Download Physics of CT

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
Points to be covered

Image quality

Noise, contrast, spatial resolution.

Radiation dose from different CT scans

Artifacts

Advances in CT
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
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
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.

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
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.
Contrast resolution
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)

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.
2. Radiation dose
U α Z3/E3
Parameter
80KV
120KV
140KV
Contrast
Noise
Penetration
Patient dose
Best
Most
Least
highest
Intermediate
Average
Average
intermediate
Poor
Least
Most
lowest
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
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
Examination
Typical effective dose
(msv)
(millirem)
Chest X-ray
0.1
10
Head CT
1.5
150
Screening mammography
3
300
Abdomen CT
5.3
530
Chest CT
5.8
580
CT colonography (virtual colonoscopy)
3.6–8.8
360–880
9.9
990
Cardiac CT angiogram
6.7-13
670–1300
Barium enema
15
1500
Neonatal abdominal CT
20
2000
Chest, abdomen & pelvis
Effective dose to the patient from different CT scans
Organ Dose
Dose to pregnant
 Head CT
1mGy (1mSv) worker.
 Thyroid - 1.9 mGy
 Eye lens - 40 mGy
 Chest CT
5mGy (5mSv ), 500
mRem)
 Breast - 21 mGy
 Abdomen CT
 Uterus – 8 mGy = 8mSv
 Gonads - 8 mGy
 Pelvis CT
 Uterus – 26 mGy
 Gonads - 23 mGy
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
a. Metallic artifact
X-ray beams pass through metal implants are
highly attenuated and detector detects no
transmission, streaks.
b. Motion artifact

Blurring , image ghosting, failure to correct u of
moving object during reconstruction ;streaks.
c. Noise artifact
Graining on image caused by a low SNR; thin slice
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
d. Partial volume effect
Floor of ant cranial fossa, as frontal bone is
irregular (bone and brain) DDx hge (bright)
e. Ring artifact
one or many "rings" appears within an image. due to
a detector fault.
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
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 2. 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
b. Multiple slice /
detector arrays
(MDCT)
(MSCT)
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
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.
Thus functional imaging
obtained by PET can be
correlated with anatomic
imaging obtained by CT.
a
b
PET CT images show metastases to left supraclavicular
lymph node a and to liver b
1. Mention the main factors that affect quality of CT
image?
2. Enumerate the factors that improve CT image
resolution?
3. What are the factors that help in reducing patient
dose during CT imaging?
4. Mention the factors that reduce noise in CT
image?