week 9- 3D methods in CT Flashcards
Why use 3D imaging?
The anatomy is 3D
The anatomy of interest does not always lie in the axial scanned plane
3D models can be used to more accurately show anatomy or pathology shape and size
Readily accessible
What are some uses of 3D imaging
Provides a more representative view of the anatomy
Virtual colonoscopy / endoscopy
Aids in the diagnosis of vascular pathology
Can be used to plan surgery
Can sometimes demonstrate pathology that is difficult to appreciate otherwise
What is the process of 3D imaging?
- Data acquisition: scan
- Creation of 3D space: voxel classification
- Processing for 3D display
- 3D image display
what does processing do?
components: segmentation and thresholding
Segmentation: identifies structures of interest
Thresholding: method of classifying tissue types such as fat, bone, soft tissue, includes MIP, final step is rendering
What are some types of 3D imaging
Multiplanar reformats (MPR)
Volume Rendered
Maximum Intensity projections (MIP)
Minimum Intensity projections (MinIP)
Surface shaded
Virtual Reality
what are scanning requirements for 3D imaging?
Isotropic voxels
To obtain a Z dimension similar to the pixel size, thin slices are required
Vendors have a number of ways of achieving this
Ideally 0.5mm slice thicknesses or less (for isotropic)
What are characteristics of MPRs
Simplest method
Acquires an image by defining a plane through the CT stack and displays those pixels in plane
MPR can be orthogonal giving usually sagittal or coronal MPR images
MPR can be any oblique plane
MPR can be a curved plane: can be used to follow the curve of the spine or a blood vessel
what are some characteristics of volume rendering
Utilises dataset from 3D space
Uses a method of “ray casting”
Enables ability to see surface and internal structures
The image can be positioned at any angle to the data set
Every pixel on the image will have its own ray
The ray will pass through multiple voxels
what are characteristics of MIPs
volume rendered images
will select the voxel on the ray that has the highest HU and then “put” that value on to the image
High CT value voxels are displayed in preference to soft tissue voxels
MIP images do not provide depth information
Loose clear delineation of different structures
what are characteristics of MinIP
volume rendered images
will select the voxel on the ray that has a low HU
Can use threshold values
Displays soft tissue voxels
do not provide depth information
Excellent for demonstrating lung or air filled structures
Looking at low HU
what is surface rendering?
- VR technique uses ray casting
- Surface rendering is an interface (steep difference in HU)
This surface can be detected and displayed in the image
what is volumetric shading
Similar to surface rendering
Rather than a ray passing through the object it is a ray “reflected” off a surface
Good for skin or bone surfaces
Can set the HU or a range of HU values for the surface for the light to be “reflected” off
Can select multiple ranges
Can give the different surfaces different colours
what is transparency visualisation
Allows the viewing of both surface and internal structures at the same time
what is 4D angio
Shows bone, soft tissue, and blood vessels at the same time to allows viewing of tortuous vessels with respect to bone
The 4th D is time
what is disarticulation
This shaded surface display technique allows the viewer to enhance the visualisation of certain structures by removing others
I.e. fractured GH; can remove humerus to better view the joint itself
