Term
| The rotating part of the CT scanner |
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Definition
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Term
| Electrical connections between the rotating gantry and the stationary components are made using ________ |
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Definition
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Term
| CT utilization rates increased starting in the late 80's due to ________ and again in late 90's due to ___________. |
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Definition
| helical scanning, multidetector array |
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Term
| Projection definition and other terms for it. |
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Definition
| The data collected at a specific angle. Also called profile or view. |
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Term
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Definition
| An individual attenuation measurement that corresponds to a line through the object. It goes from the source to an individual detector element. |
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Term
| A projection is a collection of ______. |
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Definition
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Term
| Range of maximum FOV diameter? |
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Definition
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Term
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Definition
| The angle formed from the source as it spreads across the entire row of detectors. |
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Term
| For narrow cone beam geometry, the fan beam is typically ____ degrees and the full cone angle is _____ degrees. |
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Definition
| fan beam - 60 degrees, cone beam 2.4 degrees |
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Term
| Narrow cone beam geometery |
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Definition
| Formed by the small x-ray beam divergence in the z-axis direction. |
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Term
| Full cone beam geometery has a half angle of ____ degrees and typically uses _______ detectors. |
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Definition
| 10 degrees, flat panel detectors |
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Term
| Pixel array size for a CT image. |
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Definition
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Term
| The slice thickness is defined where? |
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Definition
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Term
| Typical power rating for a CT tube? |
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Definition
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Term
| How do CT scanners reduce scatter? |
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Definition
| Anti-scatter grid - the grid septa are aligned with the deadspaces between the detector elements. |
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Term
| The anode-cathode axis runs in what direction. |
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Definition
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Term
| What advantage does having the anode rotate in the same plane as the gantry have. |
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Definition
| reduces gyroscopic effects |
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Term
| The heel effect occurs in what direction on a CT scanner? |
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Definition
| Along the z-axis, perpendicular to the fan angle. |
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Term
| Typical number of projections in a single rotation. |
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Definition
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Term
| Typical time for one rotation of the gantry. |
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Definition
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Term
| The gantry is moving during the time it takes to acquire one projection which can lead to motion blurring and a loss of spatial resolution. How is this corrected? |
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Definition
| A magnetic steering system is used to guide the electrons as they travel from the cathode to the anode to essentially rotate the focal spot in the opposite direction of the gantry rotation. |
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Term
| Typical tube voltages used in CT. |
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Definition
| 80, 100, 120, and 140 kVp. 120 is the most common. |
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Term
| At CT energies, the primary photon interaction is |
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Definition
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Term
| Compton scattering is primarily affected by |
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Definition
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Term
| The electron density is given by |
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Definition
ρN(Z/A)
ρ - density, N - Avogadro's #, Z - atomic #, A - atom mass |
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Term
| Hounsfield units (HU) are defined as |
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Definition
| HU = 1000 * (μvoxel - μwater) / μwater |
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Term
| By definition, at any energy the HU for water and air are? |
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Definition
HUwater = 0
HUair = -1000 |
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Term
HU for the lungs,
HU for adipose (fat) tissue,
HU for most organ parenchyma |
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Definition
HU lung = -500
HU adipose = -80 to -30
HU organs = 30 to 220 |
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Term
HU for soft tissue with contrast
HU for bones |
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Definition
HU soft tissue with contrast = 100 to 300
HU bones = 700 to 3000 |
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Term
| Maximum HU for a 12 bit scanner |
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Definition
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Term
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Definition
Head and torso
Torso can be broken down into chest, abdomen, and pelvis |
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Term
| CT uses a beam shaping filter. What is it called and what is it used for? |
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Definition
| Bowtie filter - it reduces the intensity of the x-ray beam in the periphery of the x-ray field where the attenuation path through the patient is typically thinner. This results in the signal levels at the detector being more homogeneous. |
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Term
| The bowtie filter ______ patient dose and ______ image quality. |
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Definition
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Term
| CT scanners use __________ __________ detectors. |
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Definition
| indirect (scintillating) solid-state |
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Term
| A common scintillation crystal used in a CT scanner. |
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Definition
Gd2S2O
Gadolinium Oxysulfide |
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Term
| What is done to the scintillation crystal to increase density and improve light output? |
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Definition
| Scintering - heating it for long periods of time. |
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Term
| CT detector modules consist of |
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Definition
| Ceramic scintillator coupled to photodiodes that are layered on substrate electronics. |
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Term
| In the old single detector array scanner, the slice thickness was determined by |
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Definition
|
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Term
| In MDCT, the slice thickness is determined by |
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Definition
| the detector configuation |
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Term
| In MDCT, the beam width is determined by |
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Definition
|
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Term
| The overall beam width is given by |
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Definition
nT
n - number of data slices, T - thickness of a data slice |
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Term
|
Definition
| faster scan times and thinner slices. |
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Term
| The downside to thinner slices is |
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Definition
|
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Term
| The trade-off in MDCT revolves around |
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Definition
| Image noise, patient dose, and z-axis resolution. |
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Term
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Definition
| Placing the x-ray beam penumbra outside of the active detectors so a relatively constant intensity beam strikes the detectors |
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Term
| Overbeaming results in a loss in ________ ________. |
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Definition
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Term
| Geometric efficiency for state of the art MDCT scanners. |
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Definition
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Term
| Axial slices are typically reconstructed to what thickness for radiologist interpretation? |
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Definition
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Term
| A CT radiograph is also called |
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Definition
| scout view, topogram, scanogram, or localizer |
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Term
| During a CT radiograph, the x-ray tube and detectors are __________. |
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Definition
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Term
| The CT radiograph is used for |
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Definition
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Term
| Describe Axial CT scanning. |
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Definition
| The gantry makes a revolution around the patient with the x-ray tube on. The tube is turned off and the patient table moves forward about the distance of the beam width. This process is then repeated. |
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Term
| Axial CT scans require _______ acquisition time than helical CT scans. |
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Definition
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Term
| Describe helical scanning |
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Definition
| The table continually moves at a constant speed while the gantry rotates around the patient. The x-ray tube is continually on during the scan. |
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Term
| The pitch of a helical scan is defined as |
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Definition
| P = table travel (feed) distance per full gantry rotation / nominal collimated beam width (nT) |
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Term
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Definition
|
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Term
| How does dose vary with pitch. |
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Definition
inversely proportional
dose ~ 1/pitch |
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Term
| Why do helical scans have to start before an axial scan would start and end after an axial scan would end? |
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Definition
| There is not enough angular coverage at the start and end of a scan to reconstruct the CT image. |
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Term
| Where do helical scans start and end vs an axial scan. |
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Definition
| Start (1/2)nT before the axial and end (1/2)nT after the axial scan. |
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Term
| The wasted dose at the ends of a helical scan increase as a percentage of the overall dose for |
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Definition
| smaller scan lengths and larger collimated beam widths |
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Term
| Advantage and disadvantage of a cone beam acquisition. |
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Definition
| Advantage - A whole organ can be imaged without table motion. Disadvantage - Increased x-ray scatter and increased cone-beam artifacts. |
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Term
| One cardiac cycle takes about _____ and to freeze cardiac motion requires an image acquisition time of ______ or less. |
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Definition
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Term
| For true CT reconstruction, how much angular data is required. |
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Definition
| 180 degrees plus the fan angle |
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Term
| Cardica CT - describe retrospective gating |
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Definition
| The ECG is acquired with the CT data. After the scan, the desired time points are selected to reconstruct the image. |
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Term
| Cardiac CT - describe prospective gating |
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Definition
| The ECG is used to trigger the CT scanner so that data is only acquired when the heart is still (at the end of diastole). |
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Term
| What is the benefit of prospective gating over retrospective gating. |
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Definition
| Much lower dose. The CT scanner is only on at certain points in the cardiac cycle. |
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Term
| A dual source CT was created for ________. |
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Definition
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Term
| The dual source CT only has to rotate through ______ degrees and this requires _______ ms. |
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Definition
| 90 degrees, 83 msec (1/3 sec ÷ 4) |
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Term
| Dual energy CT can be described as |
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Definition
| a linear combination of low and high energy CT images. |
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Term
| CTA images are often produced using. |
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Definition
| CT Angiography - produced with maximum intensity projection (MIP) techniques. |
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Term
| CT perfusion studies may require how many CT images? |
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Definition
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Term
| CT perfusion studies result in ________ patient dose. |
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Definition
|
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Term
|
Definition
| During a profusion study, the table moves back and forth so a CT image can repeatedly be made over an area that is wider than the detector array (nT). |
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Term
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Definition
| lower dose levels to be used to achieve the same image quality. |
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Term
| Tube current modulation occurs in what direction. |
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Definition
| mA is modulated as the tube rotates around the patient and also along the z-axis. |
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Term
| Projection measurements undergo _________ and _________ transformation. Why |
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Definition
normalization - to a reference detector to account for bowtie filters and different detector responses
logmarithmic transformation - so the measurement is the sum of linear attenuation coefficients instead of an exponent.
Pj = t(μ1 + μ2 + μ3 + .... ) |
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Term
| Creating projection data from central measurements is called |
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Definition
|
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Term
|
Definition
| The process of creating the image matrix from the measured projection data. |
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Term
| What problem arises with simple back-projection? |
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Definition
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Term
| What process solves the 1/r blurring found in simple backprojection? |
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Definition
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Term
| How does "filtering" work in filtered backprojection. |
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Definition
| A deconvolution kernel is applied to the measured projection data that undoes the effect of the 1/r blur funtion. |
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Term
How can the convolution of p(x) and h(x) be calculated using the Fourier transform?
Why would you do this? |
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Definition
Take the FT of each function and multiple the results together. Then take the inverse FT.
It is faster on a computer. |
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Term
| What is the FT of the deconvolution kernel used in filtered backprojection? |
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Definition
| It is the ramp filter. It is zero at zero frequency and ramps up linearly with frequency. |
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Term
| What can you do to the ramp filter to remove noise? |
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Definition
| Since noise is a high frequency component, have the ramp filter "roll-off" as frequency increases. |
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Term
| The cone beam reconstruction process is called? |
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Definition
| The Feldkampt algorithm or FDK |
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Term
| How does iterative reconstruction work. |
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Definition
| The CT image is iterated. At each iteration the forward projection data is calculated. The difference between the forward projection data and the measured projection data is called the error matrix. The goal is to construct a CT image that minimizes the error matrix. |
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Term
| What is the downside to iterative reconstruction? |
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Definition
| It is very numerically intense and requires a lot of computer power and time. |
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Term
| What is the advantage to iterative construction over filtered backprojection? |
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Definition
| It results in images with better SNR for the same dose. |
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Term
| The ACR phantom has how many bar patterns and at what frequencies for measuring spatial resolution? |
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Definition
| There are eight bar patterns. 4, 5, 6, 7, 8, 9, 10, and 12 lp/cm |
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Term
| The MTF is affected by the |
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Definition
|
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Term
| How would you measure the MTF? |
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Definition
| By imaging a wire or plane of metal foil and measuring the line spread function (LSF). |
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Term
| Spatial resolution in the z-axis is measured using the |
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Definition
| slice sensitivity profile (SSP) |
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Term
| How is slice thickness determined from the ACR phantom. |
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Definition
| There are a series of high contrast wires spaced 0.5 mm apart in the z-direction. You count the number of visible wires and multiple by 0.5 mm. |
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Term
| The MTF in the z-direction is ________ for thinner slices. |
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Definition
|
|
Term
| How is dose distribution in the z-axis measured? |
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Definition
| It has to be measured with an external device such as film or a CR plate. |
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Term
| The low-contrast module of the ACR phantom has test objects that have a ____ HU greater density than the surrounding material. |
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Definition
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Term
| In CT, the direct measurement of noise is given by |
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Definition
| the standard deviation of the HUs |
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Term
|
Definition
| the frequency dependancy of the noise. |
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Term
| Noise correlation in the (x,y) plane in CT is largely due to |
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Definition
| the filtered backprojection reconstruction kernel. |
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Term
| Primary factors affecting spatial resolution in CT. |
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Definition
| 1) focal spot distribution, 2) gantry motion, 3) detector size and sampling, 4) reconstruction filter |
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Term
| Primary factors that affect contrast resolution (noise) in CT. |
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Definition
| 1) technique factors; kV, mA, time, and pitch, 2) slice thickness, 3) reconstruction filter (filtered backprojection), 4) reconstruction method - iterative reconstruction significantly reduces image noise |
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Term
| Beam hardening artifacts appear as |
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Definition
| dark webbing between dense regions. |
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Term
| How are beam hardening artifacts reduced? |
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Definition
| By prehardening the beam with added filtration. |
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Term
| When do streak artifacts occur? |
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Definition
| When attenuation levels are excessive such as around metal fillings or cardiac pacemakers. |
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Term
| What amplifies streak artifacts? |
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Definition
|
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Term
| View aliasing is caused by |
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Definition
| to few projections acquired for reconstructing high-frequency objects. |
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Term
| What are partial volume artifacts |
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Definition
| When a voxel contains more than one type of tissue. |
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Term
| How are partial volume artifacts avoided? |
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Definition
|
|
Term
| Cone beam artifacts are generally due to |
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Definition
| not acquiring enough data in the cone beam direction. |
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Term
| Describe 1st generation CT |
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Definition
| Pencil beam, translate-rotate |
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Term
| Describe 2nd generation CT |
|
Definition
| small fan beam, translate and rotate |
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Term
| Describe 3rd generation CT |
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Definition
| Large fan beam, rotate-rotate - both the x-ray tube and the detector rotate (no more translating of the x-ray tube) |
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Term
| The diameter of the FOV is given by |
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Definition
FOV = 2S sin (α/2)
S - source to isocenter distance
α - fan angle |
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Term
| Describe a 4th generation CT scanner? |
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Definition
| The x-ray tube rotates, but the detector ring is stationary. This requires the detector ring to go all the way around. This is expensive. |
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Term
| What problem from 3rd generation scanners was the 4th generation trying to solve. |
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Definition
| Dead or improperly calibrated detectors would result in ring artifacts. |
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