0.0625% for the amount of ultrasound reflected.
The six marker was amazing!! I talked about the half lives of radioisotopes and the need for them to be short in order to reduce the radiation dose on the patient, the use of positron emitters in PET scans, the use of gamma emitters in nuclear scans, combining a radioisotope such as Iodine (which is known to accumulate in the thyroid) to a radiopharmaceutical (a chemical similar to that naturally found in the body) which is consumed by the patient and then being able to detect the gamma radiation emitted by a gamma ray camera to get a clear image of the organ being studied. I talked about the PET machine, I drew the damn thing, talking about the reaction of a positron and electron that emits 2 gamma rays in opposite directions. Talked about the fact that PET scans not only give information abotu the structure of the organ being analysed but also their function. etc...
And yes, image B was a CAT scan. You needed to talk about how CAT scans allow for the ribs to be "digitally removed", where as an X-ray shows everything in it's path. You could have talked about the CAT scan showing transverse "slices" of organs allowing for a 3D image to be built of organs where as an X-ray creates a 2D image. Talk about there being more detail in image B, CAT scans can produce images with more shades of grey therefore allowing doctors to easily distinguish between tissues. 3 points would have got you full marks. I wrote a lot more though :L loved the question.