X-Message-Number: 29518
From: David Stodolsky <>
Subject: The Ubiquitous SQUID - From Axions to Magnetic Resonance Imag...
Date: Sun, 20 May 2007 21:38:45 +0200

The Ubiquitous SQUID - From Axions to Magnetic Resonance Imaging

John Clarke



The SQUID (Superconducting QUantum Interference Device) combines the  
phenomena of Josephson tunneling and flux quantization to make an  
ultrasensitive detector of magnetic flux.  Coupled to a  
superconducting flux transformer, the SQUID achieves a magnetic field  
noise below 10-15 THz-1/2.  I briefly review a variety of SQUID  
applications.  An amplifier consisting of a SQUID with a resonant  
input circuit achieves a noise temperature within 20% of the quantum  
limit at 0.8 GHz.  This amplifier is being installed in the axion  
detector at Lawrence Livermore National Laboratory, and is expected  
to increase the frequency scan rate by three orders of magnitude.  We  
use a SQUID magnetometer, together with prepolarization of the  
nuclear spins, to obtain magnetic resonance images (MRI) in a  
magnetic field of 132 microtesla, corresponding to a proton Larmor  
frequency of 5.6 kHz four orders of magnitude lower than in  
conventional MRI.  The spatial resolution is typically 1 mm.  The  
combination of prepolarization, spin relaxation in an adjustable  
field and detection in a microtesla field enables us to obtain  
relaxation time (T1)-weighted contrast images in fields ranging from  
1 microtesla to 0.3 tesla.  Values of T1 measured in ex vivo  
specimens of surgically removed prostate tissue at 132 microtesla are  
typically 60% higher in normal tissue than in tumors.  Preliminary ex  
vivo images suggest that microtesla MRI may well have clinical  
applications.





dss

David Stodolsky    Skype: davidstodolsky

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