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Medical > CT - Chapter 2 > Flashcards

CT - Chapter 2 Flashcards

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USMLE® Step 1 flashcards
1
Q

Describe pros/cons of iterative reconstruction

A
  • Reduces image noise
    • improved SNR
  • Reduced radiation doses
  • Requires longer processing (reconstruction) times than traditional FBP
  • Improves image quality
  • Can potentially be used with any CT system
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2
Q

What are advantages of sequential (prospectively triggered) scan mode compared to conventional retrospective gating?

A
  • Reduced scan time
    • may be prolonged if HR is faster
  • Accommodating functional reconstructions
  • Reducing the radiation dose

****** Does not improve temporal resolution

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3
Q

Describe the relationship between:

  • image noise (quantum noise)
  • exposure
A

Inversely proportional

  • not in a one-to-one relationship
  • quantum noise is inversely proportional to the square root of the exposure
  • quadruple the exposure –> half the noise
  • Examples:
    • XR exposure that is too short (uses too little tube current) –>
    • poor quality, noisy XR
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4
Q

What will be the result of increasing peak tube voltage on a MDCT?

A
  • Decrease in image noise
  • Higher radiation dose
  • Lower contrast resolution
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5
Q

Describe the effects of increasing tube voltage

A

Decrease in image noise

  • image noise is inversely related to the square root of radiation exposure
  • increased peak tube voltage (exposure) –> higher energy XR’s –> decrease the noise of the image
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6
Q

What does the “p” stand for?

  • typical XR voltage for cardiac CT is 120 kVp
A

“Peak”

  • referring to the peak (maximum) energy of the XR spectrum
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7
Q

What material has a Hounsfield unit of zero?

A

Water

  • HU scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at standard pressure and temperature is defined as zero HU units
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8
Q

How can the quality of the image be improved?

A
  • Widening the window level
  • Thin slices
    • both above improve image quality
  • Sharp reconstruction kernels/filters
    • improve image resolution
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9
Q

What adjustments to acquisition or reconstruction will result in reduced image noise?

A
  • Higher tube current or voltage
  • Iterative reconstruction
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10
Q

What adjustments to acquisition or reconstruction will result in increased image noise?

A
  • lower tube current or voltage
  • increased pitch
  • FBP reconstruction
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11
Q

What is the temporal resolution of the scan?

  • dual-soucre CT scanner
  • single-segment reconstruction
  • 360 degree gantry rotation time of 280 ms
A

280 ms / 4 = 70 ms

  • dual-source scanners use 2 XR tubes and 2 detectors, arranged at 90 degree angles to each other, to simultaneously acquire data
  • 180 degrees of attenuation data that are necessary to reconstruct one image can be collected by only a qurter turn of the gantry, rather than the usual-half turn required in single source systems
  • data can be gathered in one-half the time
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12
Q

Describe how to calculate temporal resolution

  • Single-source scanner
  • Dual-source scanner
A
  • Single-source scanner
    • Rotation time / 2 –> temporal resolution
  • Dual-source scanner
    • Rotation time / 4 –> temporal resolution
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13
Q

After reviewing the images of a CCTA, you decide to reconstruct the images using a smoother filter. Where can this be done?

A

At the scanner

  • raw attenuation data are usually stored on a dedicated server attached to the scanner but separate from the CT workstation
  • can be performed using raw data acquired via any acquisition mode
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14
Q

CCTA requires using thin detector collimation to avoid partial volume averaging, but the use of thin collimation comes at what cost?

A

Increased image noise

  • very thin sections –> avoids partial volume averaging
    • absolutely required for high-resolution coronary imaging
  • comes at the cost of increased image noise
  • can be overcome with:
    • higher tube current
    • special reconstruction algorithms for coronary arteries
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15
Q

What matrix size would make full use of the spatial resolution of the scanner at this field of view?

  • CT scanner spatial resolution = 0.25mm
  • Rows of detectors = 512
  • Detector width = 0.25 mm
  • Manufacturer’s protocol for CCTA = reconstructed field of view 250 mm
A

1024 x 1024

  • main goal of image reconstruction is to properly match:
    • image pixel size = spatial resolution of scanner
      • ​one cannot resolve something smaller than one pixel
      • pixels much larger than 1/1 are essentially losing information
  • 250 mm (FOV) / 1024 (matrix size) = 0.243 mm pixels
    • closets match to CT scanner spatial resolution = 0.25 mm
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16
Q

What is the utility of multi-cycle reconstruction (multi-segment reconstruction)?

A

Improve temporal resolution

  • occurs at the expense of “averaging” data from several cardiac cycles into one single image
  • requires that each slice position is scaned during several consecutive heart beats ( at least 2 )
    • typically done in helical mode + low-pitch value
    • “oversampling” typically occur in prospective, ECG-triggered axial scanning
17
Q

What can be done to improve visualization of the proximal LAD?

A

Increase Peak Tube Voltage

  • “burns through” the dense calcium and decrease this contrast
  • calcium (partial volume averaging or “blooming” artifact) obscures the underlying contrast column within the vessel lumen –> very high photodensity gradient (contrast between the calcium and adjacent structures
  • however, gain in visualization is not justified by increased radiation dose to patient.
18
Q

What are the two ways in which dual-energy CT is performed?

A

Two raw datasets are collected for each scan –>

representing attenuation data from two different photon energies –>

differential attenuation (calcium and iodine) will allow for new differential (subtracted) images to be created

  • layered detector array (which can differentiate):
    • high energy photons
    • low energy photons
  • two XR-tubes (dual-source scanner) to scan simultaneously at two different XR energies (e.g.)
    • 80 kVp
    • 120 kVp
19
Q

Describe the difference in regards to photons for each:

  • tube voltage (kV)
  • tube current (mA)
A
  • tube voltage (kV)
    • energy level of the photons coming out of the XR tube
  • tube current (mA)
    • quantity (output) of photons

*****Lowering both –> reduced radiation exposure

20
Q

What is the definition / formula for pitch?

A

table feed (mm) per 360 degree gantry rotation

collimated beam width (mm) (or coverage)

21
Q

What is the principle determinant of spatial resolution in CT?

A

Gantry rotation time

  • determines how long it takes to acquire the attenuation data used to reconstruct a single axial slice
22
Q

What do scintillation crystals in photodetectors convert?

A

X-ray photons to light photons

23
Q

What is the size, in the z-dimension, of the voxels in the resultant reconstructed data set?

  • Prospectively ECG-triggered, 256-slice scaner
  • Detector collimation of 256 x 0.625 mm
  • FOV = 250 mm
  • Gantry rotation time = 270 ms
  • Image matrix = 512 x 512
A

0.625 mm

  • In prospective, axial scanning, the minimum voxel size in the z-axis dimension –>
  • determined by minimum slice thickness –>
  • determined by detector collimation –> 256 x 0.625 mm
24
Q

What are negative aspects of utilizing high resolution setting on CT scanners?

A

Higher heat load to the X-ray tube

  • may result in maximum heat capacity of the anode –>
  • and limit the available peak tube voltage and tube current
  • which may be needed in scanning obese patients
  • may limit scan duration
25
Q

If a scanner’s spatial resolution is reported to be 12.5 line pairs per cm, what is the equivalent spatial resolution expressed in mm?

A

0.4 mm = equivalent spatial resolution

  • 10 / (resolution in line pairs x2)
  • 10 / (12.5 x 2)
  • 10 / 25 = 0.4 mm
26
Q

How does increasing peak tube voltage decrease image noise?

A

Increases photon penetration

  • increases the number of photons that pass through the patients (photon penetration)
  • higher energy photons are less likely to be attenuated by the patient’s body
  • more photons reach the detector –>
    • better SNR
    • reduced image noise
27
Q

What must be present for multi-cycle reconstruction to occur?

A

gantry rotation time and HR must be asynchronous

28
Q

What effect will increasing peak tube voltage have on soft tissue contrast?

A

lower soft tissue contrast

  • as photon energy increases –>
  • more photons pass through the body unattenuated –>
  • tissue contrast is decreased
  • example:
    • XR in which photon energy is so high that the field is extremelly overexposed and “burned out”
29
Q

What is the effect of increasing the gantry rotation time of a CT system from 1 sec to 2 sec without changing kVp or mA?

A

Increasing the mAs by 100%

  • “tube current-time product” –> expressed in mAs, refers to the product of X-ray tube current multiplied by X-ray exposure time
    • If Gantry rotation time is doubled (increases by 100%) –>
    • time required to collect attenuation data is doubled –>
    • exposure time will be doubled –>
    • TCT product (mAs) will incerase by 100%
30
Q

What filter results in the highest spatial resolution in the setting of coronary calcifications?

A

Sharp reconstruction filter

  • improves spatial resolution by reducing “partial volume averaging” or “blooming artifact”
31
Q

What variables influence attenuation of a tissue?

A
  • density of tissue
  • thickness of the tissue
  • energy of incident photons
    • higher energy photons –> more likely they will pass through the tissue without interacting with it
32
Q

What are factors that determine temporal resolution?

A
  • gantry rotation speed
  • number of X-ray sources
  • ability to perform multi-cycle reconstruction
33
Q

Which CT acquisition (using 64-row scanner) will have higher/better spatial resolution? Assume HR, Rhythm and Body habiuts are the same for both

  • detector collimation = 64 x 0.625
  • detector collimation = 32 x 1.25
A

64 x 0.625

  • thinner detector collimation –> better ability to resolve small details because of less volume averaging
  • Both scans have:
    • collimated beam width (40 mm) –>
    • same scan time
34
Q

What is the conventional CT value (in Hounsfield Units) for air?

A

-1,000 HU

35
Q

How is pixel size calculated?

A

Field of View (FOV)

Matrix size

Medical (69 decks)

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