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Radiofrequency penetration and absorption in the human body: Limitations to high‐field whole‐body nuclear magnetic resonance imaging
Author(s) -
Röschmann P.
Publication year - 1987
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.595995
Subject(s) - specific absorption rate , nuclear magnetic resonance , materials science , radiofrequency coil , radio frequency , magnetic resonance imaging , penetration depth , electromagnetic coil , rf power amplifier , optics , magnetic field , absorption (acoustics) , magnetostatics , electromagnetic shielding , physics , optoelectronics , medicine , telecommunications , amplifier , radiology , cmos , quantum mechanics , computer science , antenna (radio) , composite material
This study presents experimental results about the effective depth of penetration and about the radiofrequency (rf) power absorption in humans as a function of frequency. The frequency range investigated covers 10 up to 220 MHz. For the main part, the results were derived from bench measurements of the quality factor Q , and of the resonance frequency shift due to the loading of the coil. Different types of head‐, body‐, and surface coils were investigated loaded with volunteers or metallic phantoms. For spin‐echo imaging at 2 T (85 MHz), the local specific absorption rate (SAR) was found to be ≊0.05 W/kg using a π pulse of 1‐ms duration and pulse repetition time T R =1 s. Measurements of the quality factor Q as a function of frequency show that the SAR depends upon the frequency f according to ∼ f 2.15 . The effective depth of rf penetration as derived drops from 17 cm at 85 MHz to 7 cm at 220 MHz. Head imaging with B 1 penetrating from practically all sides into the object should be possible up to 220 MHz (5 T) with SAR values staying within the local limit of 2 W/kg as set by the FDA. Whole‐body imaging of large subjects as well as surface coil imaging is depth limited above 100‐MHz frequency. Perturbation methods are applied in order to separate the total rf power deposition in the patient into dielectric and magnetic contributions. The observed effects due to interactions of rf magnetic fields with biological tissue contradict predictions based on homogeneous tissue models. A refined tissue model with regions of high electrical conductivity, subdivided by quasi‐insulating adipose layers, provides a rationale for a better understanding of the underlying processes. At frequencies below 100 MHz, the rf power deposition in patients is apparently more evenly distributed over the exposed body volume than currently assumed.