Principles of Imaging Week 3 Flashcards Preview

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Flashcards in Principles of Imaging Week 3 Deck (24):
1

Benefits and drawback of x-ray

Benefits
Low dose and low risk
Simple, fast, and inexpensive
High resolution

Drawbacks
Need 2 or more projections (e.g., A/P & LAT)
Limited number of views
Can’t see behind structures

2

Describe CT

X-ray source (gantry) rotates around patient
Multiple projections
Hundreds of “films”
See behind structures
Digital detector
Axial Scan or Helical
Table can move during tube rotation
Single Source: provides one energy level for morphology
Multiple Source: 2 x-ray sources with different energy levels (kV) allow simultaneous acquisition of two dif images- dif contrast

3

Benefits and drawback of CT

Benefits:
Fast scanning with good resolution ~2 mm
See behind structures
Good bone contrast
Largely replaces exploratory surgery
Drawbacks:
Limited soft tissue contrast
Higher patient dose

4

Describe nuc med:

Nuclear Medicine: gamma rays (nucleus)
Internal uptake of radioactive material
Molecular tracers tagged with radioactive atoms
Example: macroaggregated albumin (MAA) labeled with Tc99m
Tc99m is the most common radioisotope*

Nuclear imaging
Activity taken up internally (oral or I.V.)
Wait for uptake and non-specific clearance
Gamma camera captures emission

Single Photon Emission Computed Tomography (SPECT)
Single photon travels through the body and deposits in camera
Functional image only
Need CT for anatomical information
Uptake depends on several factors

5

What imaging modality has the highest radiation dose?

PET/CT combination followed by CT

6

Benefits / drawbacks of nuclear medicine:

Benefits
Sensitivity: ~100,000 times more sensitive than transmission x-ray scans
Functional imaging provides additional molecular information
Drawbacks
Limited resolution (~10 mm)
Requires CT for anatomy
Patient dose
Complex quantification

7

Describe PET

Positron Emission Spectrometry (PET)
Positron emitter: short half-life
F-18 (110 min), C-11 (20 min), O-15 (2 min), Cu-64 (12.7 hr), Zr-89 (78 hr)
Cyclotron is required
Internal uptake (IV or oral)
Positron annihilates with electron to form 2 photons traveling in opposite directions
Photons interact in tissue (attenuation) and detector
Coincidence counter
PET is intrinsically quantitative – activity/ml
FDG (fluorodeoxyglucose) is a glucose analog and is taken up by demanding cells such as brain, kidney, and cancer cells. Most common tracer for PET*.

8

Benefits /drawbacks of PET

Benefits
Sensitivity: ~10 million times more sensitive than transmission imaging (10 x SPECT)
Functional imaging

Drawbacks
Limited resolution (~2-6 mm)
Requires CT for anatomy and attenuation correction
Higher patient dose
Radiotracers are expensive

9

Describe MRI

Magnetic Resonance: radiofrequency (FM)
Earth’s magnetic field = 0.5 Gauss
1 Tesla = 10,000 Gauss

Hospital magnet = 1.5 – 3.0 Tesla

Uses non-ionizing radio frequency
Tissue heating
Same range as radio (60 – 120 MHz)
Proton (Hydrogen atom) = magnet (spin)

Proton spins align with external magnetic field
RF pulse tips proton spins by 90 degrees

Tissue contrast depends on the pulse sequence and tissue properties
Longitudinal relaxation (T1) and transverse relaxation (T2) are characteristics of tissue
Pulse sequences typically emphasize either T1 or T2 characteristics (contrast)

10

MRI benefits / drawbacks:

Benefits
Excellent soft tissue contrast
High resolution (1 mm)
Different pulse sequences generate images with different contrast
Non-ionizing radiation

Drawbacks
Long scan times (~30 min)
Expensive
Patient eligibility/safety: claustrophobic, imbedded metal (unregulated tattoos), pacemakers, etc.
Low sensitivity for molecular imaging

11

Describe ultrasound:

Most widely used imaging modality
Transducer produces high frequency mechanical pressure waves (sound)
Sound wave coupled to surface using gel
Sound is transmitted through tissue or reflected at interfaces
Reflected wave is detected by transducer
Time for reflection indicates depth of interface
Speed of sound in tissue is ~1540 m/sec

12

US benefits / drawbacks

Benefits
Inexpensive and fast
No radiation dose (safe)
Extremely portable (battlefield)

Drawbacks
Limited contrast
Difficult to see behind air-filled cavities and bone

13

Doppler

Use Doppler effect to measure and visualize blood flow
Color Doppler: velocity information is presented as color overlay

14

Examples of US use


Applications
Anesthesiology – needle placement
Cardiology – cardiac ECHO
Gastroenterology – abdomen sonogram
Neonatology – intracerebral abnormalities
*Obstetrics & Pediatrics – most popular use
Urology – pelvic sonogram
Musculosketal – tendons, muscles, ligaments
Emergency Medicine – irreplaceable detection of bleeds

15

Where do X rays originate from?

Atomic Shells

16

Where do gamma rays originate from?

Nucleus

17

What are photons?

High energy EM rays

18

Particulates?

Particles (charged = electrons, uncharged = neutrons)

19

What is contrast?

Density difference

20

Most widely used imaging modality?

US

21

What test has the highest resolution?

X rays tied with US

22

What test has the lowest resolution?

SPECT

23

What test has highest contract?

PET and SPECT

24

What test has lowest contrast?

US

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