There are several broad categories of imagers. They can be lumped into groups as follows:
Single detector/scanning systems, scanned/linear array and staring or focal plane array. Each has characteristics somewhat unique to itself, but each can be made to read temperatures (radiometric).
Scanning systems use a single detector and have a scanning mechanism in the optical path which allows an image to be constructed. A major benefit to this approach is that the single element is making all the measurements. Differences in element sensitivities or bad elements, which occur in focal plane array devices, don't occur thus thermal uniformity is quite good. The downside to scanning cameras is they take longer to image, but for stationary objects where temperatures aren't changing rapidly (as with medical imaging) this is a non-issue.
Focal Plane Array or staring array cameras (FPA's) have the benefit of providing fast scanning. Images are formed in "real-time". Each element in the detector array corresponds to a pixel element in the displayed image. Until recently, FPA's required cooling to be useful. Liquid nitrogen was used for a long time. Today closed cycle Sterling Coolers are frequently employed. Downsides to these cameras are expensive replacement cost for coolers and the fact that there are dead pixel elements and different sensitivities across the detector which must be compensated for (or assigned a value).
A new type of FPA, one that doesn't require the Sterling cooler, is becoming more popular. This is known as the micro-bolometer. It has the same issue with dead or lower sensitivity elements.
This type of detector is really a hybrid of the other two. It uses a linear array, offers higher scanning speed than the Scanning detector, but considerably slower than the Focal Plane Array detectors. To some degree it suffers from the bad pixel syndrome but since there are fewer active elements it's a smaller issue. It also needs a cooler, which can be a costly maintenance item.
The third style, the linear array, uses a line of detectors which are scanned. This is a cross between a scanned single element and a focal plane array. It provides marginally faster scans but also has some of the same bad pixel issues. Currently, linear array cameras use detectors that require mechanical coolers or use Liquid Nitrogen as the cooling element. Images, after averaging, can be quite good
Both FPA's and Linear array cameras have noisier displayed images and are generally averaged before saving an image. Signal averaging ads to the overall acquisition time thus reducing the seeming advantage over scanning systems.