X-Ray, CT and PET Scan

Question 1

What is ionising radiation?

Answer 1

Ionising radiation is radiation that causes ionisation (esentially takes or add electrons off and become charged particles) when it interacts with matter

Question 2

Why do we use ionising radiation?

Answer 2

Ionising radiation is used as they can penetrate through the body through different mediums/tissue) and be captured on our imaging detectors

Question 3

What are the two ways ionising radiation can act?

Answer 3

• Indirect action (radiation will break up water and produce free radicals)
• Direct action (directly impact DNA inside a cell)

Question 4

Describe the indirect action of ionising radiation

Answer 4

• Majority of our body is water
• Ionising radiation can break up water and result in the production of free radicals and hydrogen peroxide
  
  • H• OH• HO₂• H₂O₂
• Free radicals are highly reactive and break down DNA

Question 5

Describe the direct action of ionising radiation

Answer 5

Direct impact of the DNA inside our cells

Question 6

Direct effect of radiation damage

Answer 6

• Threshold effect (very high level of radiation exposure/ doses needed, beyond the threshold level, in order to see those effects)
  
  • E.g Erythema and hair loss

Question 7

What are the indirect effects of radiation damage?

Answer 7

• Indirect effects can increase risk of the cancer induction
• Risk of genetic change in subsequent population
• Effect is proportional to radiation dose, no threshold
  
  • All radiation has risk (the higher the radiation the higher the risk that radiation will cause cancer)

Question 8

What are positrons?

Answer 8

Posotively charged electrons which interact with matter to create gamma rays

Question 9

How are positrons formed?

Answer 9

They are emitted following the radioactive decay of an unstable nucleus

Question 10

What are positrons used in

Answer 10

PET scanning (Positron Emission Tomography)

Question 11

What are gamma rays

Answer 11

Are a type of penetrating electromagnetic radiation emitted following the radioactive decay of an unstable nucleus

Question 12

What are gamma rays used in?

Answer 12

Gamma camera imaging e.g SPECT

Question 13

What are X-rays?

Answer 13

Spectrum of electcomagnetic radiation

Question 14

How are X-rays produced?

Answer 14

They are produced artificially in an X-ray tube

Question 15

What are X-rays used in?

Answer 15

X-ray imagine

CT and radiographs

Question 16

What does X-ray attenuation mean?

Answer 16

As the high-energy photons pass through material, their energy is decreasing

Essentially there is a reduction in the intensity of an X-ray beam as it traverses matter

Question 17

What factors affect attenuation?

Answer 17

Attenuation increases with

• Higher atomic number
• Higher density

Question 18

What is used for transmission imaging and what is used for emission imaging?

Answer 18

Transmission Imaging → X-Ray

Emission Imaging → Gamma Rays

Question 19

What is transmission imaging?

Answer 19

• A patient will lie between an X-ray tube and detector
• Radiation will be directed through the patient
• A transmission map is collected (attenuation map)
  
  • Maps all the tissues that either block the signal or not
• Good at showing structure, especially between tissues of different densities or atomic number

Question 20

Describe emission imaging?

Answer 20

• Radiation is administered to the patient in the form of a tracer (radioactive material injected)
• Patient will emit radiation (e.g gamma rays)
• These will be detected outside the patient

Question 21

What is an X-Ray tube composed of?

Answer 21

• Filament, Target (anode), tube (evacuated), Voltage difference between two terminals (filament and target) to accelarate electrons

Question 22

How does an X-ray work? How are the X-rays produced?

Answer 22

1. A current will flow through a heating circuit which will cause electrons to be fired from the filament to the target (anode)
2. The electrons will accelerate through the voltage and hit the target (anode)
3. Upon hitting the target (metal tungsten), those electrons will produce a beam of X-rays

Question 23

How do we have the abillity to control X-rays?

Answer 23

• Operators can control the X-rays by changing the voltage setting
  
  • X-rays are only produced when the tube is in action (can be switched on/off)
• High voltage and current can be used to control the amount and energy of X-rays produced

Question 24

What is the effect of X-ray on bone?

Answer 24

Bone has a high atomic number and high density so therefore blocks most of the X-ray signal (high attenuation) hence appears white

Soft tissue has a lower atomic number and lower density so therefore allows more X-rays through = blacker appearance

Question 25

Explain some uses of X-rays

Answer 25

* Fractures * Dental radiology * Chest X-ray (look for pulmonary embolisms) * Mammography = X-ray of breast tissue * Uses a compression plate to reduce breast thickness. Thinner breast tissue = better resolution * Used to detect benign micro-calcifications, tumours

Question 26

What is fluoroscopy?

Answer 26

Real-time X-ray A method of viewing the X-ray directly in **real time** (moving images of the interior of an object). e.g pumping of the heart, motion of swallowing Can be used for coronary angiographies to identify areas of stenosis and occlusion

Question 27

How does fluoroscopy work?

Answer 27

1. A catheter will be fed inside an artery for example and radio opaque dye is injected 2. This will show blood flow inside vessels 3. A radiologist will inject dye with a high atomic number int the patient and can be seen more visually using X-rays

Question 28

What are the limitations of planar X-ray?

Answer 28

* Cannot distinguish between overlying tissues * E.g skull and below * Tissues other than those being observed can **reduce contrast** in the image * Historically partially solved by moving the film cassette and X-ray relative to the patient to blur out overlying tissues called 'tomography' * Superseded by Computed Axial Tomography (now abbreviated to CT)

Question 29

Why does CT supersede X-ray?

Answer 29

The CT scan is an X-ray image, but you can decipher all different tissues within the image

Question 30

What are the key elements of a CT scanner?

Answer 30

* X-ray tube * Detectors (transmission image) * Aperture/ Bore (where patient sits) * Fan beam (rotates around the patient) 1. Patient will go into the CT scanner 2. When positioned the X-ray tube will rotate around the patient in order to derive slices through the patient while the patient bed slowly moves through the gantry 3. Has the abillity to show us 3D slices

Question 31

What is the benefit of helical MSCT compaired to helical CT?

Answer 31

* Multi-slice * Faster scan * More coverage for each rotation

Question 32

What is the clinical significance of CT scans?

Answer 32

* **_Acute Diagnosis_** * We can use a CT scan to identify between a haemorrhage/blood clot (drugs for these are very different and can counteract eachother, giving the wrong drug could be detrimental ) * **_Disease Progression_** * Monitor response to therapy * **_Treatment planning for radiotherapy_** * CT is used to define the area to be treated and the direction of radiotherapy beams that are used to minimise damage to normal tissue

Question 33

Describe emission imaging

Answer 33

* Patient will be administered with a radioactive material in the form of a tracer * Emitted radiation (gamma rays) will be detected **outside the patient** by scanner * We are making an image on the distribution of the radioactive tracer. Nuclear medicine **_will only show function_**

Question 34

What does emission imaging depend on?

Answer 34

Metabolism of the tracer (not on anatomy) FUNCTIONAL IMAGING

Question 35

What are the types of emission imaging?

Answer 35

Gamma Camera and PET (positron emission tomography)

Question 36

What is the difference between a gamma camera and PET?

Answer 36

* **Gamma camera** = uses single photon emitting radionuclides * Can operate ibn 2D or 3D (SPECT) * **PET** = uses positron emitting radionuclides * Always 3D

Question 37

What has to be considered when injecting a radioactive tracer?

Answer 37

When we inject a radioactive tracer we have to consider the **radioactive decay** The tracer will decay according to its **HALF LIFE.** Half life is the time it takes for the radioactivity to be reduced to 50%

Question 38

What is the most common radionuclide tracer used in gamma camera imagine?

Answer 38

**Tc-99m (T½ = 6 hours)** Radioactive tracers will be coupled to a **biological marker** to asses the function of specified organs * Tc-99m MDP (bone scans) * Tc-99m DTPA (kidneys) * Tc-99m White Cells (infection/inflammation)

Question 39

Gamma camera in cancer

Answer 39

* Cancer has a **higher metabolic rate** than normal tissues therefore they will take up more of the tracer * This allows the radiation to be concentrated in them areas of cancer * Allows us to **image function** and not anatomy

Question 40

How us gamma camera imaging used to asses kidney function?

Answer 40

1. Camera is positioned above the patient 2. Tc-99m DTPA injected IV 3. Gamma camera records gamma rays and collects images over time 4. Functional Time-Activity curves are obtained * Compared to normal kidney functioning rates

Question 41

What is SPECT?

Answer 41

**Single Photon Emission Computed Tomography** * **3D version** of gamma camera imaging * Images are taken at different projection angles around the patient (similar concept to CT) but there is delivery of a radionuclide * Images of high contrast are obtained between layers that are not obscured by all the overlying tissues

Question 42

How does a PET (B+ particle radiation) scan work?

Answer 42

* Tracer will be injected into patient which **releases positrons** * Positrons and electrons will **interact** and are annihilated, **producing two gamma rays** at 180º to each other * **PET Scanner** detects the gamma rays in a short time

Question 43

What is PET used in?

Answer 43

* Brain imaging (e.g neurodegenerative dementia) * FDG (tracer) reflects metabolic activity

Question 44

How is flurodeoxyglucose used in PET?

Answer 44

* FDG is a **glucose analogue,** * concentrations of the tracer imaged will indicate tissue metabolic activity * can be used to explore **cancer metastasis** * FDG will be metabolically trapped because it does not undergo glycolysis like glucose

Question 45

Application of FDG is neurodegenerative dementias

Answer 45

* FDG will reflect metabolic activity * Alzheimers disese (hypometabolism, mostly in temporal and parietal regions * Picks disease (fronto-temporal hypometabolism)

Question 46

What are examples of hybrid imaging?

Answer 46

**PET**-_CT_ **SPECT**-_CT_ **PET**-_MR_ Combination of different scanners will allow you to get the combination of **functional** and _anatomical_ elements

Question 47

How does a PET-CT scan work?

Answer 47

1. Patient will go into the bore 2. Undergo transmission imaging from the CT 3. Patient injected with radioactive tracer (positrons will annihilate with bodily electtrons giving off two gamma rays detected by PET scanner)

Question 48

How does attenuation correction occur in PET-CT?

Answer 48

* Gamma rays originating frm the centre of the patient will travel through more tissue which means they are attenuated more * The CT image is used as an attenuation map to correct the PET image