Introduction to Radiology II
ž Objectives:
Explain the basic principle and uses of Radio-isotope imaging
MRI techniques and uses
Ultrasound imaging
Nuclear Medicine / PET
Ionizing radiation
Radio-isotopes attached to molecules targeting specific organs or
metabolic processes
Radio- isotopes emit gamma rays as they decay.
Spatial resolution is limited
Events to keep the dose to the patient to a minimum, the smallest
possible dose of isotope with short half life should be used.
Technetium-99m most commonly.
Tc-99m in ionic form (pertechnetate) for thyroid scan, Labelled to
complex organic phosphate for bone scan, Labelled to
magroaggregate of albumen for lung perfusion scan,…
The emitted gamma ray by the isotope detected by gamma
camera.
In selected cases emission tomography is performed. In
this technique, the gamma camera moves around the
patient. A computer can analyze the information and
produce sectional images similar to CT. Emission tomography can
detect lesions not visible on the standard views. Because only one
usable photon for each disintegration is emitted, this technique is
also known as single photon emission computed tomography
(SPECT).
Example of uses:
ž First line evaluation of biliary function evaluation
ž First line evaluation of cardiac perfusion
ž First line evaluation of solid pulmonary nodules
ž First line evaluation for many neoplasms, staging,
ž and treatment response
Positron emission tomography
Positron emission tomography (PET) uses short-lived positron-emitting
isotopes, which are produced by a cyclotron immediately before use.
The most commonly used agent is
F-18 fluorodeoxyglucose (FDG).
Because muscle activity results in the uptake of FDG, the patient should
rest quietly in the interval between injection of the FDG and scanning.
ž Current role of PET imaging:
ž Oncology:
Differentiate benign from malignant masses
Eg, solitary pulmonary nodule
Primary tumor staging eg in breast CA
Detect tumor recurrence eg in colorectal malignancy
ž Cardiac: Diffrentiation viable from non viable myocardium
CNS: charectarization of dementia disorder, localization of seizure
focus
Radionuclide examination / Main advantage:
ž Highly sensitive eg in osteomylitis
ž Provides functional and anatomical information eg DTPA renal
scan
ž Staging and follow up of lymphoma eg Gallium scan
Radionuclide examination / Limitation and disadvantage:
ž Its non specific eg isolated hot spot
ž Use IR
ž Cost
ž Extra care in handling the materials
ž Magnetic Resonance Imaging
No ionizing radiation
Utilize magnetic fields and radio waves
Contraindication: implanted devices, ferro-magnetic metals
Relative contraindication: claustrophobia
Hydrogen nuclei(protons)in water molecules and lipid are
responsible for producing images
Ability to do spectral analysis (remember organic chemistry)
Gadolinium compounds is a contrast agent used in MRI which
appear high signal on T1.
ž The nucleus of hydrogen atom is a single proton. Being a spinning,
charged particle it has a magnetic properties.
ž The first step in MRI is application of a strong external magnetic field
son the H atoms within the patient align in a direction, H spin around
the line of the field- precession at a frequency- Larmor frequency.
ž Second magnetic field is now applied at a right angle to the original
external field at a same LF ( radiofrequency pulse ) so adding energy
to the system
ž Following cessation of 2nd RF Pulse- energy dissipated ( T1 relaxation )
and the process of dephasing (T2 relaxation ).
ž Strength of signal depend on proton density and relaxation times T1
and T2.
ž Example of primary Uses:
ž First line evaluation of suspected neurologic abnormality
ž First line evaluation of soft tissue mass/ neoplasm
ž First line evaluation of joint disarrangements
ž First line evaluation of bone neoplasm
Advantage of MRI Disadvantage of MRI
Best for soft tissue imaging Expensive and time consuming
No ionizing radiation No metal can be allowed
Procedure of choice for MSK diz. Need no patient movements
during the procedure
It can be done for pregnant Patient fear and dislike (
women claustrophobia
Multiplanner imaging
MR angiogram can be performed
without use of contrast media
Ultrasound
Uses US probe with piezoelectric crystal.
Excitatio n of the crystal by electrical signal cause it to emit ultra
high frequency sound.
The sound waves are reflected back to the crystal by various
tissues.
Hyperechoic and hypoechoic are used to describe tissues of high
and low echogenicity respectively.
Pure fluid reflect no sound= anechoic=black.
ž Doppler US
Flowing blood cause alteration of frequency., this change or shift
calculated- quantification of blood flow.
Color Doppler: Flow toward= red, away= blue.
ž Growing use of U/S
ž Intracavitary scanning
ž Transvaginal is most widely used and accepted
ž Transrectal for prostate
ž Endoscopic US
ž High frequency US for MSK imaging
ž Intra operative high frequency US
ž
ž Primary applications:
First line evaluation of pregnancy and developing fetus. Some
time use 3D or 4D application .
First line evaluation for differentiation of cystic from solid masses/
structures
First line evaluation of liver and biliary tree
First line evaluation of kidneys and bladder
First line evaluation of thyroid gland
Small probes for endoscopic/laproscopic U/S/endovascular.
U/S contrast agents available.
ž
ž Advantage: Lack of IR, low cost, portability.
ž Disadvantage: US cannot penetrate bone or gas e.g US and
pulmonary conditions.
ž
References:
- Peter Armstrong- Diagnostic imaging
- Imaging for students
