MRI

The MR750 scanner has a main magnetic field strength of 3 Tesla and is equipped with fast water-cooled gradients (slew rate of 200 mT/ms, peak amplitude 50mT/m, 100% duty cycle) used for spatial encoding.  The MR 750 has 32 receive channels for improved multi-coil signal detection.  Coils include single channel (birdcage) transmit / receive (GE), 8 channel (GE), 16 channel (MRI Instruments), 32 channel (Nova Medical), 8 channel speech (Clinical MR Solutions), and an 8 channel array for nonhuman primate imaging (Clinical MR Solutions).  The image reconstruction engine is based on a solid state 192GB architecture with 24 cores operating at 2.7 GHz capable of reconstructing parallel image acquisitions during the scanning session allowing the operator to assess image quality in near real time.  The scanner has the ability to run in research mode, allowing modified research sequences both from in-house and external collaborators to be used in addition to the standard GE pulses sequences already provided.   Real-time AFNI is used to monitor head motion in real-time.  Synchronization and triggering signals are logged by the BIOPAC system described below.

Stimulus presentation can be delivered by using an advanced fiber optic goggle system (Avotec), or a high resolution back-projection system (VPixx). The VPixx system uses a high refresh rate DLP projector (480Hz in color, 1,440Hz in grayscale and 10kHz for binary images). DLP permits extremely fast access and transmission of video, in microseconds. The VPixx system can also display in stereoscopic vision up to 400Hz with circular polarizing filter and passive glasses.  Both goggles and back-projection systems are interfaced with SensoMotoric Instruments eye tracking system (sampling rate of 50 Hz). This eye tracking system yields four different analyses: percentage analysis, pictorial analysis, order of object vs. time, and pupil diameter vs. time.

MRI auditory stimuli are presented using either a pneumatic headphone system (Avotec) with a frequency response of 40Hz to 40kHz, a set of electro-dynamic headphones (MR CONFON), or Optoacoustics Optoactive system.  The Optoactive system minimizes gradient noise both actively and passively and includes a noise cancelling microphone for recordings.  Both MR CONFON and Optoacoustic systems are low profile and can be used with the 32 channel coil.   MRI sound levels are reduced in studies of sleeping young children using an acoustic foam insert.

Stabilizing patients to minimize head movement is critical for MRI studies.  The Brain Imaging Core has three head stabilizing methods: bite bars, vacuum pillow, and foam inserts.            

Collection of peripheral, physiological measures in the MRI are done using the BIOPAC MP150 system. The MP150 system provides high resolution (16 bit), variable sample rates for analog and calculation channels, 16 analog inputs and two analog outputs, digital I/O lines (automatically control other TTL level equipment), and 16 online calculation channels. The MP150 System provides high-speed acquisition (400 kHz aggregate) via an Ethernet connection to a host computer.  AcqKnowledge, the BIOPAC control and analysis software package, is used is used in conjunction with Psychology Software Tools (PST) E-Prime or PsychoPy (developed at the University of Nottingham) to control the acquisition and can be used for data analysis.

Electrodermal activity is acquired in the MRI using the BIOPAC amplifier and two 6mm Ag-AgCL non-polarizable electrodes placed either on the tips of the second and third fingers or on the thenar and hypothenar eminences of the palm, depending on the requirements and placement restrictions of individual studies.

Facial electromyography is acquired in the MRI using the BIOPAC amplifier and two 3mm Ag-AgCL non-polarizable electrodes placed over the right or left corrugator supercilli muscle (small narrow pyramidal muscle at the medial end of the eyebrow beneath the frontalis and obicularis oculi); zygomatic corrugator muscle activity is recorded via two electrodes over the right or left cheek muscle with a third electrode applied over the ipsilateral mastoid to serve as an electrical ground.

As well as BIOPAC EMG, the Core also has a custom 8-channel MR-EMG system. This MR compatible bipolar amplifier includes matched radio frequency (RF) filters, a 4th order Butterworth low-pass filter (100/300 Hz settings), and switchable gain (0.4k/2k/10k). The amplifier is battery-powered, and signal is sent out of the magnet room via optical-fiber. In order to remove artifacts caused by MR gradients and RF pulses, analog-to-digital conversion is run off the 10MHz scanner clock at a sample rate of 488 Hz and re-synchronized with the scanner acquisition with each scan volume to eliminate phase shifts.  An artifact template, calculated by averaging across volume acquisitions, is then subtracted from the data to yield cleaned EMG.

Electrocardiogram activity is acquired in the MRI using the BIOPAC amplifier and three Ag-AgCl lead electrodes, using Einthoven’s triangle potential measurement strategy.

Blood oxygen saturation levels are acquired in the MRI using the BIOPAC amplifier and a receiving photodiode.  The amplifier transmits two wavelengths of light via LEDs through a pulsating vascular bed (finger).

Skin temperature is acquired in the MRI using the BIOPAC amplifier and a thermistor transducer.  This amplifier includes a lower frequency response selection switch that permits either absolute (DC) or relative (via a 0.05 highpass filter) temperature measurement.

Carbon Dioxide gas analysis is done in the MRI using the BIOPAC amplifier and a gas sampling interface kit.  This amplifier records varying carbon dioxide concentration levels employing a single beam infrared, single wavelength, measurement technique.

Respiration data is acquired in the MRI using the BIOPAC amplifier and a respiration transducer to measure abdominal or thoracic expansion and contraction.

Blood pressure is collected in the MRI using Magmedix’s non invasive Tesla blood pressure manometer.  Diastolic, mean, and heart rate can be acquired.  This monitor can be used on neonates, pediatric, and adult subjects (Adults:  Heart Rate: 30-220 BPM, Systolic NBP: 40-260 mmHg, Mean NBP: 20-200 mmHg, Diastolic NBP: 20-200 mmHg, Static Cuff Accuracy +/- 3mmHg).

In addition, respiration and pulse oximetry data collected by the scanner can be acquired via a serial interface.

To acclimate patients to the unique environment of the MRI scanner, the Brain Imaging Core of the IDDRC houses a simulator room. This room has a mock MRI scanner using a back-projection and auditory system with two control computer along with Psychology Software Tools’ Motrak software and hardware to train subjects on reducing motion. This room is used to introduce subjects to the identical experimental procedures that they will experience in the actual scanner to ensure both comfort and data quality.