CT - Chapter 2

Question 1

Describe pros/cons of iterative reconstruction

Answer 1

• 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

Question 2

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

Answer 2

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

****** Does not improve temporal resolution

Question 3

Describe the relationship between:

• image noise (quantum noise)
• exposure

Answer 3

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

Question 4

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

Answer 4

• Decrease in image noise
• Higher radiation dose
• Lower contrast resolution

Question 5

Describe the effects of increasing tube voltage

Answer 5

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

Question 6

What does the “p” stand for?

• typical XR voltage for cardiac CT is 120 kVp

Answer 6

“Peak”

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

Question 7

What material has a Hounsfield unit of zero?

Answer 7

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

Question 8

How can the quality of the image be improved?

Answer 8

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

Question 9

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

Answer 9

• Higher tube current or voltage
• Iterative reconstruction

Question 10

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

Answer 10

• lower tube current or voltage
• increased pitch
• FBP reconstruction

Question 11

What is the temporal resolution of the scan?

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

Answer 11

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

Question 12

Describe how to calculate temporal resolution

• Single-source scanner
• Dual-source scanner

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

Question 17

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

Answer 17

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.

Question 18

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

Answer 18

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

Question 19

Describe the difference in regards to photons for each:

• tube voltage (kV)
• tube current (mA)

Answer 19

• 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

Question 20

What is the definition / formula for pitch?

Answer 20

table feed (mm) per 360 degree gantry rotation

collimated beam width (mm) (or coverage)

Question 21

What is the principle determinant of spatial resolution in CT?

Answer 21

Gantry rotation time

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

Question 22

What do scintillation crystals in photodetectors convert?

Answer 22

X-ray photons to light photons

Question 23

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

Answer 23

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

Question 24

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

Answer 24

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

Question 25

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?

Answer 25

**0.4 mm = equivalent spatial resolution** * 10 / (resolution in line pairs x2) * 10 / (12.5 x 2) * 10 / 25 = **0.4 mm**

Question 26

How does increasing peak tube voltage decrease image noise?

Answer 26

**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

Question 27

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

Answer 27

**gantry rotation time and HR must be asynchronous**

Question 28

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

Answer 28

**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"

Question 29

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?

Answer 29

**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%

Question 30

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

Answer 30

**Sharp reconstruction filter** * improves spatial resolution by reducing "partial volume averaging" or "blooming artifact"

Question 31

What variables influence attenuation of a tissue?

Answer 31

* 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

Question 32

What are factors that determine temporal resolution?

Answer 32

* **gantry rotation speed** * number of X-ray sources * ability to perform multi-cycle reconstruction

Question 33

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

Answer 33

**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

Question 34

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

Answer 34

**-1,000 HU**

Question 35

How is pixel size calculated?

Answer 35

Field of View (FOV) ------------------------------------------ Matrix size