Principles of Imaging Week 3 Flashcards
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
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
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
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
What imaging modality has the highest radiation dose?
PET/CT combination followed by CT
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
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*.
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
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)
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
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
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
Doppler
Use Doppler effect to measure and visualize blood flow
Color Doppler: velocity information is presented as color overlay
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
Where do X rays originate from?
Atomic Shells
