CT Flashcards
What is CT dose index?
The CTDI is a measure of the absorbed dose from a single rotation of the CT scanner gantry, with no movement of the patient couch. It is usually expressed in milligray (mGy). CTDI is related to the absorbed dose.
What is dose length product?
Dose-length product is simply the CTDIvol multiplied by the scanned length
DLP is proportional to the integrated absorbed dose and is useful for comparing doses between patients for the same examination.
measured in milligray centimetres (mGy cm).
What is effective dose in CT?
Effective dose is used to describe doses to patients in terms of relative biological radiation risk for an examination and takes into account the radiation sensitivity of the different organs in the body. Effective dose can also be used to compare radiation risk with scans of different body regions or other types of radiological examination.
Recall that the effective dose (E) for a radiological examination is calculated from the sum of all the individual organ equivalent doses (HT), each weighted by the appropriate tissue weighting factor (wT).
The units of effective dose are millisieverts (mSv).
Effective doses in CT can be calculated using special computer programs. These are based on computational ‘Monte Carlo’ simulations of organ doses using a mathematical model of the body
Other than kVp and mAs, what effects CTDI?
the peak dose and the shape of the dose profile depend on the CT scanner hardware configuration including Focal spot size
Collimator
Focus-isocentre distance (FID)
Where is the dose highest in CT?
Because the x-ray beam is attenuated as it passes through the body, doses are highest at the periphery and lowest at the centre.
What is CTDIw?
CTDIw is calculated as a weighted sum of the centre and average periphery measurements.
CTDIw is a good estimate of the average dose to the phantom at the central slice of a CT examination, as though it were scanned with contiguous slices over a distance of 100 mm in the z direction. It is expressed in mGy, the same units as CTDI.
What is CTDIvol?
simply the CTDIw divided by the pitch, where pitch is the couch feed per rotation divided by the nominal collimation.
A larger pitch reduces dose while a smaller pitch increases dose, so CTDIvol is adjusted accordingly. CTDIvol is an approximation to the average absorbed dose within the volume of tissue that has been scanned. Again, it is expressed in mGy.
UK-average effective doses for common CT examinations are:
Head (acute stroke) -
Chest (lung cancer) -
Abdomen (liver metastases) -
Head (acute stroke) 1.8 mSv
Chest (lung cancer) 14 mSv
Abdomen (liver metastases) 16 mSv
How much greater is the effective dose of CT than planar radiography?
typically one to three orders of magnitude greater than planar radiography of the same body region
How can sagittal or coronal slices be made?
interpolation from a stack of transverse slices.
What physical property of the tissues do the pixel values in the x-ray CT image represent?
Linear attenuation coefficient
What is the dominant physical process contributing to attenuation of x-rays?
Compton effect
The amount of Compton scatter depends on the corresponding component of the linear attenuation coefficient. This component is proportional to the electron density (the number of electrons per unit mass) of the material and, like the total coefficient, it is also proportional to the material’s physical density (mass per unit volume). Overall, therefore, it is proportional to the number of electrons per unit volume. For human tissues, the electron density is itself closely related to physical density.
Hints are provided below:
Pair production cannot occur for photon energies below 1.02 mega-electron volts.
The likelihood of photoelectric interactions decreases with increasing photon energy
What is the CT number of air and water?
water is 0 and air is -1000
How are CT numbers calculated?
Computed tomography numbers for other materials are calculated according to the difference in linear attenuation coefficient between that material and water
HU= 1000((LAC of tissue - LAC of water)/LAC of water)
What is the difference in window and level?
Level is the middle number of the window. Window is how many levels are within above or below which it is displayed as white or black.
How is data collected in a sequential CT scanner?
This type of scanner acquires only one slice at a time: after completion of data acquisition for one slice, the gantry is rotated back to its original position and the patient couch moved in preparation for the next slice.
To form a CT image, a series of projections through the patient are collected at discrete angles during rotation of the gantry around the patient.
WHat is back-projection?
To reconstruct the attenuation values, a process called back-projection is used.
With no prior knowledge of the attenuation values, we assume that the row and column totals are made up of equal contributions from each of the boxes along a ray path. Therefore, to back-project the data, we evenly ‘smear out’ the attenuation totals back across an empty grid (or matrix) of boxes.
If we do this for both projections in our example and add the results together, we get a simple attenuation ‘image’. The attenuation values are then converted into CT numbers (pixel numbers) in the form of HUs.
Differences between the reconstructed and true values in this example are due to two factors:
Too few projections
Blurring caused by the back-projection technique
how many projections are needed?
depend on many factors including scanner design, matrix size and reconstruction method. However, if too few projections are used to reconstruct a CT image then undesirable image artefacts (streaks) will result and image resolution may be degraded.
In clinical scanning, of the order of 1000 projections are typically acquired per rotation of the scanner gantry. The number of projections is not generally user-adjustable.
What is filtered back projection?
To remove the blurring from the image, the projections are first combined with a special mathematical function called a filter (sometimes called a ‘kernel’). The combination process is called convolution.
There are two main effects on the projection:
Production of negative going edges at boundaries in the object. When back-projected, these tend to cancel the back-projection ‘streaks’ to give a sharper image on a uniform background
The amplification of noise in the projection
Back-projection of the filtered projections gives an image in which both the artefacts and the blurring are much reduced
WHy is it important to choose the correct filter in FBP?
A range of filters are available on clinical CT scanners. In choosing the filter there is generally a trade-off between the sharpness of the image (good spatial resolution) and noise. It is therefore important to choose the correct filter for a specific diagnostic task.
How are images required in helical CT?
Image reconstruction in helical CT involves a further complication because the patient couch is moved continuously during the scan and the gantry rotates continuously. Thus, the x-ray beam moves in a helical path along the body
This means that we must form our transverse slices from projections that are not only at different rotation angles, but also at different longitudinal positions along the scanner axis.
This is done by interpolating projections.
This differs from sequential CT, for which all the projections for a particular transverse slice are acquired within that slice.
How are true axial slices reconstructed from single slice helical scans?
For reconstruction in single-slice helical scanning, the method used is 2 point interpolation. To generate a projection at a particular angle in the plane (slice) of interest, the two nearest projections at that angle on either side of the plane are combined in a weighted average.
How are true axial slices reconstructed from multi slice helical scans?
Multi-slice CT uses an array of x-ray detectors arranged in multiple rows side-by-side. In multi-slice helical scanning, multiple projections are recorded at each angle of the spiral, one from each row of detectors.
filter interpolation is used. Typically, this is a fixed-width interpolation. All projections at a particular rotation angle, from any detector row and within a certain distance of the plane of interest, are combined in a weighted average.
The nature of the weighting function, or ‘filter’, may vary depending on application. For 16-slice scanners and above, more involved methods may be used, but the principle of filtering the image data over a fixed length remains the same.
What is the purpose of filtering the beam in CT?
to remove low energy, or ‘soft’, x-rays, which would not penetrate through the patient, contributing only to radiation dose and not the image.
also narrows the energy spread of the x-ray beam making it more monochromatic. This is important in CT as the method of image reconstruction is based on the assumption of a single energy, monochromatic, beam.
Why can a bow-tie filter be used?
Some scanners have a filter which has a shape that is wider at the edges than at the centre.
When a view is acquired through the body, the x-rays traverse a greater thickness of tissue at the centre of the field of view than at the edges. Beam shaping filters attempt to compensate for this difference in attenuation across the field of view. This helps to reduce undesirable beam hardening artefacts in the image.
Note that the bow-tie filter shapes the intensity profile of the transmitted x-ray beam, and not its physical dimensions.
What do you think are useful functions of the collimator?
To restrict radiation dose to the patient - By restricting the x-ray beam to the active detector width, the collimator minimises unnecessary radiation reaching the patient that does not contribute to the image. The collimated width at the centre of the scanner (the isocentre) may vary between 1 and 40 mm
To minimise scatter reaching the detector
To define the imaged slice thickness
What are the 2 different types of detector element?
Solid state
Ionisation chamber
What are important properties for a CT detector?
High detection efficiency for x-rays in CT energy range
High dynamic range
High geometric efficiency (narrow gaps between active elements)
Fast response
Linearity
Stability
Low cost
Small physical size
What makes up a solid state detector in CT?
cintillator material coupled to a high-purity, temperature-stabilised silicon photodiode to convert the scintillation light to an electrical signal. Scintillator materials are cadmium tungstate, bismuth germanate or doped rare-earth ceramics.
What are the beneficial properties of SSDs?
High (~98%) detection efficiency
~80% geometrical efficiency
Small physical size of detector elements
A rapid light output with rapid decay time is desirable to allow rapid sampling of the signal as the gantry rotates around the patient.
What makes up an ionisation chamber detector in CT?
Gas-filled ionisation chamber detectors are used in some single-slice scanners.
These are filled with a high atomic number gas (e.g. krypton or xenon) at approximately 30 atmospheres pressure. The detector array is a single pressurised vessel, subdivided by tungsten septa, which also help to stop some scattered x-ray photons. Incident x-rays ionise the gas, producing a signal at the collection electrodes
What enables continuous rotation of the CT scanner?
a ‘slip ring’ system where a series of ‘brushes’ on the rotating gantry maintain contact with stationary rings, allowing power to be supplied to the gantry and the detector signals to be passed out to the computer. Slip ring technology is essential for the technique of helical scanning
What are the 2 ways in which CT scanners can acquire data?
In sequential scanning, also known as axial or ‘step-and-shoot’ scanning, the x-ray source is switched on and the gantry rotates to acquire data in a transverse (transaxial) slice while the patient couch is stationary
In helical scanning, also known as spiral scanning, the couch movement and gantry rotation are continuous while the scan region is covered. Therefore, the x-ray beam describes a helical path relative to the patient
What is pitch?
Pitch is defined as the couch travel per gantry rotation divided by the collimated beam width.
What are the advantages of helical scanning?
Avoidance of respiratory misregistration as it can be performed in one breath hold
More effective use of contrast agents - With faster scanning high vascular contrast can also be achieved
Overlapping slices can be reconstructed from the helical data at any pitch without increase in radiation dose.
Reduction in scan time and radiation dose
Why can respiratory misregistration occur?
In sequential scanning requiring a breath hold, a small lesion can be missed if the depth of breath hold is different on successive slices.
How are slices formed in multi slice scanning?
by a combination of beam collimation and electronic switching of the detector rows.
For example, consider a detector array with 16 rows of 1.25 mm detector elements. By collimating the x-ray beam to cover half of each of the two innermost detector rows, a section width of 2 x 0.63 mm can be obtained.
What are the different types of array in CT?
Linear array - A linear, or ‘matrix’, detector array has multiple rows of detectors, all of the same width.
Adaptive array - In adaptive arrays, the detector row width is narrowest in the centre and increases outwards. Using fewer elements means fewer septa dividing the active area of the detector. This helps to improve geometric efficiency and therefore dose efficiency.
Hybrid array - Hybrid arrays are similar to linear arrays, except that a central set of detector rows is narrower than the outer detector rows.
What are the 2 different types of pitch in multi slice scanners?
Pitchx is defined as the couch travel per rotation divided by the total collimated beam width.
Pitchd is defined as the couch travel per rotation divided by the detector subgroup width.
For example, if a scanner is operating in 16 x 1 mm mode, the width of each detector row subgroup is 1 mm. With a couch movement of 24 mm per rotation, pitchx is 1.5 (24/16) while pitchd is 24 (24/1).
