explaing about CT artifacts and CT image quality

1. Lavanya s r
1st Year Msc.MIT
KMIO, Bangalore

2.  Image quality refers to the visibility of diagnostically
important structures in the CT image .CT image quality
is influenced by mainly technical parameters.
1. Spatial Resolution.
2. Image contrast.
3. Temporal resolution.
4. Image noise.

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 display more details.

4.  Spatial resolution is the ability to discriminate adjacent
objects.
 It depends on pixel size, focal spot size, detector size,
choice of image reconstruction filters and sampling
frequency.
 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(lp/mm).

5.  Focal spot – Smaller focal spots give a higher resolutions. Thus , the
Spatial Resolution improves.
 Detectors size – if the width of the detector is small, it gives a
higher resolutions. 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.
 FOV – smaller FOV gives a finer resolution.
 Pixel size – smaller pixel sizes gives a finer resolutions .

6.  It is the ability of the CT scanner to display the object with
considerably different in density from surrounding objects.
 Ct contrast is the difference in the HU values between tissues.
 Contrast increases as tube voltage decreases, not affected by
tube current or scan times.
FACTORS AFFECTING CONTRAST RESOLUTION
 Increases in mAs ,improves contrast resolution.
 Decreases in pixel size decreases contrast resolution.
 Increases in slice thickness improves contrast resolutions.
 Decreasing FOV improves contrast resolutions.

7.  Temporal resolution is an indication of CT systems
ability to freeze motion of the scanned object.
 In cardiac CT , it is necessary to image the beating heat
in milliseconds.
 In cardiac CT the temporal resolutions of 3 second
means that a single image is acquired within 3 seconds
of the cardiac cycle.
 Increasing scan speed will improve the temporal
resolution

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

9.  The number of x-ray photons detected per pixel is also often referred
to 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.
FACTORS AFFECTING CT NOISE
 Decreases in pixel size increases the noise.
 Increases mAs, decreases the noise.

10.  Artifacts can degrade image quality, affect the
perceptibility of detail, or even lead to misdiagnosis.
 Artifacts is “ a distortion or error in an image that is
unrelated to the subject being studied”.
 Artifacts arise from a number of sources, including the
patient, inappropriate selection of protocols,
reconstruction process, problems relating to the
equipment such as malfunctions or imperfections.

11.  Artifacts in CT can be classified according to
cause and appearance.
a. Streak
b. Shadings
c. Rings and bands.

12. Appearance Cause
 Streaks
 Shading
 Rings and
bands
o Improper sampling of data, partial volume
averaging, patient motion, metal, beam
hardening , noise, spiral/helical scanning,
mechanical failure.
o Partial volume averaging, beam hardening,
spiral/helical scanning, scatter radiation,
off-focal radiation, incomplete projections.
o Bad detector channels in third –generation
CT scanners.

13. Patient Motion Artifacts
 Patient motion can be voluntary or involuntary.
 Voluntary motion is directly controlled by the patient, such as
swallowing or respiratory motion.
 Involuntary motion is not under the direct control of the patient,
such as head motion with injury, peristalsis, and cardiac motion.
Corrections-
 Positioning aids
 Shorten the scan time

15.  The presence of metal objects in the patient also causes artifacts.
 Metallic materials such as prosthetic devices, dental fillings, surgical
clips, and electrodes gives a streak artifacts.
 The loss of information leads to the appearance of typical star-
shaped streaks.
Corrections
 Increasing the kvp .
 Image reconstructed with modified projections are likely free of
metal artifacts.

17.  Beam hardening refers to an increase in the mean
energy of x-ray beam as it passes through the
patient.
 The radiation beams have different path lengths
through the object.
 This artifacts are corrected by performance of a
polynomial mapping of the measured projections
before the reconstruction.
 Calibration correction also reduce beam hardening
artifacts.

19.  CT numbers are based on the linear attenuation coefficients
for a voxel of tissue.
 Partial volume artifact occurs when tissues of widely different
absorption are encompassed on the same CT voxel producing
a beam attenuation proportional to the average value of these
tissues.
 The latest generation of CT scanners with an associated
reduction in the volume of a voxel has substantially reduced
the occurrence of this artifact.

20.  Occurs in 3rd generation scanner, due to miscalibration of any
one of the detectors.
 The detector will record incorrect data in each angular
position.
 Detectors will record incorrect data in each angular position.
 Detectors towards the center of the detector array contributes
ring artifact that is small in diameter than detector in
periphery.
 REMEDY-
 Detector calibration
 Detector replacement
 Selecting the correct scan field of view