Apples and oranges. HiRes CT is a term usually reserved for discontinuous non-helical images obtained through the lung to assess for interstitial lung disease. It is reconstructed with a high resolution (i.e., high frequency) filter with edge enhacement to bring out the finer details of high-contrast (air-lung tissue) interfaces. This can be achieved pretty well by single detector, multidetector, or even older generation scanners. A lot of doctors outside of radiology think that since it's "high-resolution", then "it must be better". Can't be more wrong. Do a CT for PE with "hiRes" and you won't see half the emboli. We routinely get requests for "Hires CT" for patients who don't need it and we just ignore the request and do what's right. Same goes for contrast studies. A lot of physicians want a contrast study when it's clearly not indicated or even contraindicated. They just think a study is better because it has "contrast". Can't be more wrong in many many disease categories.
MDCT (multidetector CT) on the other hand, should more accurately be called multichannel CT. It is a CT scan in which more than one row of detectors is acquiring data at the same time. Think of it as parallel processing. The current "16 detector scanners" actually have 24-32 detectors, but acquire images through "16 simultaneous channels".
Examples: larger area of coverage for arterial phase images, whole brain CT perfusion instead of 3-6 cm slabs of CT perfusion, less need for negative chronotopics (i.e., betablockers) for cardiac CT. Of course it not that simple at all. When there are too many detectors laid out, there is the mathematical problem of beam scatter and fanning, and the optimum algorithms for correcting these sometimes not so subtle gemetric distortions in the CT images is a hot research topic. So, it's very simplistic to assume "hey, what the heck, lets have 2000 detectors and scan the whole body in 0.5 seconds". It doesn't work that way. More detectors are fraught with the above aproblem and a whole slew of technical difficulties to make accurate images without distortion of image shape, false density measurements, and inaccurate Hounsfield units in the reconstructed image.
Along the same lines as above.
For the first one, interstitial lung disease for the most part. MDCT is not really different in indications from just a "CT", except it is faster and allows certain applications, such as better CTA, better imaging of moving parts (e.g. heart, delirious patient), better CT perfusion.
Fishman and Jeffrey - Multidetector CT: Principles, Techniques, and Clinical Applications - 2003.
