Q1.Reducing the field of view (FOV) while keeping matrix size constant will: A. Increase pixel size and improve spatial resolution B. Decrease pixel size and improve spatial resolution C. Increase pixel size and improve contrast resolution D. Decrease pixel size and worsen spatial resolution
B
Q2.Doubling slice thickness in CT will:
A. Decrease SNR and contrast resolution
B. Increase partial volume averaging and improve Spatial Resolution
C. Increase SNR and contrast resolution, but reduce SR
D. Improve both spatial and contrast resolution
C
Q3.In CT, high kVp (120–140) is routinely used because:
A. It improves spatial resolution
B. It reduces scatter and improves Spatial Resolution
C. It reduces voxel size
D. It ensures Compton scatter dominates, improving Contrast Resolution in dense tissue
D
Q4.A higher helical pitch will:
A. Improve both Spatial Resolution and Contrast Resolution
B. Reduce slice sensitivity profile width and improve SR
C. Increase slice profile width and reduce SR
D. Have no effect on image quality
C
Q5.Compared to conventional CT, cone-beam CT typically offers:
A. Higher spatial resolution but poorer contrast resolution
B. Better soft tissue contrast
C. Lower spatial resolution
D. Higher dose and wider field of view
A
Q6.Image noise is most strongly influenced by (CT):
A. Matrix size
B. kVp
C. mAs
D. Pixel size
C
Image noise in Computed Tomography (CT) is primarily determined by the number of X-ray photons detected. The mAs (milliampere-seconds), which is the product of the tube current (mA) and the scan time (s), directly controls the quantity of X-ray photons produced and, by extension, the patient dose. An increase in mAs leads to more photons and, consequently, lower image noise (noise is inversely proportional to the square root of mAs).
What is the pitch if a couch travels 3cm per rotation and acquisition width of the simultaneously acquired slices is 2cm?
Pitchx= couch travel per rotation /total width of simultaneously acquired slices= 1.5
2) Which of the following statements about CT scanner components is CORRECT?
A)The collimator increases scatter outside the desired slice to improve image detail.
B) Bow tie filtration is used to equalise transmitted intensities across the patient.
C) Inherent window filtration increases low-energy X-rays to improve contrast.
D) Older scanners used photon counting detectors instead of xenon gas detectors.
B
3)Which is the most common type of ct scanner generation?
3rd
4) Which CT generation has the ring artefact?
3rd
5) Which of the following statements about assumptions in basic back projection CT reconstruction are TRUE?
1-The X-ray focal spot is considered a point source.
2-Photon emission and detection are assumed to follow a uniform distribution without statistical noise.
3-Each detector is assumed to be identical in geometry and sensitivity.
4- The cone-beam geometry of the X-ray beam is modelled exactly as it is in practice.
A) 1, 2, and 3 only
B) 1 and 4 only
C) 2 and 3 only
D) 1, 2, 3, and 4
A
HU of muscle
Typical Muscle: 50-70
HU of GM and WM
cerebrospinal fluid ± 5 – 10 HU;
white matter ± 25 – 30 HU; gray matter: ± 35 – 45(50) HU.
HU of cancellous and cortical bone
cancellous- 200-400
cortical- 1000+
HU of lung: inflated, hypoinflated, atelectatic
Normal, inspiratory: -846
Normal, expiratory: -713
poorly aerated: -500 to -101
atelectatic: -101 to +100
how many shades of grey in 8 bit?
2 to the power 8= 256
ww/wl for lung
1400/ -500
ww/wl for brain
120/40
ww/wl for bone
4500/450
ww/wl for liver
160/60
effect of sharp/ bone kernel on image quality
Sharp (bone) kernel: highlights edges → ↑ spatial resolution, but amplifies noise → ↓ CR.
Smooth (soft tissue) kernel: suppresses noise → ↑ contrast resolution, but ↓ Spatial Resolution.
Always match kernel to study type (bone vs soft tissue).
effect of mAs on image quality and dose CT
mA (photon flux)
Controls number of photons.
↑ mA = ↑ photons/pixel = ↑ SNR = ↑ contrast resolution (less “grainy” image).
No direct effect on spatial resolution(detail limited by pixel size, not photon count).
Doubling mAs increases SNR by √2 (~41%). Dose ↑ proportionally
effect of kVp on image quality CT
Determines beam energy and penetration. CT assumes attenuation is dominated byCompton scatter, which is nearly linear with density at high kVp.
High kVp (120–140) used routinely:
↑ penetration → less beam hardening → bettercontrast resolutionin dense tissues.
effect of pitch on ct image quality and dose
High pitch (table moves faster) → broader slice sensitivity profile → ↓ spatial resolution.
Lower pitch = better detail but ↑ dose and scan time.
