Image registration 2 COPY

Question 1

What is the difference between a T2 and T1 weighted image?

Answer 1

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)

Question 2

Define MRI?

Answer 2

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

Question 3

What is Spin-Echo?

Answer 3

↑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
• also good for anatomy contrast e.g. differentiating bladder and prostate

Question 4

What colour is fat on a T2 weighted FSE?

Answer 4

fat is bright a T2 fast spin echo (FSE) image

Question 5

What is gradient echo?

Answer 5

short acquisition time, minimises motion artifacts, ↓spatial resolution; used for multi-phased imaging, oxygenation levels

Question 6

Limitations of MRI?

Answer 6

• 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

Question 7

Define PET?

Answer 7

Pharmaceuticals attached to positron emitting radioisotopes which interact to produce two 511kev photons

Question 8

What is the radio nuclide used in PET?

Answer 8

fluoro-D-glucose (18FDG)

Question 9

What has a high uptake in PET?

Answer 9

Brain, bladder and bowel (some examples)

Question 10

Limitations of PET?

Answer 10

• High uptake in benign pathologies
• Image resolution much poorer than CT – How accurately can we mark up tumour volumes?
• PET is a metabolic image

Question 11

What are examples of benign pathologies that can have a high uptake in PET?

Answer 11

Inflammation • Abscess • Sinusitis • Acute pancreatitis • Healing bone • Osteon-arthritis. • Tuberculosis. • Sarcoidosis.

Question 12

Why does PET have poor image resolution?

Answer 12

Due to the large voxel size

Images appear pixelated compare to CT

Question 13

What is the value used to quantify uptake in PET?

Answer 13

Standardised uptake values (SUVs)

Question 14

What are the image registration techniques?

Answer 14

• Manual – Rigid registration

- Automated – Rigid – Deformable

Question 15

What is (manual) rigid registration? What does it require?

Answer 15

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

Question 16

What is semi-automated and automated rigid image registration ?

Answer 16

• Use of automatic translational functions to match landmarks

Where as automated rigid uses algorithms

Question 17

What are limitations of CONTOUR semi automated rigid image registration?

Answer 17

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)

Question 18

What are the limitations of semi-automated AMATOMICAL registration/Fiducial marker registration?

Answer 18

identification of the points is user dependant – however the more points used the more any errors in point definition are averaged out

Question 19

What is Automated rigid image registration ?

Answer 19

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)

Question 20

What are the two most common automated rigid image registration algorithms used

Answer 20

• Cross correlation

* Mutual information

Question 21

what is cross correlation?

Answer 21

-registering images with similar information
-Best used for intra-modality images (CT-CT or MRI-MRI registration)
(BUT Can be used for CT - MRI)

Question 22

What is mutual information?

Answer 22

• registering images with dissimilar information
• Most commonly used for CT-PET or MRI-PET registration

can be used for all imaging modalities

This is the most used one because it’s not sensitive to the image modality used. Uses the probability distributions of the grey values in each image (the probability that features from both images represent the same point in the patient)
• Can fail when air-tissue interfaces are present, if you haven’t generally aligned the images

Question 23

What is the most used automated algorithm?

Answer 23

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.

Question 24

What does all image registration require and what are the two validation methods supported in the literature?

Answer 24

-Fiducial markers • Visual inspection

ALL require visual analysis

Question 25

What is Deformable image registration?

Answer 25

Image warping:

Used when distance between two points of the body are not the same (due to different patient positioning).