Medical City · Device Lab 6 of 69
PEM
Two small detector heads a few centimetres apart with the breast lightly compressed between them. The patient sits at the machine and each view takes about ten minutes. Because the detectors are close to the tissue, the resolution in the plane of the image is about a millimetre and a half, roughly three times better than a whole-body scanner manages.
- 10 parts
- 2 configurations in the Device Lab
- Also called: Positron Emission Mammography
Configurations
The versions in the Device Lab.
Opposed-planar PEM scanner
Two small detector heads a few centimetres apart with the breast lightly compressed between them. The patient sits at the machine and each view takes about ten minutes. Because the detectors are close to the tissue, the resolution in the plane of the image is about a millimetre and a half, roughly three times better than a whole-body scanner manages.
Dedicated breast PET ring
A complete detector ring, with the patient lying prone and the breast hanging through an aperture into it. No compression at all. Lines of response arrive from every direction, so the volume reconstructs properly in three dimensions rather than as two flat views.
How it works
Explore it in the Device Lab.
Two photons, back to back, and what follows from it
A positron is emitted, travels a short distance losing energy, and annihilates with an electron. The rest mass of both becomes two photons of 511 keV each, and because momentum has to balance they leave in opposite directions.
What the sharpness is actually made of
The resolution figure quoted for a system is not one thing. It is four independent blurs combined in quadrature and then multiplied by a factor for the reconstruction itself.
The one line that separates the two machines
Take the in-plane resolution of a system and divide it by the sine of half its angular coverage. That gives the resolution along the axis it does not watch.
Why more tracer stops helping
True coincidences rise in proportion to the activity. Random coincidences, two unrelated events landing inside the same timing window and being paired, rise as its square, because either one of two independent rates can supply either half of the pair.
The lesion that fades instead of vanishing
A lesion smaller than the resolution does not disappear. It is smeared across a larger volume, so the total counts belonging to it are preserved while the peak value falls. What is left of its true contrast is the recovery coefficient.
The parts
Part by part.
Scintillator crystal array
A block of small transparent bars, each about two millimetres square and a centimetre or so deep, packed side by side behind the face of every detector. This is the only part of the machine that a 511 keV photon ever touches, and everything after it is dealing with the consequences.
Photosensor array
The layer directly behind the crystal that turns its flashes of light into electrical pulses. On a modern breast system these are solid-state devices a few millimetres square rather than the glass photomultiplier tubes that older scanners used.
Front-end electronics
The boards immediately behind the photosensors, one set per detector block. They take a small fast charge pulse and turn it into two numbers: how much energy, and at what time.
Coincidence processor
A single board or module, usually inside the cart rather than in the detector, that receives every individual detected event from every block and decides which of them belong together in pairs.
Out-of-field shielding
Lead or tungsten plate built into the sides and back of the detector assembly, and often a shaped shield between the detectors and the patient’s torso. It is heavy, it is invisible from outside, and it is a large part of why the assembly is as massive as it is.
Reconstruction workstation
A computer inside the cart, with more cooling than its size suggests, that turns a list of several tens of millions of lines of response into the images on the review screen.
Review display
The screen the operator and the reporting radiologist actually look at, mounted on an arm above the cart, showing either a page of small plane images or a set of orthogonal views depending on which configuration produced the study.
Keyboard and operator console
A keyboard, a pointing device and a small set of dedicated controls on the cart top, from which the study is set up, the patient identified and the acquisition started and stopped.
Column and vertical drive
The upright mast rising from the cart, carrying the detector assembly and setting its height. A scale runs up the front of it and a flexible chain of links runs up the back.
Cart and wheeled base
The wheeled body that everything else stands on. It holds the reconstruction computer, the coincidence electronics and the power supplies, and it carries the connections that an electrical safety test starts from.