13. X-Ray CT PET Flashcards Preview

Clinical Pathology > 13. X-Ray CT PET > Flashcards

Flashcards in 13. X-Ray CT PET Deck (33)
Loading flashcards...
1
Q

What is ionising radiation? and why use it?

A

Radiation that causes ionisation when it interacts with matter

Types used for medical imaging are:

  • Gamma rays
  • X-rays

=> Because they penetrate through the body (through the different mediums/tissues) and can be captured on our imaging detectors

2
Q

Describe the two ways in which ionising radiation acts.

A

INDIRECT:

  • the radiation interacts with the water in the body
  • It will split the water molecules into free radicals
  • these free radicals have an impact on the cell

DIRECT:
- the gamma radiation/proton can directly imapct the cell DNA strand and cause breaks, and therefore impact and have an effect on the cell

  • Both these methods may or may not have a biological effects, such as:
    • cell death (necrosis)
    • cell loses ability to regulate cell growth
    • causes genetic mutations
3
Q

What are the two types of radiation damage and risk?

A

DIRECT EFFECT:

  • only at high radiation dose, not noticed at usual diagnostic doses
  • threshold effect
  • e.g. Erythema and hair loss

INDIRECT EFFECT:

  • risk of cancer induction
  • risk of genetic change in subsequent population
  • effect is proportional to radiation dose, no threshold -> all radiation has risk
4
Q

What are the three types of ionising radiation?

A
  • POSITRONS: Positive electrons interact with matter to create gamma rays
  • PET scanning
  • GAMMA RAYS: Penetrating radiation
  • Gamma camera imaging
    e. g. SPECT
  • X-RAYS: Spectrum of electromagnetic radiation
  • X-Ray imaging e.g. radiographs, CT
  • Positrons and gamma rays are emitted following the radioactive decay of an unstable nucleus
  • X-rays are artificially produced in an X‑ray tube
5
Q

How is attenuation important in x-rays?

A
  • Attenuation means to stop
  • X-Rays are essentially an attenuation map
  • Attenuation increases with
    • Higher atomic number
    • Higher density
6
Q

Describe emission and transmission imaging.

A

TRANSMISSION IMAGING: (X-rays)

  • Radiation is directed through the patient
  • A transmission map collected is essentially an attenuation map
  • Good at showing structure, especially between tissues of different densities or atomic number

EMISSION IMAGING:

  • The radiation is administered to a patient in the form of a tracer (patient is injected)
  • Emitted radiation is detected outside the patient
7
Q

How does an x-ray tube work?

A

There are some key components to an x-ray: the evacuated (vacuumed) tube, the target (anode) , the filament and the heating circuit. The target and the filament are separated by a potential difference. The heating elements produces electrons, which then hit the target, causing it to emit x-rays.

  • X-rays only produced when tube is in action i.e. can be switched on/off
  • We have control over the amount and energy of x-rays produced
8
Q

List some detectors for planar x-rays

A
  • FILM HARDCOPY:
  • Film processor with tanks of chemicals
  • High resolution
  • COMPUTED RADIOGRAPHY COMPUTER COPY:
  • Phosphor plate
  • Special laser scanner or CR reader that reads and digitizes the image
  • Digital enhancement and archiving
  • DIGITAL RADIOLOGY (DR):
  • Flat panel detector, fully digitised system
9
Q

In mammography x-ray screening, why do we need a compression plate on the breast?

A
  • Compression plate used to reduce breast thickness
  • Improves resolution
  • Lowers radiation dose (used as a screening tool)
10
Q

Describe fluoroscopy as an x-ray imaging technique.

A
  • Real-time imaging
  • A catheter is fed inside an artery and radio opaque dye is injected
  • Show blood flow inside vessels and can be used to assist with interventions
11
Q

Describe coronary angiography as an x-ray imaging technique

A
  • Real-time imaging using an image intensifier called fluoroscopy
  • A cardiac catheter is fed inside the aorta
  • Radio-opaque contrast agent used to identify areas of occlusion
  • Treatment may be either balloon angioplasty or insertion of a stent
12
Q

What are the limitations with planar x-rays?

A
  • Cannot distinguish between overlying tissues
  • Tissues other than those being observed 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” (from Greek “part/slice” - “write”)
  • Superseded by Computed Axial Tomography, now abbreviated to CT
13
Q

What is helical scanning, and how has it improved?

A
  • There was continuous rotation of the x-ray and continuous table feed.
  • Now, we have helical MSCTs. These are multi-slice rotating round, making it more efficient (takes about 30 seconds for a full-body scan) and gives it more coverage each rotation.
14
Q

How are x-rays used in A&E?

A

With the case of a suspected haemorrhage or blood clot, the opposite treatment can be fatal for the condition. For example, Clot busting drugs may increase bleeding, which would be fatal if it was actually a haemorrhage.

  • So, x-rays are used for the urgent diagnosis required for treatment
15
Q

How are x-rays used in disease progression?

A
  • X-ray imaging is used for monitoring response to therapy
16
Q

How is CT scanning used in treatment planning?

A
  • External beam radiotherapy irradiates normal tissue as well as tumour
  • Multiple beams are used to spare normal tissue
  • CT is used to define area to be treated and the direction of the radiotherapy beams that are used to only affect the tumour area and not the surrounding tissue.
17
Q

How does nuclear medicine: emission imaging work?

A
  • You inject radioactive tracer, patient is emitting the gamma rays which are picked up by the detector
  • Image depends on the metabolism of the tracer: Functional Imaging
18
Q

Describe the difference between a gamma camera and a PET scanner.

A

GAMMA CAMERA:

  • Uses single photon emitting radionuclides
  • Can operate in 2D (planar) or 3D (SPECT)

PET (Positron Emission Tomography):

  • Uses positron emitting radionuclides
  • Always 3D
19
Q

Describe nuclear medicine imaging.

A
  • We are making an image of the distribution of a radioactive tracer
  • Nuclear Medicine only shows function
  • It may reflect anatomy but without metabolism, the tracer will not be taken up
  • Nuclear Medicine is a functional modality
20
Q

What does half-life mean?

A

Half-life is time taken for the radioactivity to reduce to 50%

21
Q

Describe gamma camera imaging.

A
  • Gamma cameras have imaging “heads”
  • For radionuclides that decay with direct emission of gamma rays
  • Most common radionuclide is Tc-99m (T1/2 = 6 hours)
22
Q

Give examples of some tracers used in gamma camera imaging.

A
  • Tc-99m MDP (bone scans)
  • Tc-99m DTPA (kidneys)
  • Tc-99m White Cells (infection/inflammation)
23
Q

Describe a dynamic renal transplant scan.

A
  • Tc-99m DTPA injected IV
  • Camera positioned above the patient
  • Gamma camera records gamma rays and collects image over time
  • Functional Time –Activity curves are obtained

Detects how well the kidneys are functioning

24
Q

What is a SPECT?

A
  • Single Photon Emission Computed Tomography.
  • Essentially the same as gamma camera imaging, but makes the image 3D as it reconstructs the transaxial slices it took.
  • It can acquire up to 64 images from around the head.
25
Q

How can SPECT be applied for a brain scan?

A
  • We can perform a SPECT “Datscan”
  • We use Ioflupane-123 FP-CIT, which binds to dopamine transporters (DAT) on the neurones
  • Can be used in diagnosing Parkinson’s disease ~ reduced uptake in the Putamen differentiates from Essential Tremor
26
Q

Describe β+ particle radiation

A
  • Both the positron and electron are annihilated.

- This results in 2 gamma rays that are created at 180° to each other

27
Q

How is the metabolism of FDG useful in radioimaging?

A
  • FDG is a glucose analogue which enters cells in the same way as glucose
  • Good reflection of the distribution of glucose uptake and phosphorylation by cells in the body
  • ? FDG reflects metabolic activity. It can be used to help diagnose:
  • Alzheimer’s disease (hypometabolism, mostly in temporal and parietal regions)
  • Pick’s disease (fronto-temporal hypometabolism)?
28
Q

Describe a PET scanner.

A
  • A ring of scintillation detectors supported in a fixed gantry
  • Operated in “coincidence mode” - only photons emitted from an annihilation event are recorded
29
Q

Describe PET coincidence

A
  • 2 gamma rays originate from one annihilation event
  • Both are detected within a short time (a few ns)
  • Defines ray path for subsequent reconstruction of image
30
Q

What are some examples of hybrid imaging?

A
  • PET-CT
  • SPECT-CT (used to localise uptake)
  • PET-MR (very expensive)
31
Q

How is the hybridisation of PET-CT helpful?

A
  • Fused PET and CT show the exact location of the “hot spot”
  • Gamma rays originating from the centre of the patient will travel through more tissue which mean they are attenuated more
  • The CT image is used as an attenuation map to correct the PET image
32
Q

What is SPECT-CT used for?

A

To localise uptake

33
Q

Expand on WB PET-MR

A
  • First WB system
  • Biograph mMR
    • 3T magnetCrystal
    • 4x4x20mm
    • 64 rings -> axial length 26cm
  • Very expensive

Decks in Clinical Pathology Class (62):