Functional Imaging

Question 1

What is functional imaging?

Answer 1

Maps (in 3D) the DISTRIBUTION of the tumour, tissue or functional features.
Provides information about the CLINICAL RESPONSE of tumours or healthy tissues to ionising radiation.

Includes: SPECT, PET, MRI

Question 2

What are different types of MRI functional images?

Answer 2

Magnetic Resonance Spectroscopy Imaging (MRSI)
Dynamic Contrast Enhanced MRI (DCE-MRI)
Diffusion weighted MRI (DW-MRI)
Fast pulse sequences 
Hyperpolarisation

Question 3

How does nuclear medicine work?

Answer 3

Inject the patient with gamma emitting radiotracer. Then detect where the radiotracer goes in the body (measure the distribution of radionuclide in the body) - using a GAMMA camera (for SPECT).
The distribution of radionuclide should correlate with biological processes.

Question 4

What is SPECT?

Answer 4

Single Photon Emission Computed Tomography
(Shows function of organs)

Allows a 3D distribution of radionuclide to be reconstructed from multiple 2D projections (using cameras)

Attenuation of gamma rays within the patient, gamma ray scatter and the detector response can impact and degrade the projection data but we are able to correct for that.

Using: filtered backprojection attenuation correction (CT).

Question 5

What is PET how does it work?

Answer 5

Positron Emission Tomography.
Fluoride (18) decays by emitting positrons.
Postrons annihilate (collide) with electrons and this creates 2 annihilation photons.
Photon energy is equal to 511keV each. (180 degrees to each other).

Detection of two photons in COINCIDENCE allows one to reconstruct a tomographic image.

Question 6

What is a TRUE coincidence?

Answer 6

Coincidences simultaneously detected on both detectors resulting from the same annihilation of a positron and corresponding to the 511 keV energy photons not having undergone any scatter.

The true signal that one wants to detect.

Question 7

What is a RANDOM coincidence?

Answer 7

Photons emitted by different annihilations but detected in the same time window

Background noise

Reduced by lowering coincidence time window

Question 8

What is SCATTERED radiation (in relation to PET)?

Answer 8

Photons from the same annihilation
Due to scattering the assumption that the annihilation
took place on a line joining the two detection points is incorrect

Energy of scattered photon will be lower than 511 keV

Question 9

What is SUV

Answer 9

Standardised Uptake Value.
Definition: an index of tracer uptake that can be compared between subjects.
SUV = TU/ Q
TU is the tumour uptake from the image
Q is the injected dose per unit mass of the patient

Question 10

What are the different radionuclides used in PET and their function?

Answer 10

O(15) - has a half life of 2 minutes. Tracer: Water, used for cerebral blood flow.

C(11) - Half life: 20 minutes, Tracer: Methionine, used for Tumour protein synthesis.

N(13) - Half-life: 10 minutes. Tracer: Ammonia, used for: Myocardial blood flow.

F(18) - Half-life: 110 minutes, Tracer: Fluoro-deoxyglucose, used for: Glucose metabolism

Question 11

What are applications of PET/CT in radiation therapy?

Answer 11

Non-small cell lung cancer: PET allowed for more accurrate target volume delineation and tumour staging. Revealed unsuspecting nodal disease.

Hypoxia and glucose metabolism: can show better correlation of tumour hypoxia (important as hypoxic cells have increased x3 radio and chemo resistance). Also shows glucose metabolism.

Breast Cancer: Can detect bone and supraclavicular lymph node metastasis.

Question 12

What is DCE-MRI?

Answer 12

Dynamic Contrast Enhanced MRI.
Sequential imaging Following the injection of a suitable para-magnetic contrast agent (Gd based).
Image the uptake and washout the contrast agent.

Question 13

What is DW-MRI?

Answer 13

Diffusion weighted MRI.
Image contrast that correlates with differences in tissue-water mobility

Measures treatment response by looking at changes in water molecules (ADC - apprarent diffusion co-efficient) due to cell swelling, tumour lysis and necrosis.

Question 14

What is MRSI?

Answer 14

Magnetic Resonance Spectroscopic Imaging

Question 15

What are applications of functional MRI in radiotherapy (Brain)?

Answer 15

Used to assist with GTV and CTV delineation.
fOAR (functional OAR) for conformal avoidance
Measurement of response to therapy (using diffusion maps)

Question 16

What are applications of functional MRI in radiotherapy (Lung)?

Answer 16

Fast image sequences can give high spatial and temporal resolution. Up to 10 images per second.

Can see tumour mobility (from dynamic imaging).

Can see effects of motion on dosimetry using probability density function (PDF).

Hyperpolarised He-3 to determine lung function (allows you to avoid functional lung during planning).

Measurement of response to radiation (Tumour and normal tissue response, also allows for ADAPTIVE RT during treatment).

Question 17

What are applications of functional MRI in radiotherapy (Prostate)?

Answer 17

MRI is a superior imaging modality for the prostate.
Used for determining extent of disease and contouring.

Multiple combinations of MRI sequences (T2, DCE, DW and MRS all have a role in improving cancer detection and mapping).

Blood Oxygen Level Dependent (BOLD) sequences for measuring hypoxia.

Question 18

Advantages and Disadvantages of Functional Imaging in RT?

Answer 18

Advantages:

• Used for better staging, planning and treatment response.
• Non-ionising radiation - no dose constraints.
• allows for real time adaptive radiotherapy. (especially with MRI linac).

Limitations: Longer acquisition time than CT.
Still need CT for electron density information.

Question 19

What are problems with CT based Anatomical Imaging?

Answer 19

Staging:
- CT has limited spatial resolution and sensitivity. Often cant resolve metastatic disease or nodal involvement.

Treatment Planning:

• CTV delineation not always accurate
• Assumes uniform radiosensitivity
• OAR contouring: function usually non considered and assumed to be uniform.
• Plan optimisation.

Treatment Response:

• Where was the dose actually delivered?
• Doesnt really give response of disease and normal tissues both during and post-treatment.