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Posted: February 6, 2008
Harvard claims NMR system is smallest built
(Nanowerk News) Harvard University and its medical school have built the smallest complete nuclear magnetic resonance (NMR) system, claimed the team.
Described at the International Solid-State Circuits Conference (ISSCC) in San Francisco, the NMR weighs 2kg and occupies 2.5 litres.
It can detect biological samples tagged with magnetic nanoparticles (which speed the loss of phase coherence - see box) in tiny quantities of liquid. For example, 4nmol of chemical in 5Ál of liquid. "This is 60x more sensitive than a commercial bench top NMR," said the team, comparing it to a system weighing 120kg.
Key to the system is a CMOS RF transceiver chip which drives and receives from a 3mm diameter 500nH planar RF coil which is held inside a fist-sized commercial 0.5T magnet.
The chip's receiver is a fully differential heterodyning design with Gilbert mixers tuned to 21.3MHz, a frequency suitable for protons in this magnetic field.
Its LNA offers a voltage gain of 110 with noise below 2.5nV/VHz - dominated by channel thermal noise of the input transistors which are operated at 4mA tail current.
The planar coil arrangement (Q=16) was selected because it was easier to make, but tests with a Q=200 300nH solenoid coil "showed spin echoes more clearly", said Harvard.
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.