Difference between a PET scan and CT scan

I am totally confused. Someone please explain the difference between a CT scan and a PET scan.
My husband had a CT scan back in March. When it showed abnormalities in his esophagus we went to MSK and a PET scan was ordered. He received 4 treatments of chemotherapy and then last week he went for a F/U CT scan. The radiologist stated that a comparison between the both is not an adequate way to make a impression.
When I read the CT report it was based on the original CT scan
from an outside facility. The PET scan only gave impressions and no measurements were included in the report. The current CT scan has included measurements and shows minor improvement. The oncologist that we are going to recommends that he has a repeat PET scan to do a comparison.
Has anyone else experienced the same problem or shouldn't the doctor know that you can't compare CT scans with PET scans?

CT and PET scans are different.  F/U CT scan means a follow-up CT scan.  At the beginning of my journey I got CT scans and PET scans separately, but for several years now I had combined scans all done in one machine at the same time.  The CT scan shows the "organs" and any abnormalities.  The PET scans are usually done with a radioactive injection after a "protein only" diet for 48 hours.  When the PET portion is done, the patient is given some sugar substance and since cancer thrives on sugar the tumors will "suck up" the sugar/carbs and with the radioactivity it will "glow" when seen on the resultant PET scan.  So one scan (PET) is measuring the "activity" of the cancer and the other (CT) is measuring the size and position of the tumor.

I was happy when they began to give me the combined scan.  My doctor's also told me that the CT scans and x-rays are somewhat "muddy" after radiation and they cannot see the images as well.  That is why they give so many and do comparisons.  One of my CT scans showed abnormalities to my liver, but thankfully I found out that it was simply inflamation and the cancer had not spread. 

My understanding is that the radiation is done to "shrink" the tumor.  The chemo is to "kill off" cancer cells.  Once they "shrunk and killed" the cancer cells in my EC tumor, I went to surgery to remove the damaged esophagus. 

Perhaps someone else can explain this better, but that is how they explained it to me.  I do hope your husband makes good progress.  Hang in there.  It is a real nasty process, but hopefully the end result will be as so many of us have experienced ... no more cancer.  God bless.

PET/CT is a better diagnostic but is not available everywhere because it is a newer machine design. They are done separately but use the same machine so the CT and PET can overlay each other and you get a combination of anatomical information overlaid with the metabolic read obtained by the PET. It makes it a lot easier to determine if the anatomic area they are are concerned with is metabolically active which can be an indication of cancer because you do not have discrepancies from using different equipment, or being positioned differently. There are also times growth has not gotten large enough to show on a CT, but you might see the metabolic activity that indicates growth. There is a much missed detail which is that PET is not infallible. You can have a cancer which has diffused widely, and it may bot be concentrated enough in any one area to show up at all on PET, so at times they go to do surgery indicated by a clear PET, but may find someone full of cancer and be unable to complete surgery.  False positives and negatives can occur with PET, but most of the time you get an accurate picture of what's going on, so they are a very valuable diagnostic tool.

CT shows you anatomical features such as bone and some soft-tissue (example: organs) information.  PET shows you information about the biochemistry going on (example: metabolism).  So, since cancer is usually both anatomical (has a size, location, shape, etc.) and biochemical (it can metabolize sugar at a different rate as compared to the rest of your body), both of these imaging techniques tell you about what's going on.  In very simplified terms, CT tells you about your structure (different densities of materials in your body), and PET tells you about your chemistry (different chemistry of materials in your body).  For instance, your bone will show up very nicely on a CT and you won't see it at all on a PET scan (generally, unless you're using a probe that reacts with bone).