PHIP is a method that provides a strong enhancement of the MRI signal and allows the acquisition of images from nuclei other than protons. The hyperpolarization of cell metabolites is key to their imaging due to the 30,000 to 100,000 signal amplitude increase after hyperpolarization, but hyperpolarizing a metabolite is not a trivial process. Hyperpolarization of metabolites lead to a number of new challenges. LabVIEW, a high level graphical programming language efficiently adapts to hyperpolarization process challenges and allows for a more natural intuitive man-machine interaction than text based languages. Furthermore, as hyperpolarization of metabolites difficulties arise, LabVIEW’s hardware control versatility and easy to program graphical user interface (GUI), reduce costs associated with the hyperpolarization process automation. In reviewing the literature, no LabVIEW software applications in para-hydrogen induced polarization (PHIP) instrumentation have been published.
The LabVIEW software provides a graphical application development environment developed by NI in 1986 for Apple Macintosh. LabVIEW is composed of several sub-tools targeted at making the development and prototyping of instrumentation applications very simple, flexible, and efficient. This prototype was developed with software flexibility in mind, allowing for easy and intuitive changes to its graphical code.
The signals acquired in this project are; Temperature, Pinch Valve Status, Gases Pressure, and Bo Current. These signals were acquired through two data acquisition boards. First, the NI PCI-6221 with two 16-bit analog outputs and 24 digital I/O lines (Dev1) and the NI PCIe-6351 with 16 analog inputs and 24 digital I/O lines (Dev2). Dev1 controls and acquires data from pinch valve and VCCS controllers. Dev2 controls fan heaters and RF output sequence and acquires gas pressure data. Once the signals were acquired, the data was available to LabVIEW for further analysis and software front panel (SFP) presentation.
The hyperpolarization of metabolites is temperature dependent. Thus, every circuit board designed in our lab and installed in the PHIP prototype comes with a temperature sensor. In total, there are six temperature sensors installed throughout the prototype box. In order to minimize signal noise, the temperature data acquired was further processed by LabVIEW using a Butterworth low pass digital filter with a cut off frequency at 100 Hz.
The control of sample and gas flow was accomplished using modified Cole-Palmer 12VDC two-way normally closed solenoid pinch valves. A disposable 1/4"OD tubing was used for the sterile manipulation of gas and sample throughout the system (Fig 1). Each pinch valve was controlled and monitored using Dev1 through a controller circuit designed in our lab. This controller was designed to minimize pinch valve heating and stray magnetic fields.
The Honeywell MLH150PGL06B gas pressure sensor was used to monitor nitrogen, hydrogen, and reaction chamber gas pressures. This pressure sensor is an all metal amplified sensor with temperature compensation with a range of 0 bar to 10 bar.
The hyperpolarization of a 13C based metabolite is sensitive to shifts in the
13C resonance frequency, or Larmor Frequency (eq.2.3). Therefore, the monitoring
of the hyperpolarization solenoid’s Bo is of the utmost importance. Exciting a
metabolite sample with an incorrect RF sequence frequency will lead to poor polarization
results. The Larmor Resonance Frequency is measured indirectly through a voltage measurement
provided by a current controller board (designed in our laboratory). Moreover, the voltage
data (V) is collected using Dev2 (Fig 2)
The derivation of the Larmor Frequency is as follows:
The signals generated in this project are 1H and 13C sequences. These signals are produced by Dev2’s two 2.86 MS/s analog outputs. The output waveforms are sequences built in advance and stored in technical data management streaming (TDMS) file format for later retrieval. The main program reads these TDMS files and when instructed, sends the data to the output coil encompassing the reaction mix capsule. The TDMS file contains data on the sequence’s sampling rate, waveform amplitudes and length. The sequences shape and length is dictated by the type of hyperpolarizing sequence used (Goldman, Hakee, Kadlecek, etc)
In this PHIP instrument prototype, we have used LabVIEW as a platform for acquiring, controlling, and processing data leading to the hyperpolarization of metabolites.
The software structure of the PHIP instrument prototype is shown in fig 3. With 46 controls and 36 indicators on the SFP, the program is too large for linear programming. Instead, the program is event driven, allowing for more efficient program execution. Each button and knob control on the SFP is linked to an event. When a button or knob is changed by the user, an event is triggered executing different program sections. If no event is triggered, the event timer times out at 100ms and collects Gases Pressure, B o Voltage Drop, and instrument Temperature data. The events available are Start Experiment, individual Pinch Valve control, and Tab Control Setup. At time out, the following events are available; Gases Pressure, Bo voltage drop and Instrument Temperature