How to open MRI images on Mojave

That is false. You can use a third-party application developed by OsiriX, which is a highly trusted DICOM viewing company that has its programs used by professionals all around the world. Since you're not using the program as a medical professional, I believe you can download the lite version for macOS.

Keep in mind, this is not Apple software. 

There's a free DICOM viewer in the App Store. Miele-LXIV, DICOM Workstation and Viewer.

It's entirely possible to do. Unfortunately most of the medical community can't see past their own keyboard when it comes to things like this.

MRI images are typically stored in the DICOM format.

Osirix https://www.osirix-viewer.com has a free Mac version that can read these and show you the scans.

With the app open, press Command+O. Navigate to the CD and whatever folder within that to where the actual images are. Choose one and click Open.

Personally, I just drag and dropped the .dcm files onto the app's icon.

I downloaded the app and a few sample DICOM images. I had no problem opening a few randomly selected .dcm files without having to pay anything to view them. Here's one:

I've use the free image viewer from Horosproject.org for a couple years now.

It works pretty well.