a low-cost multichannel spectrometer for magnetic resonance imaging

8/12/2019 A Low-Cost Multichannel Spectrometer for Magnetic Resonance Imaging

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A Low-cost Multichannel Spectrometer for MagneticResonance Imaging

Weinan Tang, Weimin Wang

Institute of Quantum Electronics, School of ElectronicEngineering and Computer Science, Peking University,Beijing, Peoples Republic of ChinaE-mail: [email protected]

Abstract: A low-cost multichannel spectrometer is proposed for

magnetic resonance imaging. There are three distinct features of

the instrument: (1) the scalable parallel acquisition with

extension to 16 channels is achieved upon high-speed, wideband

digital downconverters (DDCs). (2) high frequency range up to

400MHz is allowed by using two quadrature digital upconverters

(QDUCs) as a radio-frequency (RF) source and a local oscillator,

respectively. (3) the design is based on a system on chip (SOC)

integration with PowerPC architecture. Besides, by combiningfield programmable gate array (FPGA) and digital signal

processor (DSP) technique, the spectrometer is built in a digital

manner with high performance and accuracy. The device consists

of inexpensive and highly-integrated components which make it

low-cost and compact. The spectrometer has been successfully

designed and demonstrated by the experimental result.

Keywords: magnetic resonance imaging, multichannel spectrometer,

parallel data acquisition

I. INTRODUCTIONMagnetic resonance imaging (MRI) is the most important

of diagnostic imaging modalities, providing highly contrastedimages of the body tissue with features of exquisite spatialresolution and oblique imaging. As a non-invasive and non-ionizing imaging method, MRI has found numerousapplications in the clinical field, such as functional imaging,tumor detection, and angiography [1-3].

The design of a spectrometer is critical to the developmentof an MRI system. As an electronic console, the spectrometeris mainly responsible for running pulse sequences, transmittingand receiving RF signals, and controlling the generation ofgradient waveforms. In general, the key revolutions indeveloping a modern MRI spectrometer are widespread shift tohigher field imaging and further increases in the number ofchannels [4]. In addition, the cost, size, and complexity of aspectrometer are also considerable factors in the design.

Many standard commercial spectrometers, such as Brukerand Varian, deliver state-of-the-art performance. However,these systems are generally rather complex and expensive dueto the use of highly dedicated subsystems, for example, theProgrammed Test Sources (PTS) and distributed pulse

programmers. With the development in the technique ofelectronics and computer science, a number of groups areworking to design varies MRI spectrometers aiming at low-cost,small size and simplicity [5-9]. Inexpensive direct digitalsynthesizer (DDS) chips are often used to replace highly

specialized RF synthesizers [7,10]. The FPGA and DSP makethe receiver more flexible and integrated [11,12].

In this work, we present a new spectrometer with up to 16acquisition channels and high frequency range (


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Figure 1. Block Diagram of the multi-channel MRI spectrometer

Later, the DSP fetches the file, distributes the digitalwaveforms, and initiates the hardware within other modules.The interconnection design becomes simple since externalmemory interface (EMIF) of the DSP provides a gluelessinterface to a variety of memory and peripheral types;including SRAM, DPRAM, and FLASH as well as FPGA andASIC devices.

A quadrature digital upconverter (QDUC) chip (AD9957,Analog Devices) is used as a digital modulator that shifts thefrequency of the baseband spectrum up to the desiredresonance frequency fr. The QDUC takes in the digital

baseband I and Q data, which is amplitude modulated if softpulse is needed. The baseband data, at an I/Q sample rate of fIQ,is generated from an FPGA (XC3S2000, Xilinx) using a directdigital synthesis method. The algorithm of digital quadraturemodulation is described according to Eq. (1)-(3), as listed

below.

RF pulse is a narrow band modulation signal with carrierfrequency equal to resonance frequency:

IQLOzr ffzGBf +=+= )(2 0 (1)

where is the gyromagnetic ratio, 0B is the magnitude of the

magnetic field, zis slice location, zG is gradient magnetic field

along the slice direction, and LOf is the frequency of a

quadrature local oscillator signal in the QDUC. The basebandmodulation signal can be written as:

))(2sin()())(2cos()()( nnfniAnnfnAnS IQIQ +++=

)()( niQnI += (2)

where )(nA is the amplitude and )(n is the initial phase. Then

the time domain output of the QDUC takes the form:

)2sin()()2cos()()( tftQtftItS LOLORF =

))()(2cos()( ttfftA IQLO ++=

))(2cos()( ttftA r += (3)

The advantage of using this direct RF architecture ratherthan the conventional super heterodyne approach is: (1) I and Qmatching can be extremely accurate because the quadraturemodulation is manipulated within the digital domain. (2) rapidswitching and fine tuning resolution of frequency and phase is

achievable due to the DDS technique. The AD9957 can supporta system clock and DAC sampling rate of up to 1GHz,therefore allowing for RF transmission approaching 400MHz.

We use another AD9957 as a single tone DDS in themixing stage of the receiver. The NMR signals from multi-channel coils are first amplified by the preamplifier and thendown mixed to intermediate frequency (IF) signals. The 14-bitADCs (AD9251, Analog Device) directly acquire the IFsignals at a sampling rate of 50MHz. Then the digital data isquadrature demodulated and filtered by parallel DDCs(AD6636, Analog Device). The AD6636 has 4 independent

processing channels for real inputs. So four AD6636 chips areused to build a digital receiver with up to 16 channels. Afteroversampling, the subsequent processing decimates the datainto a lower rate to gain higher signal-to-noise rate (SNR). Thefilter chain of AD6636 provides programmable decimation ratewith range from 1 to 32768, which leads to a minimumreceiver bandwidth of 1.53KHz. Since NMR signal occupies anarrow spread of frequencies of 5KHz to 500KHz, thisdecimation range is enough for most magnetic resonanceapplications. Considering the tradeoff between signal-to-noise

ratio (SNR) and spatial resolution, we define the passband ofoverall filter response as 30% of the receiver bandwidth andstopband attenuation as 100dB. Fig. 2 shows an overall filterresponse with a receiver bandwidth of 27.778KHz. Once thesignal for data acquisition is triggered, the state machine insidethe FPGA stores the processed data into its embedded DPRAMin a time domain multiplexing (TDM) fashion. The data iswritten into one-half of the RAM and read out by the PowerPC

processor from the other half. This double buffering is realizedby ping-ponging mode between the memory halves. ThePowerPC processor performs data accumulation and combinesdata from more than one channel into a single data stream. Alarger buffer memory (SDRAM) is used for temporary storage

(a)

(b)

Figure 2. The overall frequency response of digital filter with an output sampleperiod of 36us. (a) The attenuation above the stopband frequency (19.45KHz)is larger than 103dB. (b) The maximum ripple in the passband is 0.007dB.


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a low-cost multichannel spectrometer for magnetic resonance imaging

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