Okay, so the most common reason that men have a decreased flow is some type of obstruction of the urethra (urine tube from bladder to outside). In most men this is due to an enlarged prostate that pushes in on the urethra and causes it to narrow. You are a little young for an enlarged prostate, but it certainly happens. Sounds like your prostate and bladder were normal by ultrasound, but it would be interesting to know how big the prostate measured to be.
Another thing that can happen in younger men and also in women is a tightness at the bladder neck (outlet of bladder to urethra). This is usually due to a tightness of the muscles in this area, and it can cause decreased stream and other symptoms similar to enlarged prostate.
Men can also develop scar tissue or strictures along the urine tube causing blockage. Sometimes this will cause spraying of the stream, split stream, etc., but not always. This is diagnosed either by a scope of the urethra/bladder or by a special xray where dye is injected into the urethra.
Rarely people develop nerve/muscle damage to the bladder where it is not able to contract normally. Usually this is a side effect of another medical problem such as diabetes, back problems, or other neurologic problems.
If your problem is one of the first two (prostate or bladder neck), a medication such as Flomax or Uroxatral, will help to relax the muscles in this area so that the urethral tube can open up and the urine can flow out better. Both of these medications are quite safe with minimal side effects so that would be my initial thought. To see if this would help you. Otherwise, you might need further evaluation with a scope or other bladder tests to evaluate the cause. Doubt that an MRI would be helpful unless there were concerns about back problems or other neurologic disorders.
Hope this helps. Please let me know if any other questions.