ct image reconstruction hounsfield envisioned dividing a slice into a matrix of 3-dimensional...
TRANSCRIPT
COMPUTED TOMOGRAPHY I –
RAD 365
CT - SCAN
Prepared By:Ala’a Ali Tayem Abed
CT Image Reconstruction
CT IMAGE RECONSTRUCTION
Hounsfield envisioned dividing a slice into a matrix of 3-dimensional rectangular boxes (voxels) of material (tissue). Conventionally, the X and Y directions are within the plane of the slice, whereas the Z direction is along the axis of the subject (slice thickness direction). The Z dimension of the voxels corresponds to the slice thickness. The X and Y voxel dimensions, depend on the size of the area over which the x-ray measurements are obtained as well as on the size of the matrix (the number of rows and columns) into which the slice is imagined to be divided.
The objective of CT image reconstruction is to determine how much attenuation of the narrow x-ray beam occurs in each voxel of the reconstruction matrix. These calculated attenuation values are then represented as gray levels in a dimensional image of the slice.
CT Image Reconstruction
The Radon Transform is widely applicable to Tomography, the creation of an image from
the Projection Data associated with cross-sectional scans of an object.
The Radon Transform represents the projection data obtained as the output of a
Tomographic scan. Hence the Inverse of the Radon Transform can be used to reconstruct
the original density from the Projection Data, and thus it forms the mathematical
underpinning for tomographic reconstruction, also known as Image reconstruction.
Ray, Ray Sum, View & Attenuation Profile Ray – Imaginary line between Tube & Detector. Ray Sum – Attenuation along a Ray. View – The set of Ray Sums.
For example, consider a simple 2-row by 2-column reconstruction Matrix. Views are collected at 4 angles, 0 (left to right), 90 (top to bottom), 45 (diagonal), and 135 (diagonal), and each measurement is expressed as the sum of the voxel attenuation values along each ray. In this case, there are 10 equations:
U1 U2
U3 U4
U1
3U2
4
U3
1U4
8
NoNoNo
No
No
No
45
90
135
o
X1 = U1 + U2
X2 = U3 + U4
X3 = U1 + U3
X4 = U2 + U4
X5 = U2
X6 = U1 + U4
X7 = U3
X8 = U1
X9 = U2 + U3
X10 = U4
ART algorithm for 4-voxel ‘‘patient.’’
Attenuation measurements
U1
3U2
4
U3
1U4
8
11
4
4 12 18
53
7
9
7 7
9 9
11
1
4
18 11
10 20
4 + 7 = 1111 + 7 = 1811 + 9 = 201 + 9 = 10
18 11
10 20
1244 + 18 = 22 4 + 10 = 1412 + 11 = 2312 + 20 = 32
22 23
14 32
25 28
19 40
22 23
14 32
8
35
3 + 22 = 255 + 23 = 285 + 14 = 198 + 32 = 40
7 + 9 = 16 Initial x-ray scan combined
25 - 16 = 928 - 16 = 12 we subtract the total initial x-ray
19 - 16 = 3 combined from the value of each box.
40 - 16 = 24
9 / 3 = 3 because we have the scan not once
12 / 3 = 4 but three times we returned others.
3 / 3 = 124 / 3 = 8
25 28
19 40
9 12
3 24
3 4
1 8
Image ReconstructionThe acquisition of volumetric data using spiral CT means that the images can bepostprocessed in ways appropriate to the clinical situation.Multiplanar Reformatting (MPR) – allows images to be created from the original axial plane in either the coronal, sagittal, or oblique plane.
The three images demonstrate a haemoperitoneum, shatteredright kidney and a lacerated spleen in Axial (A), Sagittal (B) and Coronal (C) Planes.
(A)
(C) (B)
Three Dimensional Displays
3D displays represent a scan volume in a single image which has generally beenmanipulated to enhance a specific characteristic. This can only be done successfullywhen one high-contrast structure (like the skeleton) is to be displayed.
Computed Tomography (CT) Volumetric Rendering Techniques such as:
1. Shaded Surface Displays (SSD).
2. Minimum Intensity Projection (MinIP).
3. Maximum Intensity Projections (MIP).
4. Volume Rendering (VR).
5. Perspective Volume Renderings Technique (pVRT), or Virtual Endoscopy (VE).
6. Curved Plane Reconstructions.
Shaded Surface Displays (SSD).
(A) Coronal Multiplanar volume rendering and (B) minimum intensity projection
(A)Maximum Intensity Projection and (B)Volume Rendered, Images of the Abdomen.
Dental CT reconstructions. By drawing a manual trace on an axial CT scan (A), it is possible to obtain shaded surfaced display volume rendered (B), curved Multiplanar reconstruction
CT IMAGE QUALITYFundamentally, image quality in CT, as in all medical imaging, depends on 4 basic factors: Image Contrast, Spatial Resolution, Image Noise, and Artifacts. Depending on the diagnostic task, these factors interact to determine sensitivity (the ability to perceive low-contrast structures) and the visibility of details.
1. CT Image ContrastCT image contrast depends on Subject Contrast and Display Contrast. Because CT Display Contrast is arbitrary (depending only on the window level and width selected), it will not be discussed further.
As in radiography, CT Subject Contrast is determined by Differential Attenuation: that is, differences in X-Ray Attenuation by Absorption or Scattering in different types of tissue and thus resulting in differences in the Intensity of the X-Rays ultimately reaching the Detectors. Because of the high peak Kilovoltage and relatively high beam filtration (beam hardness) used in CT, the x-ray/tissue interactions (except in bone) are overwhelmingly Compton-scattering events. Differential attenuation for Compton scatter arises from differences in tissue density, which in turn are due primarily to differences in physical density. Thus, subject soft-tissue contrast in CT comes mainly from differences in physical density. That the small differences in soft-tissue density can be visualized on CT is due to the nature of the image.
2. CT Spatial ResolutionSpatial resolution in CT, as in other modalities, is the ability to distinguish small, closely spaced objects on an image.
A. Motion introduce blurring, although the more important effect of motion is the potential creation of artifacts. B. Matrix Size and Pixel Size. (A bigger matrix brings better resolution. When pixel size is smaller creates a sharper image).
3. Image NoiseCT image noise is this associated with the number of x-rays contributing to each detector measurement. To understand how CT technique affects noise, one should imagine how each factor in the technique affects the number of detected x-rays. Examples are as follows:
X-ray Tube Amperage: Changing the mA value changes the beam intensity—and thus the number of x-rays—proportionally. For example, doubling the mA value will double the beam intensity and the number of x-rays detected by each measurement.
Scan (Rotation) Time: Changing the scan time changes the duration of each measurement—and thus the number of detected x-rays—proportionally. Because amperage and scan time similarly affect noise and patient dose, they are usually considered together as mA × s, or mAs.
Slice Thickness: Changing the thickness changes the beam width entering each detector—and thus the number of detected x-rays—approximately proportionally. For example, compared with a slice thickness of 5 mm, a thickness of 10 mm approximately doubles the number of x-rays entering each detector.
Peak Kilovoltage: Increasing the peak Kilovoltage increases the number of x-rays penetrating the patient and reaching the detectors (increasing the Energy of X-Ray Photons). Thus, increasing the Kilovoltage reduces image noise but can (slightly)
reduce subject contrast as well.
4. Image ArtifactsArtifacts may be defined as any structure that is seen on an image but is not representative of the actual Anatomy.
Most types of CT artifacts fall into 1 of 3 categories: 1. Shading Artifacts.2. Ring Artifacts.3. Streak Artifacts.
The most common type of 1. Shading Artifact is Beam-Hardening Effects. Because the attenuation of bone is greater than that of soft tissue, bone caused more beam hardening than an equivalent thickness of soft tissue. The beam-hardening phenomenon results from the increase of mean energy of the x-ray beam when it passes through object, CT numbers of certain structures change and this confuses the reconstruction algorithm.
2. Ring or Partial Ring Artifacts are associated with third-generation scan geometry. Ring artifacts arise from errors, imbalances, calibration drifts, or other measurement inaccuracies in an element of a detector array relative to its neighbors.
3. Streak Artifacts may occur in all scanners. Although arising for many reasons, most are due to inconsistent or bad detector measurements. Factors causing inconsistencies include:
1. Motion (anatomy in different locations during different parts of the scan).
2. Metal (The primary reason that streaks occur from metal objects is because the objects exceed the maximum attenuation value that a CT system can image).
3. Partial -Volume Effects (arise when a voxel contains many types of tissue. It will produce a CT number as an average of all types of tissue. This is the source of partial volume effect and will appear as bands and streaks), The posterior cranial fossa is the most critical region to produce partial volume artifact. Using thinner slice and some computer algorithms can reduce partial volume artifacts.
4. Insufficient X-ray Intensity (leading to high random errors).
5. Malfunctions (tube arcing or system misalignment).
6. Out of Field Artifacts are caused by anatomy that is out side of the selected scan field of view. For example, if the size of a person’s chest is 50 centimeters (cm) and themaximum scan field of view is 40cm, the CT system can only image 40 of the 50 cm.Unfortunately, The "extra" tissues are blocking detectors and attenuating photons. Therefore, streak artifacts occur throughout the entire image.To avoid the artifacts caused by out of view, we need ensure that scan field of view is larger then the object to be scanned.
Beam-hardening artifact caused by unusually severe hardening of x-rays passing though thick Bone.
Ring Artifact
Partial Ring Artifact
Streak Artifacts
Motion ArtifactsMetal Artifacts
Partial Volume Artifact
A. 60 mA, 120 kVp, slice thickness 5 mm B. 440 mA, 120 kVp, slice thickness 5 mm
Insufficient X-ray Intensity Artifact
