1. Advanced Biomedical Imaging
Lecture 7
Factors affect CT image quality
&
Advanced CT machines
Dr. Azza Helal
A. Prof. of Medical Physics Faculty of Medicine Alexandria University

2. Points to be covered
Image quality
Noise, contrast, spatial resolution.
Radiation dose from different CT scans
Artifacts
Advances in CT

3. 1. Image quality Image quality also depends on:
Nature of x-ray source & detectors,
Number & speed of measurements.
Method of reconstruction & method of displaying the image.
The more measurements the more accurate the reconstructions.
Ability to reconstruct axial images into coronal, sagittal or oblique planes

4. A pixel is 2D element of the image A voxel is 3 D element of the image
Slice thickness is the 3rd dimension of the voxel.
Kanal

5. Noise (Grainy image)
variation in no of x ray photons absorbed / voxel.
Causes: Any cause decreases no of photons
Zoom enlargement and narrow window.
The decrease in slice thickness and scan time
Obese/ Patient thickness and high attenuation materials as bone and prosthesis in slice.

6. It limits image quality (decreases CR & SR).
Noise is reduced by
Any cause increases photons no absorbed / voxel.
S/N ratio kv
MA or scan time (patient dose )
FOV, slice thickness or pixel size, SR is worse

7. Spatial resolution (SR)
Minimum distance between two points that imaging system distinguishes them as being separate.
SR is improved by using (scanner design)
Smaller focal Spot & detector width
Smaller slice thickness, pixel size & FOV (small voxels)
More projections & lower Pitch
Reconstruction & decreased Patient motion.

8. Contrast resolution Patient size (thin patient) . low KV (U α Z3/E3)
ability to detect small difference in HU of adjacent structures.
It is superior to plain film due to smaller scatter & removal of superimposed anatomy, very fine x-ray beam, double collimation & Windowing
Contrast Resolution is improved by
Causes increase no of photons (dose increases) as:
More mAs,
increasing FOV, pixel size, slice thick.
Patient size (thin patient) low KV (U α Z3/E3)

9. To ½ noise, dose increases by 4
Not possible to achieve good CR & SR at same time except by deliver high unaccepted pt dose.
To improve contrast by 2 pt dose increases by 4.
To improve SR by 2 patient dose increases by 8.
To half slice thickness pt dose increases by 2
To ½ noise, dose increases by 4
Typical effective dose is in range of 5-10msv.

10. Factors affect patient dose 2. mAs Doubling mAs double dose,
U α Z3/E3
2. Radiation dose
140KV
120KV
80KV
Parameter
Poor
Intermediate
Best
Contrast
Least
Average
Most
Noise
Penetration
lowest
intermediate
highest
Patient dose
Factors affect patient dose
1. KVp Low Kv increase dose & contrast
2. mAs Doubling mAs double dose,
3. Pitch Dose α mAs/pitch.
4. Slice thickness thin slice increases dose
5. Scan time Increase time increase dose

11. How to reduce the dose in CT scan?
Decrease mA or current
Increase Kv
Increase slice width
Decrease the time, 1/2 the time, 1/2 the dose.
Double the pitch 1/2 the dose.
Reducing patient dose AFFECT image quality
Increase noise
Decrease contrast & SR
Thick slice
So decrease patient dose As far as not affect image quality
and hence diagnosis

12. Typical effective dose (msv)
(millirem)
Typical effective dose (msv)
Examination 10
0.1
Chest X-ray
150
1.5
Head CT
300 3
Screening mammography
530
5.3
Abdomen CT
580
5.8
Chest CT
360–880
3.6–8.8
CT colonography (virtual colonoscopy)
990
9.9
Chest, abdomen & pelvis
670–1300
6.7-13
Cardiac CT angiogram
1500 15
Barium enema
2000 20
Neonatal abdominal CT
Effective dose to the patient from different CT scans

13. Organ Dose
Head CT
Thyroid mGy
Eye lens - 40 mGy
Chest CT
Breast - 21 mGy
Abdomen CT
Uterus – 8 mGy = 8mSv
Gonads - 8 mGy
Pelvis CT
Uterus – 26 mGy
Gonads - 23 mGy
Dose to pregnant
1mGy (1mSv) worker.
5mGy (5mSv ), 500 mRem)

14. 3.Artifacts
Appearance of signal in an image location not representative of actual properties of object it is necessary to understand why artifacts occur, appearance, cause, and how to prevent/ avoid
Artifacts

15. a. Metallic artifact
X-ray beams pass through metal implants are highly attenuated and detector detects no transmission, streaks.

16. Graining on image caused by a low SNR; thin slice
Blurring , image ghosting, failure to correct u of moving object during reconstruction ;streaks.
Graining on image caused by a low SNR; thin slice
b. Motion artifact
c. Noise artifact

17. d. Partial volume effect
(PVA)
a high contrast object occupies part of voxel (bone).
scanner is unable to differentiate between a small amount of high-density material (e.g. bone) and a larger amount of other tissue densities (brain).
The processor average out the two structures, it raises CT No of pixel & appears higher than it is.
It is avoided by thinner slice & smaller pixel
DR Azza Helal

18. d. Partial volume effect
Floor of ant cranial fossa, as frontal bone is irregular (bone and brain) DDx hge (bright)

19. one or many "rings" appears within an image. due to a detector fault.
e. Ring artifact

20. (high energy), low u, low ct no along path of x ray beam,
f. Beam hardening
Attenuation of bone is greater than that of soft tissue, bone causes more beam hardening (low energy x ray are attenuated ) than an equivalent thickness of soft tissue
The beam becomes hard
(high energy), low u, low ct no
along path of x ray beam,
black signal,
post fossa black, e.g Petrous bones
cupped appearance
This is easily corrected by filtration

21. 4. Advances in CT
a. Helical CT (6th generation)
pitch value determines how fast the body is moved
Body moves continuous through x-ray beam. creates a continuous data which sliced during reconstruction.
1. Fast scan low dose (rad. is less concentrated)
But; reduced contrast & image details in Z axis, More noise & more heating of the tube so need low Ma & PVA occurs

22. b. Multiple slice / detector arrays (MDCT)
(MSCT)

23. MSCT can be used for:
Fast imaging for larger tissue volume
Fewer motion artifact
Efficient use of x-ray beam & dose reduction.
Reconstruction in different slice widths
Possibility of isotropic Imaging (better MPRs
and 3D images with reduced image artifacts).
Thinner slices for better z-axis resolution

24. c. PET/CT
A medical imaging device which combines in a single gantry system both PET and CT components.
Images acquired from both devices can be taken sequentially, in the same session from the patient and combined into a single image.

25. Thus functional imaging obtained by PET can be correlated with anatomic imaging obtained by CT.
PET CT images show metastases to left supraclavicular lymph node a and to liver b

26. Questions
Mention the main factors that affect quality of CT image?
Enumerate the factors that improve CT image resolution?
What are the factors that help in reducing patient dose during CT imaging?
Mention the factors that reduce noise in CT image?

27. Thank you