The longer gadolinium contrast is used, the more medical professionals are becoming concerned about its use. Researchers are coming to the conclusions that 1) gadolinium contrast stays in your body, 2) there is currently no way to remove it, and 3) it is toxic. This article will help you make an intelligent choice as to whether you need it.
First, let me state up front that I am not a healthcare professional. I cannot diagnose or treat diseases. That said, I AM an intelligent and well-read person. I have read the medical guidelines on contrast for MRIs and summarized them for you. I have also included links to the guidelines I used.
Wikipedia defines medical guidelines as such: A medical guideline (also called a clinical guideline or clinical practice line) is a document with the aim of guiding decisions and criteria regarding diagnosis, management, and treatment in specific areas of healthcare…A healthcare provider is obliged to know the medical guidelines of his or her profession, and has to decide whether to follow the recommendations of a guideline for an individual treatment. 
According to A1 Medical Imaging (a radiology lab), non-contrast MRIs are sufficient in most cases. Below are lists of conditions for which contrast may or may not be needed. (This list is not comprehensive) 
Conditions that need contrast:
- Detecting benign and malignant tumors
- Determining the stage of tumors of the central nervous system and major body organs
- Suspected brain aneurysms
- Benign and malignant pituitary disease
Conditions where contrast may be helpful
- Acoustic neuromas
- Central nervous system aneurysms
Conditions that don’t need contrast to diagnose:
- Bone tumors
- Multiple sclerosis
In some conditions, MRIs are recommended with and without contrast. Often the illness can be diagnosed with no contrast, but contrast is needed to identify progress of the disease. In the case of Multiple Sclerosis, a non-contrast MRI will identify existing lesions. Contrast may be needed to differentiate between old and new lesions and track the illness’s progress. [My opinion: If two MRIs are recommended for the condition, one with no contrast and one with contrast, decide if you and your doctor have enough information after the non-contrast MRI. Only get the contrast MRI if you need more information. Ask your doctor what he expects to learn from the contrast MRI.]
A more complete list of conditions and recommendations for contrast can be found at baystatehealth.org, a hospital company. This is a link to the guidelines from their radiology department 
And finally, guidelines from my local radiology lab, Radia. These include recommendations for contrast with CT scans, too. 
I will end this article with a tidbit I learned reading through the documentation about how to administer contrast. If you take Metformin for diabetes, tell your radiology technician. You will not be able to take your dose for a period of time (at least a day). Here’s a link to patient information.