CT IMAGE QUALITY AND IMAGE RESOLUTION ARE DESCRIBED IN DETAILIN THIS PPT. CONTENT TAKEN FROM MUTIPLE BOOKS AND GENERALS.

1. CT IMAGE QUALITY AND IMAGE
RESOLUTION
MR. ROHIT BANSAL
ASISTANT PROFESSOR
MAMC (AGROHA)

2. Image quality refers to the visibility of diagnostically
important structures in the CT image. CT image quality is
influenced by many technical parameters.
The image quality is influenced by five basic factors:
1. Spatial Resolution.
2. Image Contrast.
3. Temporal Resolution.
4. Image Noise.
5. Image Artifacts.

3. Resolution is a term used to describe the ability to
differentiate between structures.
In diagnostic imaging, resolution refers to the number of
pixels in an image.
A high-resolution image has more pixels, which allows its
to display more details.
Resolution

4. It is the ability of the CT scanner to display separate images
of two objects which are placed close together.
In digital imaging, it depends on the size of the pixel used.
A large pixel size will be unable to differentiate between two
near-by objects as compared to a small pixel size.
Spatial resolution is measured in line-pairs per millimeters.
Spatial Resolution

5. Focal Spot- Smaller focal spots give a higher resolution. Thus, the
Spatial Resolution improves.
 Detectors width- If the width of the detector is small, it gives a higher
resolution.Thus Spatial Resolution improves.
Number of Projections- A larger number of projections gives a finer
resolution.Thus Spatial Resolution improves.
Slice thickness- Thinner slices produce sharper images. If the slice is
thinner, it gives a higher resolution.Thus Spatial Resolution improves.
Kernels- Sharp kernels have better spatial resolution than soft kernels.
Pitch- Lower pitch ratio improves spatial resolution.
Pixel Size- Smaller Pixel Size gives more detail. Thus Spatial Resolution
improves.
FOV- Smaller FOV gives a finer resolution.Thus Spatial Resolution.
Patient Motion- If the patient's motion decreases, thus the Spatial
Resolution increases.
Factors Affecting Spatial Resolution

9. It is the ability of the CT scanner to display the object with
considerably different in density from its surrounding
objects.
In digital imaging, it depends on the bit-depth of the
system.
The 8-bit system shows less gray values - 256 gray values,
and the 12-bit system that shows 4096 gray values.
If the system can clearly show two near-by gray value
intensities, the system will have a high contrast resolution.
Contrast Resolution

10. 1. Increases in mAs, improve contrast resolution.
2. Decreases in pixel size decreases contrast resolution.
3. Increases in Slice thickness improves contrast
resolution.
4. Increases in FOV improves contrast resolution.
Factors Affecting Contrast Resolution

12. Temporal resolution is the duration of time for the
acquisition of image.
In cardiac CT, it is necessary to image the beating heart in
milliseconds.
In cardiac CT, the temporal resolution of 3 second means
that a single image is acquired within 3 seconds of the
cardiac cycle.
In a fast MDCT scanner, the temporal resolution will be
better and the motion artifacts will be lesser.
Temporal Resolution

13. Noise is defined as the grainy appearance of cross-sectional
imaging.
Noise decreases in the picture quality and reduces the
contrast resolution.
Noise is caused by a low photon count in an image.
Noise is measured by the signal to noise ratio (SNR).
NOISE

15. The number of X-ray photons detected per pixel is also
often referred to as signal-to-noise ratio (SNR).
The higher the ratio, the less noise is present in the image.
The noise is caused by many factors; the most common is
quantum mottle.
Quantum Mottle - Quantum mottle occurs when there is
an insufficient number of photons detected by the detector.
Signal to Noise Ratio (SNR)

16. Decreases in pixel size increase the noise.
Increases in mAs, decreases the noise.
Larger patients will absorb more radiation hence fewer
photons will reach the detector, which will reduce the signal-
to-noise ratio.
Factors Affecting CT Noise

17. CT scanner generates a high dose of radiation.
These radiations alter the tissue and produce free radicals,
which can lead to cancer.
Factors Affecting CT Dose
1. Increases in mAs, increase the patient dose.
2. Increases in kVp without a decrease in mAs, increase the
patient dose.
3. Increases in image quality, increase the patient dose.
4. Decreases in slices thickness increase the patient dose.
Patient Dose

18. The patient's dose inCT scan examination can be reduced by following
steps
1. By reducing the tube current (mA)
2. By Increasing the table pitch.
3. By adjusting the mA settings manually, according to patient weight.
4. By reducing the number of multiple scans.
Effective Radiation Dose in Adults for CT Scan Procedure
• ComputedTomographyAbdomen and Pelvis-10 mSv (Approximate)
• ComputedTomographyChest-7 mSv (Approximate)
• ComputedTomographyAngiography- 12 mSv ( Approximate)
● ComputedTomography without contrast Head- 2 mSv (Approximate)
Reduction of Patient dose in CT Procedure

19. ICRP EFFECTIVE RADIATION DOSE IN
ADULTS
ABDOMINAL REGION Procedure Approximate effective
radiation dose
Comparable to natural
background radiation
for:
ComputedTomography
(CT)–Abdomen and Pelvis
7.7 mSv 2.6 years
ComputedTomography
(CT)–Abdomen and Pelvis,
repeated with and without
contrast material
15.4 mSv 5.1 years
ComputedTomography
(CT)–Colonography
6 mSv 2 years
Intravenous Urography
(IVU)
3 mSv 1 year
Barium Enema (Lower GI X-
ray)
6 mSv 2 years
UpperGI Study with Barium 6 mSv 2 years

20. BONE Procedure Approximate effective
radiation dose
Comparable to natural
background radiation for:
Lumbar Spine 1.4 mSv 6 months
Extremity (hand, foot, etc.)
X-ray
Less than 0.001 mSv Less than 3 hours
CENTRAL NERVOUS
SYSTEM
Procedure Approximate effective
radiation dose
Comparable to natural
background radiation for:
ComputedTomography
(CT)–Brain
1.6 mSv 7 months
ComputedTomography
(CT)–Brain, repeated with
and without contrast
material
3.2 mSv 13 months
ComputedTomography
(CT)–Head and Neck
1.2 mSv 5 Months
ComputedTomography
(CT)–Spine
8.8 mSv 3 years

21. CHEST Procedure Approximate
effective radiation
dose
Comparable to
natural background
radiation for:
Computed
Tomography (CT)–
Chest
6.1 mSv 2 years
Computed
Tomography (CT)–
Lung Cancer
Screening
1.5 mSv 6 months
Chest X-ray 0.1 mSv 10 days
DENTAL Procedure Approximate
effective radiation
dose
Comparable to
natural background
radiation for:
Dental X-ray 0.005 mSv 1 day
Panoramic X-ray 0.025 mSv 3 days
Cone Beam CT 0.18 mSv 22 days

22. HEART Procedure Approximate
effective radiation
dose
Comparable to
natural background
radiation for:
Coronary Computed
Tomography
Angiography (CTA)
8.7 mSv 3 years
Cardiac CT for
Calcium Scoring
1.7 mSv 6 months
Non-Cardiac
Computed
Tomography
Angiography (CTA)
5.1 mSv Less than 2 years
MEN'S IMAGING Procedure Approximate
effective radiation
dose
Comparable to
natural background
radiation for:
Bone Densitometry
(DEXA)
0.001 mSv 3 hours

23. NUCLEAR MEDICINE Procedure Approximate effective
radiation dose
Comparable to natural
background radiation
for:
Positron Emission
Tomography–
ComputedTomography
(PET/CT)Whole body
protocol
22.7 mSv 6 years
WOMEN'S IMAGING Procedure Approximate effective
radiation dose
Comparable to natural
background radiation
for:
Bone Densitometry
(DEXA)
0.001 mSv 3 hours
Screening Digital
Mammography
0.21 mSv 26 days
Screening Digital
BreastTomosynthesis
(3D Mammogram)
0.27 mSv 33 days

24. ACR EFFECTIVE RADIATION DOSE IN
ADULTS
Single Radiograph Effective Dose, mrem (mSv)
Skull (PA orAP)1 3 (0.03)
Skull (lateral)1 1 (0.01)
Chest (PA)1 2 (0.02)
Chest (lateral)1 4 (0.04)
Chest (PA and lateral)2 6 (0.06)
Thoracic spine (AP)1 40 (0.4)
Thoracic spine (lateral)1 30 (0.3)
Lumbar spine (AP)1 70 (0.7)
Lumbar spine (lateral)1 30 (0.3)
Abdomen (AP)1 70 (0.7)
Abdomen3 53 (0.53)
Pelvis (AP)1 70 (0.7)
Pelvis or hips3 83 (0.83)
Bitewing dental film3 0.4 (0.004)
Limbs and joints3 6 (0.06)

25. Complete Exams Effective Dose, mrem (mSv)
Intravenous pyelogram (kidneys, 6 films)1 250 (2.5)
Barium swallow (24 images, 106 sec fluoroscopy)1 150 (1.5)
Barium meal (11 images, 121 sec fluoroscopy)1 300 (3.0)
Barium follow‐up (4 images, 78 sec fluoroscopy)1 300 (3.0)
Barium enema (10 images, 137 sec fluoroscopy)1 700 (7.0)
CT head1 200 (2.0)
CT chest1 800 (8.0)
CT abdomen1 1,000 (10)
CT pelvis1 1,000 (10)
CT (head or chest)2 1,110 (11.1)
PTCA (heart study)3 750–5,700 (7.5–57)
Coronary angiogram3 460–1,580 (4.6–15.8)
Mammogram3 13 (0.13)
Lumbar spine series3 180 (1.8)
Thoracic spine series3 140 (1.4)