Kennedy Krieger Institute
F.M. Kirby research Center for Functional Brain Imaging

7T Projector

Background - 1.5T/3T Method

For the 1.5T and 3T scanners, we use a standard conference room quality projector with the main lens removed and replaced with a long focal length lens. The projector is outside the scan room, projecting from the back of the scanner through a 7 inch waveguide aligned with the bore of the scanner. This provides an image of the proper size on a rear projector screen mounted in the back end of the bore of the 3T scanner. The projector to screen distance is about 6 feet - the projector is outside the 5 gauss line.

Adapting to 7T Environment

The projector needs to be at 50 gauss or less for long term reliable operation. With the 7T scanner, this would put the projector about 6 feet behind the back wall of the scan room. Due to the relatively small space between the top of the volume transmit coil and the top of the scanner bore, the rear projection screen needs to be mounted on the coil rather than near the back end of the scanner bore. As a result, the total projector to rear projection screen distance would end up being about 20 feet with a projected image size of about 13x10 cm. We found that finding a lens to mount of the projector to meet these requirements with acceptable image quality was very difficult.

In Room 7T Projector

As a solution to this problem, we developed a projector that could be used inside the 7T scan room, allowing it to be closer to the rear projection screen. This makes meeting the lens requirements easier. One of the relatively new pico projectors was tested and found to would work at fields up to 3 Tesla. Due to it's small size, it has almost no magnetic pull and can safely be used inside the scan room. Probably due to it's low operating power, it radiates no detectable RF noise. Combined with a high quality projector lens and non-magnetic tripod, this projector provides quality display with no effect on MR image quality. The prototype is shown on the following pages. This has been replaced with a unit with the projector enclosed in a RF shield box and lens mounted externally. VGA and power are fed through a single shielded VGA cable.

Side View

projector

The lens is mounted onto a T-shaped plywood base. The projector is mounted on a separate plywood base attached on the lens base using 3 adjustable screws with wingnuts. This allows the projector platform height and tilt to be adjusted to center the projector on the lens. The hole drilled in the projector base is excessively large and the bolt that holds the projector on has a large fender washer. This allows the projector to be adjusted left-right and closer-farther to the lens to optimize the image quality. All hardware on the projector mount is non-magnetic stainless steel. Note that the projector base tilts slightly down because the image coming out of the projector is, by design, projected slightly upwards.

The projector base is mounted on a tripod that is predominately aluminum and plastic. All steel tilt and swivel adjustment screws and nuts in the tripod head have been replaced with non-magnetic stainless steel nuts, bolts and wingnuts. The yellow cord is used to tether the projector and tripod to an eyebolt on the back wall for safety. Pull on the system is very low.

Top View

projector

Note that the projector is positioned slightly off-center due to the location of the imager in the projector body. The self-locking nuts on top of the projector base adjustment screws are for safety to prevent the projector and base from being pulled off my the magnet. The projector has a very slight pull.

The Projector

projector

The projector used is an Optoma PK102 with it's internal lens removed. The VGA port is on the opposite side with the supplied VGA cable installed. The composite video input jack (not used) is visible on the near side. The power on/off button and a menu button are on the opposite side.

VGA is fed into the scan room through a HD15 feedthrough connector mounted in our lab peneration panel. The projector runs off battery for about 1 hour. One oddity is that the projector cannot be powered on in place behind the scanner (at ~2T). It must be moved to the scanner back door (~1000G), turned on, then moved into place about 1 meter behind the scanner.

The projector native resolution is HVGA 480x320 but can handle up to 800x600 SVGA input signal. The composite video input can handle NTSC or PAL. This projector currently sells for about $250 US. The LED lamp can't be replaced - it's specified to last for 20,000 hours, then throw the whole projector away and get a new one.

Detailed dimensions of projector and lens.

Epson Lens

The lens was removed from a PowerLite 7300 series projector. The zoom and focus allows the image size to be adjusted from 13.8 x 9.2 to 18 x 12 cm at a distance of 280 cm from isocenter. This is about 120 cm from the back wall of the scan room.

projector

Back Projection Screen with Mirror

This is a copy of a design used at University Medical Center Utrecht. The screen is attached to the volume TR coil using velcro. The first surface mirror is mounted on adjustable arms that permit the mirror to be moved up and down, in and out and tilted.

projector

Back Projection Screen with Mirror

Screen and Mirror Mounted on the Nova Medical coil.

coil front