Brain Mapping Center MR Imager Bid Specs

Sept 7, 2001

Index

Physical Configuration

The design necessitates that the subject have their head positioned inside of and RF coil, which is then placed concentrically inside of a "gradient coil" of somewhat larger diameter. While the RF coil is basically an open frame, the gradient set will probably be a light-proof solid device. Increasing the diameter of the gradient set will generally come at the price of decreased spatial resolution and performance.

It will be possible to bring both light and forced air into the "gradient coil" to improve patient acceptance. Presently, we expect that the RF coil will travel with the patient into the instrument, while the gradient coil will be fixed in place inside of the magnet, as shown below in cross section.

Note that any stimulation, recording and interventional devices must be placed within the approximately 1.5 inch clearance outside of the RF coil and inside of the gradient set. For reference, note that the whole body aperture in a standard clinical instrument is about 55 cm, as opposed to the approximately 80 cm in this drawing. For mechanical reasons, the vendor may decide to limit the new instrument to the lesser dimension as well (e.g., they may require the insertion of an additional tube of 55 cm diameter, primarily to ease the development of the patient table and comfort systems.

 Magnet

 Gradients

 Acoustical environment

No explicit mention was made in the prior specifications of the acoustical requirements. While we envision the use of active noise cancellation techniques to moderate the noise of the gradient set, it will be necessary as well to rigidly mount the gradient coil apparatus to reduce vibration. A sound pressure level of less than 65 dB, A-weighted, for the gradients operating in EPI sequences (e.g. 1400 Hz oscillation at 3.5 gauss/cm) is required with the vendor supplied sound attenuation equipment).

Head Aperture Dimensions

Based on data from NASA, the USAF and the RAF, in order to fit 90% of subjects, the head coil requirements have been specified as outlined below.

As shown in the figure, the required imaging volume is approximately cylindrical, with dimensions 24 x 24 x 20 cm (X x Y x Z). Within that volume, the RF receive/transmit uniformity is to be ± 10% P-P using a loaded phantom. The gradients are to be linear within ± 7.5 % over the same region.

In the sketch, a circular gradient set is assumed, with an aperture, at the inferior end, of 35 cm. The imaging volume is to be centered 22 cm above the opening. Obviously, a number of other possibilities exist to fulfill the applications requirement. For example, it would be reasonable to consider a larger linear volume along the z-axis, but with its isocenter displaced superiorly, compared to the drawing. A larger diameter would be entirely acceptable, too, in order to achieve the required transverse linearity.

A problem we have seen in prior implementations of gradient insert systems is that the magnet isocenter is not aligned with the physical center of the brain (as the brain is not in the center of the head) in the A-P dimension. As a result, the usable field of view does not always cover the brain (as indicated above).	As this is unacceptable, we propose an alternative in which the RF head coil is properly slotted to allow the head to be displaced anteriorly, as sketched. A cushion is used to support the head, as needed.

Gradient Notes

Due to the high operational frequency (128 MHz) and to the field-dependent deleterious effects of magnetic susceptibility on the images, the instrument will be required to operate with a relatively short echo-planar readout period. We anticipate that a scan duration of 360 µsec per line will be adequate (1400 Hz EPI oscillation frequency). We desire a pixel size of better than 3 mm with a 360 microsec readout. The gradient-time product required is calculated as: At=1/(gamma x d), where A is the gradient strength in gauss/cm, t is the readout time in seconds, d is the resolution (0.3 cm) and gamma is the gyromagnetic ratio of 4258 Hz/gauss. Thus, At is 0.78283 gauss-ms/cm. For a sinusoidal readout waveform, and a 360 microsec readout, this gives a gradient strength of: Pi x (0.78283 gauss-ms/cm)/2(0.36 ms) = 3.416 gauss/cm, which is increased by 5% to enable tune-up and fractional k-space acquisition for a final gradient strength of 3.6 gauss/cm for the 1400 Hz sinusoid. Other waveforms with equivalent area (0.783 gauss-ms/cm) are acceptable. For example, a trapezoid with a 100 microsec rise and fall time and a 160 microsec flat top, having an amplitude of 3.15 gauss/cm would be equally acceptable.

ADDITIONAL SPECIFICATIONS & CLARIFICATIONS:

• The system shall have a total noise figure of 1.6 dB or less (1.4 dB target)
• The penetration panel must include additional RF-filtered connections for:

  3 DB-25 connectors

  3 DB-9 connectors

  4 BNC connection

  User accessible waveguide of at least 5 cm diameter (or multiple ports with equivalent total area)

• Similarly, the compilers, licenses, libraries and sample source to enable incorporation of special purpose image reconstruction into the standard data path must be included. (E.g. an asymmetrical gradient will require field of view offset correction. The tools to do so must be included with the system.
• The manufacturer should supply a list of potential suppliers for compatible RF coils (including phased array and other surface coils) who have been given access to any necessary proprietary vendor specifications for such development.
• The manufacturer must provide a research development package which includes access to source codes of all available scanner control programs, imaging pulse sequences and reconstruction programs, and permission to develop pulse sequences for research. The manufacturer must provide formal training (with no less than 100 hours of instruction) for at least two persona from UCLA to be proficient on the use of development tools for research pulse sequences and image reconstruction and displaying methods. Telephone and email assistance shall be available.
• The manufacturer must specify means to ensure that operations fall within IRB-specified specific absorption rate guidelines. For example, a means of monitoring forward power during the imaging sequences must be detailed.
• The MRI system must provide high-speed ethernet connections (100BaseT or similar).
• Training for system maintenance should be available to UCLA BMC personnel
• Cardiac and respiratory gating shall be available with both ECG and peripheral monitoring. Trigger points shall be selectable and variable. The system shall include all necessary MRI compatible ECG leads and electrodes.
• The system must employ a method to closely monitor RF energy deposit to be below the SAR limit set by the FDA.
• Failure to meet any of the performance specifications at any time will result in an extension of the warranty period, or a no cost extension of the full service contract for a time equivalent to the period during which the scanner is out of spec. This will include times that the scanner is in use outside of normal service hours. However, if the staff of the Brain Mapping Center refuses to turn over the equipment for service, the time during which the scanner is unavailable will not be counted against the warranty or service period.