Area where the image has good detail
- Controlled by fulcrum level
- Plane is parallel to IR
- Width of the focal plane
- Controlled by the exposure amplitude & tomographic amplitude
- Increased amplitude = thinner cuts
- Decrease amplitude = thicker cuts
- inversely proportional
- 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
Blur Affected by:
- Tomographic Amplitude
- Distance from Fulcrum
- Distance from Image receptor
- Orientation of tube motion
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
Objects that are perpendicular to the tube motion have increased blurring
- Images that do not correspond to existing structures
- Produced during complex tube movements
- Exposure amplitude = phantom images
- ¤Increase amplitude, decrease phantom images
Where did the term CAT scan originate from and is accurate for most modern CT units?
Computerized axial tomography (No)
What were 1st generation CT scanners designed for
Head scans only
List 3 advantages of helical scanning CT units.
- Scanning of entire body without stopping tube
- Shorter scan times
- Reduced possibility of motion artifacts
What are the CT advantages to the patient?
- Less contrast media
- Examination can be done on one breath hold
- Pt throughput
- Less repeats
What is pitch?
Extension of contraction of the helix is the ratio of the distance the table moves (feed) to total beam collimation
How is pitch calculated
- P=1/b Pitch =table increment /360°
- Beam width in mm
What is a major concern with a pitch that is greater than 1?
That some data may/is being missed
What is the advantage to a greater pitch value?
Permits a greater field of view in a shorter time
Why are CT tube failures fairly common?
Because CT examination produce massive amounts of heat
Section thickness using an MSCT scanner is determined by the width of the detector and not the width of the collimator?
TRUE or FALSE
How are MSCT units rated
According to the # of detector rows that can be exposed from a single beam collimation.
What are the 3 parameters that determine detectors dose efficiency?
- Capture efficiency
- Absorption efficiency
- Conversion efficiency
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
Of the 3 types of detectors which one was typically used in earlier models of CT scanners
What is the major disadvantage of this type of detector
- Size & interspace material (they could be placed close together, less effective)
Referring to Solid-State detectors what is responsible for (pick-up) receiving the emitted photons?
What does the photodiode do
Take light from the scintillator and converts it into an electrical signal
What does the scintillator do?
It received x-ray photon and emits light
Why must there be a small gap between adjacent detectors?
Because of cross-talk between detectors
Within the MSCT helically scanned beam where is the highest resolution captured?
In the centre of helically scanned beam
Describe both pre and post patient collimators
- Pre (before patient near the tube port)
- Post after patient just before detectors
What is one of the main concerns if a detector is even slightly misaligned
Is an artefact “Ring Artifact
What is the primary mathematical method used in the creation of computerized medical images
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)
Is noise always present with a digital image
Noise on a CT image is directly related to the
Amount of data collected by the detector
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
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.
What are the 3 steps involved in the production of a CT image?
- Image acquisition
- Image reconstruction
- Display image
What happens to section thickness as the tomographic amplitude increases
What happens to the amount of object blur as the tomographic amplitude increases
Greater the blur
Where is the best detail recorded during a linear tomographic exposure
What are the main components of a typical CT imaging system
The gantry ,patient couch(table) , operators console , the computer
What is the gantry
The moveavble frame work on the CT unit mount that it surrounds the patient in a vertical plane
What imaging components are located in the CT gantry
- X ray tube
- high tension generator
- detector elements
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
who is generally given credit for the development of CT
- Godfrey N. Hounsfield and Alan M Cormack
- In 1979