The presence of certain metallic implants or devices is a well-recognized contraindication for undergoing an MRI scan. MRI operates using powerful magnetic fields and radio waves to generate images of internal structures in the body. Metallic implants, such as pacemakers, cochlear implants, or certain aneurysm clips, can be affected by the magnetic field, potentially causing malfunction or displacement of the device, which poses a serious risk to the patient.
In contrast, while pregnancy is a consideration and may warrant caution when deciding on MRI due to the lack of long-term safety data, it is not an outright contraindication. History of allergic reactions to iodine is relevant in relation to CT scans that use iodine-based contrast but does not apply to MRI, which primarily uses gadolinium-based contrast agents. Recent weight loss does not directly impact the safety or effectiveness of an MRI scan. Therefore, the presence of certain metallic implants or devices is the primary concern that could make obtaining an MRI unsafe.