Re: MRI vs CT

["Gibbs, S Julian" <s.julian.gibbs@vanderbilt.edu>]

As previously stated, this is an "apples and oranges" 
comparison.  MRI uses steady state magnetic field, (ca 
0.5-1.5 tesla) plus pulsed magnetic field gradients and RF 
(ca 15-45 MHz).  Despite extensive research, no risks have 
been identified from the magnetic fields.  No risk from RF 
as long as SAR is kept low enough to avoid heating.  
Scanners generally will not operate if calculated (from 
patient age, sex, and weight plus pulse sequence 
parameters) SAR approaches a danger level.
CT uses x rays, typically 100-140 kV, tightly collimated to 
minimize contribution of scatter to the image.  Effective 
dose for a typical CT scan of the trunk is ca 5 mSv, and 
typical scan of the head ca 2 mSv or less.  All on RADSAFE 
are aware of the LNT controversy, i.e., are these doses 
harmful?
In any event, no responsible authorithy or regulatory or 
advisory body recommends avoiding a clearly clinically 
indicated and justified MRI or CT scan on grounds of risk 
to the patient.  If the study is well justified, then 
potential of benefit to the patient outweighs any possible 
risk.  The benefit is deterministic, while the risk is 
stochastic.
Hope this helps.
***********************************************************
S. Julian Gibbs, DDS, PhD               Voice: 615-322-3190
Professor of Radiology                    FAX: 615-322-3764 
Dept. of Radiology & Radiological Sciences
Vanderbilt University Medical Center
Nashville TN 37232-2670 Email:s.julian.gibbs@Vanderbilt.Edu

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