Image registration 2 Flashcards
What is the difference between a T2 and T1 weighted image?
T1 weighted images (spin-lattice or longitudinal relaxation time) • Depicts anatomy well (low sensitivity for disease without contrast) FAT IS BRIGHT
– T2 weighted images (spin-spin or transverse relaxation time) • Depicts diseases with high water content well (low specificity though) FLUID IS BRIGHT
(PENILE BULB CONTOURING, high grade glioma)
Define MRI?
Magnetic resonance imaging (MRI) is a technique that uses a magnetic field and radio waves to create detailed images of the organs and tissues within your body
What is Spin-Echo?
↑Spatial resolution and reduces artifacts caused by magnetic field inhomogeneities •
fast/turbo SE preferred for RT – much quicker scan; reduces motion artifact
-Good for identifying gold seeds in prostate
What colour is fat on a T2 weighted FSE?
fat is bright a T2 fast spin echo (FSE) image
What is gradient echo?
short acquisition time, minimises motion artifacts, ↓spatial resolution; used for multi-phased imaging, oxygenation levels
Limitations of MRI?
- Image distortion due to non-uniformities in the magnetic field
- Image quality dependence on coils – Can be a problem for RT imaging requirements
- Poor imaging of cortical bone
Define PET?
Pharmaceuticals attached to positron emitting radioisotopes which interact to produce two 511kev photons
What is the radio nuclide used in PET?
fluoro-D-glucose (18FDG)
What has a high uptake in PET?
Brain, bladder and bowel (some examples)
Limitations of PET?
- High uptake in benign pathologies
- Image resolution much poorer than CT – How accurately can we mark up tumour volumes?
- PET is a metabolic image
What are examples of benign pathologies that can have a high uptake in PET?
Inflammation • Abscess • Sinusitis • Acute pancreatitis • Healing bone • Osteon-arthritis. • Tuberculosis. • Sarcoidosis.
Why does PET have poor image resolution?
Due to the large voxel size
Images appear pixelated compare to CT
What is the value used to quantify uptake in PET?
Standardised uptake values (SUVs)
What are the image registration techniques?
- Manual – Rigid registration
- Automated – Rigid – Deformable
What is (manual) rigid registration? What does it require?
Involves manually aligning image features
- The same images to have the same voxel and spatial resolution
- Assumes the distance between two points of the body are the same independent of imaging modality
What is semi-automated and automated rigid image registration ?
• Use of automatic translational functions to match landmarks
Where as automated rigid uses algorithms
What are limitations of CONTOUR semi automated rigid image registration?
highly dependant on surface identification which in turn is dependant on the user and the image quality to define the surfaces
-There are multiple symmetries multiple matches are possible (these may be incorrect)
What are the limitations of semi-automated AMATOMICAL registration/Fiducial marker registration?
identification of the points is user dependant – however the more points used the more any errors in point definition are averaged out
What is Automated rigid image registration ?
Algorithm completes registration using raw image data (uses voxel intensities of both image sets) thus no need for user to identify matching points.
(this reduces error)
What are the two most common automated rigid image registration algorithms used
- Cross correlation
* Mutual information
what is cross correlation?
-registering images with similar information
-Best used for intra-modality images (CT-CT or MRI-MRI registration)
(BUT Can be used for CT - MRI)
What is mutual information?
- registering images with dissimilar information
- Most commonly used for CT-PET or MRI-PET registration
can be used for all imaging modalities
What is the most used automated algorithm?
Mutual information- As cross correlation relies on the relationship between the grey scales of two separate images. WHILE, mutual information is based on the probability that features represent the same point within the patient.
A
What does all image registration require and what are the two validation methods supported in the literature?
-Fiducial markers • Visual inspection
ALL require visual analysis
What is Deformable image registration?
Image warping:
Used when distance between two points of the body are not the same (due to different patient positioning).
