Lecture 2 - Foundations of Image Analysis

ONCOL 306 - Imaging. University of Alberta

Image registration

the process of aligning two images to each other

what treatment techniques mainly use image registration

IGRT, multi-modality treatment planning, and adaptive treatment planning

image fusion

the combined display of two images after they have been registered to each other

what are three types of image registration

1. rigid registration

2. affine registration

3. deformable registration

rigid registration

a best-fit registration where image are not modified in order to match each other

• shapes are rigidly maintained, so translation and rotation of image occur as a whole

affine registration

modifications change the distance between points in an image, but do not change the relationship between the lines

• translation and rotation occur but there will be some scaling, sheering and plane reflection

deformable registration

a complex registration process where one or more objects are reshaped in order to accurately align with each other

• this can correct for organ motion as we deform one image to match another

what is the purpose of IGRT

to use in-room imaging before the radiation treatment in order to improve the precision of radiation delivery

• these images are not used to plan treatments, just to check set-up

4 pros of IGRT

1. improve accuracy of treatment

2. allows for identification and correction of set-up discrepencies

3. allows for assessment of anatomical or tumor changes to aid replanning

4. decreases margin size resulting in less patient side effects

what is image matching?

the process of aligning the treatment image to the DRR in order to localize the treatment location

what happens once the images are aligned after matching?

shift information is used to move the treatment couch, putting the patient in the correct location relative to the machine isocenter

what does the RT need to be able to see to perform a match well, what can be used to help this?

the RT must be able to see key structures clearly (thus we often use kV images for better contrast)

• image filters can be used to emphasize certain objects or structures that may be poorly visable on the standard view setting

• we can only apply one image filter at a time

what types of structures can help the RT make an optimal match

bones, organs, fiducials, or foreign objects

what is a primary match structure and give an example

structures that best represent the treatment target or close proximity to the target

• in prostate set-ups, we can either use the prostate itself or the pubic symphysis

what is the best primary match structure?

the best match structure is the tumor or target itself, if this is not visable, we look for surrounding anatomical landmarks that can provide a reliable reference for alignment.

what is a secondary match structure and give an example

structures that help with the image match but alone don’t accurately represent the target, helps confirm match

• when matching prostate, a secondary match structure may be the pelvic brim

what is a manual-match

the imager software enables the RT to perform a matching of the treatment image to the DRR

what is an auto-match

the imager software performs a match and attempts to best match all anatomy in the image

what is an ROI (region of interest)?

the area to be searched during a 2D auto match

• referred to as a VOI (volume of interest) in a CBCT automatch

is it better to have a large ROI or smaller?

the smaller the ROI focuses the automatch algorithm and speeds up the automatch

• we don’t want to the computer to prioritize non-relevent information (ex: exclude head of femurs from prostate match)

what are the three P’s that an RT needs to understand to have the best possible image match

• patient

  • diagnosis and staging will change the match: late stage prostate will need to account for nodes

• plan

  • understand the technique, parameters, field and targets

• protocols

  • department information on sequences of imaging, frequency of imaging, etc

give a general workflow of best-practice image guidance

• review all patient and treatment related documentation

• review prior images (if applicable)

• determine imaging parameters

• acquire quality images

• assess anatomy

• assess treatment positioning

• perform image match (reference image) or image assessment (BEV)

• initiate corrective action (adjust set-up, perform shifts, other)

what are the three types of patient rotation (non-translational adjustments)?

• pitch

• roll

• yaw

describe pitch

the rotational movement about the lateral axis, causing the patient to tip anteriorly or posteriorly

describe roll

patient rotation about the longitudinal axis which causes the patient to tilt side to side.

describe yaw

patient rotation about the ant/post axis causing the patient to turn left or right.

unless the treatment couch is 6DOF, how must patient rotations be adjusted

by adjusting the patient's position using additional supports or manual repositioning.

what are translational adjustments

Adjustments made to the patient's position along the x, y, or z axes to achieve the desired treatment alignment.

• we can move the couch, not the patient to achieve precise positioning

what determines if the patient/couch needs to be shifted to ensure the treatment area is correctly placed within the path of the beam

the image match

what is Online Set-up Correction?

set-up correction done in real time with the patient on the treatment couch

• will reduce both systematic and random errors through real-time matching and applying bed shift corrections within a treatment fractions

what two action levels are relevant to online set-up corrections

shift action level and image repeat action level

what is the shift action level (SAL)

a couch shift threshold for an image match at which the bed shifts should be applied

what is the value of an SAL

commonly half of the PTV margin

• if PTV is 1 cm, SAL is 5 mm

what is the image repeat action level

a couch shift threshold for an image match at which bed shifts should be applied and subsequently verified with a repeat image

what is the value for an image repeat action level

typically equal to the PTV margin

• if PTV is 1 cm, image repeat action level is 1 cm

what is Offline Set-up Correction

a method of set-up error correction done with the patient off the treatment couch (in-between treatment fractions)

• we evaluate a series of shifts we have done for the patient to see if we can make an adjustment somewhere: if we are always moving the patient 1 cm superior, we should just have the patient move 1 cm sup in the field before we image (move from tattoo)

what errors does offline set-up correction account for? what does it not account for

offline set-up correction reduces systematic errors by evaluating set-up errors and applying a correction to a subsequent fraction. however, random set-up error is not accounted for

What is a No-action level protocol (NAL)

NAL is an offline set-up correction strategy

• patient set-up errors are averaged over a predetermined number of fractions and corrected for

• the mean shift is calculate and applied to the patient by remarking the isocenter

what is systematic set-up error?

set-up deviations that occur in the same direction and magnitude throughout the treatment course of RT

• will remain unchanged over the course of treatment for a given patient

systematic set-up error is defined as the ___ of set-up errors and results in a ___ of dose from the treatment plan

mean value of set-up errors, results in a shift of dose from the treatment plan

give an example of systematic set-up error

After CT Sim images with BBs, they must be removed and a tattoo is placed instead. this placement may not be completely perfect

another example: perfect bladder fullness and empty rectum in CT Sim will never be accomplished 100% during image matching

what is random set-up error?

set-up deviaitons that can vary in direction and magnitude

• less serious than systematic set-up error but PTV will still get underdosed

random set-up error is defined as the_____ of set-up errors, and results in a _____ of dose in relation to the treatment plan

standard deviation of set-up errors, resulting in a blurring of dose (in a non-constant direction)

what is residual set-up error

the remaining error after performing a correction