Camera-in-a-pill takes pictures of digestive tract
Able to provide physicians with an internal view of a patient's digestive path, Given Imaging Ltd's PillCam is an ingestible diagnostic tool that provides images of the small intestine without invasive surgery or probes.
The traditional endoscopic process relies on a tethered apparatus. Given Imaging used ultra-efficient CMOS design (to keep power consumption to a minimum) and dense optoelectronic packaging to devise an endoscopy approach that goes down a little easier: The patient swallows the camera pill, the capsule progresses via natural processes through the digestive tract and image data is relayed to an external receiver belt for eventual download and diagnostic review by a doctor.
While not quite at the level of Harry Kleiner's 1966 movie classic "Fantastic Voyage," PillCam is an intriguing step forward.
The PillCam is about the size of a megavitamin, at 26.4mm (almost exactly 1 inch) long and 11.3mm wide?not teeny but ingestible by most patients. It weighs a mere 4g and contains all image-capture, illumination, activation, transmission and control circuits within a two-piece biocompatible plastic capsule. An opaque lower shell surrounds most of the electronics, while a clear dome is fused to the lower shell to provide a view-through window for the camera lens located inside. The clear dome also allows a set of six miniature white LEDs surrounding the lens to illuminate the gastrointestinal (GI) tract as the "pill" moves through it.
Now it's our turn to look inside the pill. After an incision at the seam of the two case halves, the capsule can be popped open and the stack of electronic components removed. Five distinct strata form the capsule internals, with various forms of interconnect between the layers of circuits.
Starting at the top level?that closest to the transparent portion of the capsule?is the lens/illumination layer. An annular PCB surrounds the single plastic molded lens, supporting the LEDs and their associated current-limit resistors. Below this lens level is the imager layer, home to a 256-by-256pixel CMOS color image sensor. Marking on the chip indicates it is a custom device from Photobit, a company acquired by Micron Imaging in 2001. Combined with the plastic lens, the camera offers a claimed 140? viewing angle and 0.1mm feature resolution within the GI tract being imaged.
Behind the imager layer is a pair of Eveready No. 399 silver oxide watch batteries, wired in series to create the sole 3V supply for the PillCam. The two button cells provide 3V at 55mA-hr, or 165mW-hr of total available energy. Since the device runs for up to eight hours, a time-averaged power draw of approximately 20mW is implied.
The switch layer located behind the batteries provides the means to preserve precious battery energy before the PillCam is ingested by the patient. A reed switch mounted on the switch layer circuit board is held open by a magnet in the PillCam's shipping holster, interrupting the battery connection. When the package is opened and the capsule is removed from its holster for swallowing, the reed switch closes and power to the PillCam begins to flow.
The final strata of the PillCam?the transmitter layer?is home to the only other IC, a custom ASIC developed by Given and of unmarked foundry origin. The chip must provide system control along with radio transmission. A 27MHz crystal located on the reverse side of the transmitter layer is consistent with both functions. The 3.2-by-3.5mm flip-chip ASIC contains a small block of logic, a very small memory array and a variety of mixed-signal circuits. Since the output from the image sensor is presumed to be preconverted to digital form, the radio and LED drive circuits are the likely functions included in the analog portion of the ASIC.
RF emission guidelines
Per FCC filings, the transmitter operates at either 432.13MHz or 433.94MHz, with minimum-shift-keying modulation. MSK has the general benefits of providing constant-envelope modulation, transmitter simplicity and good spectral efficiency. A simple air coil is the radiating antenna element, tucked into the rounded capsule end opposite the camera. Transmit power is held low to manage power consumption, as the receiver antennas are in close proximity with the waist-worn monitor. Nevertheless, FCC filings indicate the PillCam stays within emitted RF guidelines only when the pill is inside the body. The minute or so that it takes the pill to go from activated/depackaged form to ingestion is apparently given a waiver as part of the PillCam's regulatory approval.
Image capture, switch and transmitter layers are all fabricated on a single rigid-flex PCB. Delayering the board among the three islands of functionality creates flex circuits to interconnect those regions. The assembly is folded up around the batteries, and a pair of gold-plated coil springs distributes power from the imager layer to the lens/illumination layer through holes in the lens barrel.
The 8hr PillCam lifetime provides up to 57,000 images at a 2fps rate, with the LEDs flashing only during image capture. The combination of low-power CMOS imagers, spartan logic, efficient radio design and pulsed white LEDs for illumination are critical given the limited available energy.
Bringing all these pieces together to create an ingestible camera-bot is an amazing benchmark in technical progress. The body's motility provides the PillCam locomotion, but advanced electronics make the rest possible. It certainly brings new meaning to the phrase "internal medicine."
- David Carey