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In our previous blog we have seen about generations of ct scanners.
Please check out for the link below
Today’s blog we are going to see about ct technology and artifacts in ct scanners .
Invention of Computed Tomography is a revolution in the healthcare sector. The contribution of Computed Tomography in diagnostic and therapeutic fields in immense. Therefore it is important for every healthcare professional to know in detail about the evolution of CT scanners.
Let’s get into the topic.
INTRODUCTION
Computed Tomography was introduced by G.N. Hounsfield
at Central Research Laboratories. The idea of computed tomography raised when
G.N. Hounsfield was trying to identify what is inside a box by taking X rays at
various angles around a box. Computed Tomography was commercialized in 1972.
WHY COMPUTED
TOMOGRAPHY?
Computed Tomography provides 3 Dimensional cross
sectional images of internal organs and structures. Computed Tomography gives
three dimensional informational on a single plane. It is used to view images of
internal organs, bones, soft tissues and blood vessels. Gives information on
the size and location of organs.
WHAT IS THE
TECHNOLOGY OF
COMPUTED TOMOGRAPHY?
The ct technology is a updated version of X ray technology. In a conventional X ray machine uses a fixed X ray tube
that send X ray beams in only one direction whereas in ct scanners, it uses a
motorized X ray source that shoots narrow beams of X ray as it rotates around
the patient. Special X ray detectors are located exactly opposite to the motorized
X ray source. When X ray beams pass through the patient, it is captured by the
detectors and send to the processing unit (Computer). The image slices can be
displayed individually in a 2 Dimensional form or they can be stacked together to
form 3 Dimensional image for examination.
HOW IMAGES ARE ACQUIRED?
In ct scanners Images are taken in axial or transverse plane perpendicular to long axis of the body. The detectors of ct scanners measure the transmission of a thin beam of X rays through a full scan of the body. The images of the section is taken from different angles and allows to retrieve the information about the third dimension, depth. Visual representation of the raw data obtained is called SINOGRAM. Raw data is processed using reconstruction technique.
Pixel is a mean attenuation coefficient of tissues.
Pixel size is based on the matrix size and field of view. It is equal to field
of view divided by matrix size. The matrix size is like 128x, 256x. The smaller
a pixel size, greater the image spatial resolution. Voxels are 3 Dimensional
Pixels and a combination of pixel size and slice thickness, representing a
value in a three dimensional space. Closer a part is to the origin of X ray
source, smaller the voxel and higher the resolution. Further the part is from
the X ray source, larger the voxel size and lower the resolution.
WHAT IS CT NUMBER?
Every region of CT image is expressed in terms of Hounsfield unit (ct number) based on the X ray attenuation. CT numbers are displayed as gray scale pixels on the monitor. White represents the pixels with high ct number (Bone). Shades of gray are assigned to intermediate level of ct number (Soft tissues, fluid and fat). Black region is represented with low ct number (Lungs and air filled organs).
WHAT ARE THE ARTIFACTS
OF COMPUTED
TOMOGRAPHY?
MOTION ARTIFACT
Motion artifact is due to the voluntary or involuntary movement of patients. Images which appear as blurring, shading, streaking are due to patient movement during image acquisition.
BEAM HARDENING ARTIFACT
X ray beam has photons of varying energy levels revolving around a mean beam energy level. In beam hardening artifact, low energy X ray beams are attenuated. Because lower energy beams are absorbed more rapidly than higher energy beams. So only high energy X ray beam passes through the patient. It has high mean beam energy level. As a result, the remaining X ray beam is hardened. It acts similar to a high pass filter.
CUPPING ARTIFACT
The beam hardening artifact produces another type of artifact called cupping artifact. X ray beam passes through the object. If the centre of the object is thickest, then the beam will become harder in the centre than at the periphery. There is a less brighter appearance along the periphery of an object.
STREAKING ARTIFACT
Streaking artifact is a type of beam hardening artifact. In streaking artifact dark streaks can appear between two dense objects in an image. Streaking artifact happens when a portion of a beam passes through one of the objects at certain tube positions is hardened less than when it passes through both objects at other tube positions. This type of artifacts commonly occur near materials like metals or bony regions of the body.
NOTE: The beam hardening artifact can be reduced using filtration technique. A metallic material is used to pre-harden the beam by filtering out the lower energy beams before it passes out the patient.
Bowtie filter is used to harden the edges of the beam, which will pass
through the inner parts of the patient.
PHOTON STARVATION
Photon starvation usually seen in objects with high attenuation. So only insufficient X ray beams reach the detector. As a result during reconstruction process, these regions are magnified leading to the streaks in the image.
The photon starvation can be reduced by using tube current modulation (increased mAs) and adaptive filtration.
PARTIAL VOLUME EFFECT
Tissues of different absorption are encompassed on the same CT Voxel, producing beam attenuation proportional to average value of these tissues. That is each voxel in an image represents more than one tissue types.
Partial volume effect can be minimized by reduction in the volume of voxel.
RING ARTIFACT
Ring artifact is due to the failure of one or more detector elements in a ct scanners. Most commonly seen in cranial CT images. Ring artifacts are concentric rings superimposed on the tomographic images.
Ring artifact can be corrected by calibration or replacing the concerning detector.
NOTE: Dear friends!!! ...Please do comment a topic related to Biomedical, so that we can discuss it in future blogs.
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