I will say that I voted to close this question, although I do not remember why. I think it was because it was an engineering question (most likely) or too broad.
But I can tell you for sure I did not think "Ah a question about MR imaging...this must be about engineering." I think there are many topics on this site that could be great physics questions as well as not physics questions at all. I don't think the topic alone determines if it is on-topic or not. Therefore, saying "MR imaging questions (or any subject) are on/off - topic" is not something I think anyone should say.$^*$
You have to take it case-by-case. In this case there are two issues. I think that understanding how one builds an image from MRI data is not a physics question. I say this because one could probably think of multiple ways to take the actual data and make some sort of image, but the physics behind MRI will be the same in all cases. Therefore, this question, to me, seems more like an engineering question. How do we take what we know about the physics of MRI, take our data, and produce a useful image? That "the application of scientific knowledge to construct a solution to solve a specific problem."
I think this question might have been a little bit too broad, but I am not sure if it really is too broad, or if it is just a question that requires a lengthy answer (as you can tell from what was submitted). I do not think lengthy answer necessarily means too broad, but I could see how this question would be taken as too broad.
Based on some discussions in the comments, I think I need to clarify my position in more than just a comment's-worth of space:
I think this is an interesting discussion. I am always trying to understand more about how this site operates and what its community wants. I agree that the distinction is not always clear. As I have been saying though, I do not think a question should be considered on- or off- topic based on its subject but rather based on the question in its entirety. So I agree that MRI, LHC, LIGO, etc. questions definitely deal with physics, but questions asked on this site about these topics (or really any topic) could be on-topic or could be off-topic. The question itself needs to be looked at in each scenario. In case I am not being clear again I will reiterate: I am not saying MRI, LHC, LIGO, etc. should be considered to be “not physics”, “less physics”, etc. I cannot speak to others who originally voted to close, but I am being honest in saying this; I have no reason to be deceptive here. I did not vote to close because the question is about MRI. I am not arguing whether MRI should be considered to be physics.
In my opinion this question was not specifically asking about physics concepts. To me it even seems like the question is saying “I understand the physics of MRI, but I do not understand how the image is formed.” Of course, on its own it is a good and important question. It has attracted many great answers then even still discuss the physics of MRI. However, as others have also stated, a question should stand on its own, not be based on the answers or potential answers associated with it. If good answers or the potential for good answers determined if a question was on-topic, then we would need to restructure many of the VTC reasons. Users should be able to post homework questions without prior work if someone can give an excellent solution. Users should be able to post broad questions that leave possibility of amazing manifestos to posted. Users should be able to post opinion-based questions so that other users can express amazing opinions with amazing discussions. Why not allow really good pure engineering and mathematics questions? Those could garnish amazing answers too.
And I think this is where the issue is. Great answers or potential for great answers cannot determine if a question is on-topic. A line has to be drawn somewhere, and if I see a question that, to me, seems to not really ask about a physics concept (once again, not because it is MRI, but because of what was stated in the question) then I am going to vote to close since that is the policy of the site. Am I being too strict? Possibly sometimes. Could we lose great questions and answers? Most likely, yes. But to me it is less subjective than basing a question on answers or potential answers. Typically questions that could have great answers just belong somewhere else. This is not the only site for amazing questions and answers.
Although, I will say that my answer here is not a petition for this question to be closed. The above is my reasoning and my own thinking. I do not expect (nor want) everyone to agree with me, which is why I am grateful that 5 votes are needed to close a question, and why others can vote to reopen questions. If the community thinks this question is fine, then that is fine with me. But that doesn’t change my own opinion on this question nor why I originally voted to close it, which is what this answer was intended to be about.
Once again, just in case someone still feels like saying this, I am not saying MRI does not involve physics, nor am I saying MRI questions do not belong on this site. :)
$^*$For a different example, projectile motion seems like a great physics topic. However, a question asking how one tracks the position of an object undergoing projectile motion I would say should be closed as an engineering question. Also, I would say a question asking "What are common problem solving techniques for projectile motion problems?" should be closed as being too broad. The subject doesn't determine if it is on/off -topic. The actual question determines that.