Biomedical Engineering Reference
In-Depth Information
9.5.3
Clinical System
Our third-generation NMR system is designed for practical applications in a
clinical environment (Fig. 9.8 )[ 57 ]. Its small size, easy accessibility, and high
robustness provide end users with fast and stable measurements of biological
samples. The new NMR system is composed of three core parts: a newly designed
probe for clinical samples, NMR electronics, and a user-friendly software.
The system design is based on that of previous generations of NMR and con-
sists of a small portable magnet (B 0 D
0:5 T) and a solenoidal coil for higher SNR
(Fig. 9.8 left ). A custom-made PMMA (polymethyl methacrylate) housing cages
the magnet, the microcoil, and RF matching circuit. With its major improvement
focused on clinical translatability, this new system uses disposable thin-walled
polyimide tubes to load biological samples for measurement, thereby eliminating
potential contamination of the NMR probe. The disposable tubes are filled with
samples (
5L) and are inserted into the coil bore for NMR detection. Modular
coils made in a variety of sizes can be plugged into the system to optimally
accommodate available sample volumes (1-100L).
A small form-factor (20
5 cm) NMR electronics is implemented using
off-the-shelf integrated circuit (IC) chips to achieve cost-effective (<$200)and
highly programmable NMR platform (Fig. 9.8 middle ). The NMR electronics
generates the NMR pulse sequences, acquires the NMR signal, and communicates
with external terminals (computer, mobile devices). It has three main parts: a
microcontroller unit (MCU), an RF transmitter, and a signal receiver. The MCU
(TMS320F28235, Texas Instruments) controls overall RF transceiver operations as
well as data communication with external terminals. As an RF transmitter, a direct
digital synthesis chip (AD9954, Analog Devices) is employed to generate two RF
signals with 90 ı phase difference that are modulated by voltage-controlled switches
(ADG1419BRMZ, Analog Devices). The RF heterodyne system is implemented
to process the NMR signal. First, the signal is amplified by low-noise amplifier
(AD604, Analog Devices) and down-converted to baseband (1-10 kHz) by a mixer
(ADE-6, Mini-Circuits). Baseband signals subsequently pass a low-pass filter and
are digitized by an analog-to-digital converter (AD7625, Analog Devices). Mobile
devices (e.g., iPhone TM ,iPad TM ), which are connected to the MCU via a standard
communication channel (e.g., USB, Bluetooth), receive digital data and show
graphical outputs.
The NMR software incorporates graphical user interface (GUI), data acquisi-
tion/process, data logging/sharing, and a temperature compensation engine, which
deliver a user-friendly interface and contribute to the robustness of the new NMR
system (Fig. 9.8 right ). The software is programmed with Objective-C using Cocoa
and Cocoa Touch frameworks and operates on iOS TM and OS X TM .Inorderto
implement a graphical representation of R 2 relaxation curve, open-source plotting
framework (Core Plot) is cross-linked with Cocoa and Cocoa Touch frameworks.
Time domain NMR data are acquired at the negative edge of spin echoes and
processed real time. Negative edge-triggered data acquisition reduces the size of
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