Part 8: Different types of scanners

The CT scanner in tx planning.

1. What does it do?

2. Year?

3. Which body region was initially the only one that could be scanned?

1. Images 360 cross-section of the body

2. 1972

3. head region

Typically, the patient must be in their treatment position for CT contour. What is an exception to this rule?

give examples.

If a limb has to be abducted out of the tx field because the CT bore is too small to extend the limb.

e.g. breast tx

e.g. hodgkin’s mantel field

What is the shape of the table of a typical CT unit? What adjustments do we need.

Concave

A table attachment to make the table flat

CT is useful in 2 aspects of tx planning. List them.

1. providing quantitative data for tissue heterogeneity

   Translation: providing large amounts of data about different densities of tissue

2. Delineation of target volume and surrounding normal structures (localization)

In CT the image scale must be accurate in which dimension?

X and Y dimensions

Spiral CT pros

Produces images faster = ↓ patient movement artifacts

MRI

1. aka

2. Year

1. MR

2. 1980s

How does MRI work?

Turns body’s atoms into radio transmitters that come together to form an image

Planes in which the following machines form images

1. CT

2. MRI

1. Axial (transverse)

2. transverse, sagittal, coronal and obliques

Advantages of MRI vs CT

1. Wider variety of views (i.e. planes)

2. no ionizing radiation

3. higher contrast

4. better imaging of soft tissues

the ability to record small images that are close together as separate images

resolution

Disadvantages of MRI compared to CT

1. lower resolution

2. Not as good for bones and calcifications

3. longer scan times

4. smaller magnet hole (bore)

5. fewer contrast agents (used to be none)

provides cross-sectional info of internal structures in relation to contour. These images are generally not nearly as good as CT or MR (or even IVP).

Transverse Tomography

Tomographic image quality

not as good as CT or MRI

Ultrasound

1. Image quality

1. Poor (compared to MRI or CT)

Ultrasound Advantages

1. no ionizing radiation

2. lower cost

Ultrasound uses in RTT

1. obtaining chest wall depth

2. Localization of:

   1. Breast

   2. Abdomen

   3. Retroperitoneum(behind

   4. Scar Boosts

Chest wall depth?

Why would be want to know it?

Form skin surface to lungs

varies from person to person

For a mastectomy patient to avoid treating lungs

Ultrasound frequencies used

1-20 MHz

how does ultrasound work.

1. How are the images produced?

2. How does it differentiate between tissues?

1. Produces images by transmission/ reflection (usually echo reflection)

2. Reflections (echos) are caused by variations in acoustic impedance (bc some materials block more sound waves than others)

impedance mismatch

↑ difference in Z# between 2 tissues = ↑ impedance mismatch = ↑ contrast (better image)

The highest mismatches occur at interfaces (junction) of:

1. air/tissue

2. tissue/bone

3. chest wall/ lung

The physics effect responsible for the production of ultrasound images

Piezoelectric Effect

In a nutshell, what is the piezoelectric effect

A transducer converts electrical energy into ultrasound and vice-versa

PET stands for

Positron Emission Tomography

How do PET scans work?

1. Patient receives a radionuclide that resembles a substance found in the body (e.g. sugar)

2. Positrons (anti-matter: e+) collide with nearby electrons to produce annihilation rxn (matter/anit-matter rxb)

3. Gamma rays produced by annihilation rxn are emitted from the patient’s body

4. The gamma rays produce the images seen in scans

Common Radionuclides

fuck.cn.org

Flourine-18 (HCC)

Carbon-11

Nitrogen-13

Oxygen-15

Rubidium-82

Gallium-68

What can a PET scan image?

Structure AND function

Mental function and psychiatric abnormalities

(e.g. brain: taken up in areas that consume sugar)

What produces the radionuclides used for PET scans?

Cyclotrons

PET/CT fusion

1. aka

2. What does it do?

1. image fusion

2. forms images with the “best of both words” Structure AND function seen together = more dynamic view

Nuclear Medicine Scanning

What does SPECT stand for

Single-photon emission computed tomography

BEV stand for

Beam’s Eye View

What is BEV?

1. A tx display in which tx plan is viewed from the POV of the radiation source

2. Images reconstructed from CT data that represent the patient’s anatomy and defined volumes from the POV of the tx beam