Can you have an MRI scan with dental implants? Heating, image distortion and magnetic dentures

MRI works with two kinds of energy: a strong, static magnetic field and the radio waves sent while the image is being taken. Three questions are asked about a metal in the body: does the magnetic field pull it or twist it, do the radio waves heat it, and does the metal distort the image? The first two are safety questions, the third a diagnostic one. For dental implants the answers differ from one another.

On the question of being pulled, titanium is reassuring. A study in Japan that examined titanium implants in 1.5 and 3 Tesla scanners writes that titanium is paramagnetic, that is, there is no problem of the static magnetic field pulling it. In a study that tested 18 different dental materials in a very powerful research scanner of 7 Tesla, too, implants and top parts showed a deflection of only between 5 and 6.5 degrees; this is a very small effect. In metal crowns made of alloys such as cobalt-chromium and nickel-chromium, the deflection was more pronounced.

On the question of heating, measurements have been made. In the same Japanese study, in a tissue-like model, the temperature of titanium implants rose by 0.4 degrees at most in 1.5 and 3 Tesla scans. In the 7 Tesla study, an implant 50 mm long heated up by as much as 1.5 degrees, particularly in scanning sequences that use high energy; in shorter implants and in top parts the rise was smaller. These measurements were made in models known as phantoms, not directly in the human mouth; for this reason the Japanese authors, too, recommend that the scan be carried out with care.

This is why long implants deserve a separate mention. Zygomatic implants, which extend to the cheekbone, and some strategic implants that anchor in cortical bone are longer than conventional implants. Because heating can be related to length, if you have an implant of this type, and particularly if the scan is going to be carried out in a powerful scanner, it is useful to tell the radiology team the type and length of the implant. The scanners used in day-to-day hospital practice are mostly 1.5 and 3 Tesla.

Image distortion is the real practical issue. The FDA writes that implants can distort or interfere with MRI and X-ray images. Metal warps the magnetic field around it and creates distortions (artefacts) that appear in the image as a void or a bright area. In a study carried out in Denmark, titanium created more magnetic field distortion and image distortion than zirconia. The field distortion grew as scanner strength increased; this measurement was made at 0.55, 1.5 and 3 Tesla in an artificial model (phantom). The scan in a volunteer was carried out at 0.55 Tesla only, and the distortion was seen mostly in the crown area close to the implant. In a study carried out in Germany on dry lower jawbones, too, the volume of distortion was largest with the combination of a titanium implant and a cobalt-chromium crown, and smallest with a zirconia implant and a ceramic crown.

How far this distortion affects the diagnosis of the brain or the neck is not fully known. A systematic review that examined nineteen studies writes that in MRI of the head and neck the most distortion is caused by stainless steel orthodontic brackets, while for implants and prostheses the data are insufficient and inconclusive. The review states that distortion can affect the diagnosis of brain and facial structures as well as the oral cavity, and that its severity depends on the material, the position in the mouth and the scan settings. In an MRI scan of the knee or the lower back, an implant is not expected to affect the image; the issue is scans of the head, face and neck.

Magnetic dentures are a separate topic. Some removable dentures hold on to a stainless steel keeper screwed onto the implant by means of a small magnet inside the denture. This steel part, unlike titanium, is strongly pulled by the magnetic field of MRI: in tests carried out at 7 Tesla the deflection angle exceeded 90 degrees; in the 3 Tesla tests, too, all the deflection angles exceeded 45 degrees. By contrast, in a measurement made in Japan at 1.5 and 3 Tesla, the force acting on the keeper stayed far below the bonding force that fixes the keeper to the root cap. This measurement was made with keepers bonded to a natural tooth root; it cannot be transferred directly to a keeper screwed onto an implant. In the 3 Tesla tests, heating was measured at below 1.5 degrees. The authors of the 3 Tesla study, however, write that the deflection forces tended to exceed the acceptable limit, and for safety they recommend that magnetic attachments be inspected before and after the scan.

The magnet itself can be affected too. In a study carried out in a 3 Tesla scanner, when the magnets inside the denture entered and left the scanner at a certain angle, their magnetic strength fell significantly and retention dropped. For this reason the authors recommend that magnetic removable dentures be taken out before an MRI scan. Removing large prostheses made of cast alloys that contain a magnet is also recommended. In practice this means: take the denture out and leave it outside the MRI room, and tell the team about the steel keeper on the implant and that the denture is magnetic.

In the situations in the first list, stating your implant on the form is enough. In the situations in the second list, the radiology team knowing in advance may call for preparation or an adjustment of the scan settings.

The steps below are for everyone who has an implant. If the hospital has its own instructions, those apply.

1. 1

   Get your implant details and keep them

   The FDA recommends that you ask your dentist for the brand and model of the implant and keep this information in your records. Keep the number of implants, their type (conventional, zygomatic, strategic), their material and the type of prosthesis on top on a card or on your phone.

2. 2

   Say so when you book the appointment

   When you book the MRI appointment, state that you have implants and a fixed or removable prosthesis in your mouth. Particularly if a scan of the head and neck is going to be carried out, this information helps the team choose the appropriate scanner and scan settings.

3. 3

   Fill in the safety form in full

   The form completed before an MRI scan, as the NHS describes it, asks about your surgical history and about metal parts in the body, including fillings and bridges. Write your implant, your prosthesis and, if you have one, the magnetic keeper on this form.

4. 4

   Take out your removable denture

   The NHS asks for jewellery and items containing metal to be removed. Take out your removable denture before the scan, especially if it is magnetic, and leave it in a safe container outside the room. Fixed teeth and the implant stay in place.

5. 5

   During the scan

   If you feel heating, pulling or discomfort in your mouth during the scan, let the team know. The NHS writes that in scans where a contrast agent is given there can be a metallic taste in the mouth; this has nothing to do with the implant.

6. 6

   A check after the scan with a magnetic denture

   The authors of the studies recommend that magnetic attachments be inspected before and after the scan. If the denture's retention has fallen after the scan, show it to your dentist; the strength of the magnet may have decreased.

Each imaging method interacts with an implant differently. The comparison below shows what to watch for in which investigation.

The following are decisions that lead to unnecessary worry or to an avoidable problem.

The sequence below is for a typical MRI appointment; the hospital's instructions take priority.

1. When you book the appointment

   Give the details of the implant, the type of prosthesis and, if you have one, the magnetic keeper. If it is a scan of the head and neck, stress this in particular.

2. On the day of the scan, beforehand

   Fill in the safety form, and take off metal items and take out your removable denture. Fixed teeth and implants stay in place.

3. During the scan

   If you feel heating or pulling in your mouth, let the team know with the call button. Heating was found to be small in measurements, but saying something you feel is always right.

4. After the scan

   Put your magnetic denture in and check its retention. The NHS writes that results usually arrive within 1 to 2 weeks; if the report has a note of distortion in the jaw area, this is most often the expected effect of the implant.

An implant does not change the MRI scan itself. But the preparation and the quality of the image are determined by these factors:

The area to be scanned

In scans of the head, face and neck, the implant's effect on the image matters; in other areas of the body no effect is expected.

Implant and prosthesis material

Titanium, zirconia and cobalt-chromium create different levels of image distortion.

How the prosthesis is held in place

Magnetic attachments require additional reporting and a check after the scan; screwed or cemented fixed teeth do not.

The strength of the scanner

Image distortion and measured heating increase from 1.5 Tesla towards 3 and 7 Tesla.