Thursday, February 7, 2008
Harvard Univ. and its medical school claim to have built the smallest complete nuclear magnetic resonance (NMR) system.
Described at this week’s International Solid-State Circuits Conference (ISSCC) in San Francisco, the NMR weighs 2 kg and occupies 2.5 litres.
It can detect biological samples tagged with magnetic nanoparticles in tiny quantities of liquid. For example, it can detect 4 nmol of a chemical in 5 µl of liquid. According to the team, the instrument is 60 times more sensitive than a commercial bench top NMR of about 120 kg.
The instrument was made by Yong Liu and Nan Sun, part of Donhee Ham's Group at Harvard Univ., in collaboration with Ralph Weissleder's Group at Harvard Medical School.
The key to the system is a CMOS radio frequency (RF) transceiver chip which drives and receives from a 3 mm dia 500 nH planar RF coil which is held inside a fist-sized commercial 0.5 T magnet.
The chip's receiver is a fully differential heterodyning design with Gilbert mixers tuned to 21.3 MHz, a frequency suitable for protons in this magnetic field.
Its LNA offers a voltage gain of 110 with noise below 2.5 nV/VHz—dominated by channel thermal noise of the input transistors which are operated at 4 mA tail current.
The planar coil arrangement (Q=16) was selected, says the team, because it was easier to make. But tests with a Q=200 300 nH solenoid coil showed spin echoes more clearly.
Nuclear magnetic resonance Protons in water in a static magnetic field can be excited by an RF field at certain frequencies. If the RF field is switched off, the nuclei precess around the static magnetic field axis at the input frequency, while slowly loosing phase coherence. The coil used to introduce the RF field can also detect the loss of phase coherence which shows up as damping of the oscillations. Concentration and other characteristics of solutions can be deduced from the damping and other responses.
A full description of the instrument is available here:
http://www.seas.harvard.edu/~donhee/ISSCC_2008_7.3.pdf SOURCES: Electronics Weekly, Harvard Univ.