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Guide 8 min read

T1, T2, and why the TMJ goes to MRI

A working explanation of MRI contrast for people who read CBCT, and why the one structure everyone wants to see in the joint is the one CBCT cannot show.

Here is a fact that catches people off guard. You can take a beautiful, high-resolution CBCT of a temporomandibular joint, scroll through every slice, and never once see the articular disc. The condyle is crisp. The eminence is crisp. The single most clinically important soft tissue in the joint, the disc whose position defines internal derangement, is simply not there. It is not hidden by artifact. CBCT cannot register it at all. To see the disc you have to change the physics entirely and go to MRI, and understanding why is a good way to understand what MRI is actually doing.

MRI is not an x-ray, at all

Everything in dental imaging up to this point is x-ray based. A beam goes through the patient, denser tissue stops more of it, and the shadow builds the picture. MRI throws that whole model out. There is no ionizing radiation. Instead the patient goes into a strong magnetic field, on the order of one and a half to three tesla, tens of thousands of times stronger than the earth’s field. That field lines up the hydrogen protons in the body, and since the body is mostly water and fat, hydrogen is everywhere.

A radiofrequency pulse then knocks those protons out of alignment. When the pulse stops, they relax back, and as they do they emit a faint signal that the scanner listens for. That is the entire trick. MRI maps where the hydrogen is and, more usefully, how quickly it settles back down, because different tissues settle at different rates. There is no dose. The image is built from the tissue broadcasting its own signal.

T1 and T2, the two clocks

The relaxation happens along two independent tracks, and the two clocks are where all of MRI’s contrast comes from.

T1 is the recovery clock. It measures how fast the protons realign with the main magnetic field after the pulse. Fat recovers quickly, so on a T1-weighted image fat is bright. Watery tissue and fluid recover slowly, so on T1 they are dark.

T2 is the decay clock. It measures how fast the protons fall out of sync with each other. Fluid holds its coherence a long time, so on a T2-weighted image water and fluid are bright, which is the opposite of what fluid does on T1. The quickest way to tell the two apart is to find the cerebrospinal fluid or any pocket of fluid: dark means you are looking at T1, bright means T2.

You choose which clock the image emphasizes by setting two timings, the repetition time between pulses and the echo time before you listen. A short repetition time brings out T1 contrast; a long echo time brings out T2. That is why a radiologist orders a specific sequence for a specific question. T1 and proton-density images show you anatomy and the shape of things. T2 images show you fluid and inflammation, which is to say they show you pathology.

One more piece completes the picture for anyone coming from CBCT. Cortical bone is nearly black on both T1 and T2. It has very little mobile hydrogen and what it has loses signal almost instantly, so bone that lights up brilliant white on your cone beam scan is a signal void on MRI. The two modalities are close to photographic negatives of each other. CBCT is a bone camera that cannot see soft tissue. MRI is a soft-tissue camera that cannot see cortical bone. This is not a flaw in either one. It is the reason you sometimes need both.

Back to the disc

Now the opening fact makes sense. The articular disc is a biconcave pad of dense fibrous tissue, thick at the front and back bands and thin through the middle. On CBCT it has essentially no contrast against the surrounding soft tissue and no dense mineral to catch the beam, so it never appears. On MRI it has a characteristic look, a low-signal bowtie sitting on top of the condyle, and you can actually read where it is.

In a healthy joint with the mouth closed, the thick posterior band sits right at the top of the condyle, around the twelve o’clock position, with the thin middle zone tucked between the condyle and the eminence where the two bones come closest. When you image the joint you do it in an oblique sagittal plane angled to the long axis of the condyle, and you add a coronal plane to catch sideways displacement. And critically, you scan the joint twice, once with the mouth closed and once open, because the whole question of internal derangement lives in the difference between those two positions.

With reduction and without

Anterior disc displacement is common, and it comes in two flavors that only the open and closed views together can distinguish.

In displacement with reduction, the disc sits forward of the condyle when the mouth is closed, but as the patient opens and the condyle translates forward, the disc snaps back into its normal spot on top of the condyle. That recapture is the click the patient feels and the clinician hears. On MRI you see the disc anterior on the closed view and back in position on the open view.

In displacement without reduction, the disc stays stuck in front of the condyle no matter how far the patient opens. It never recaptures, and it physically blocks translation, which is why these patients present with limited opening and a jaw that deviates toward the affected side. This is the closed lock. You can only call it by comparing the two positions and seeing that the disc that was displaced when closed is still displaced when open.

That comparison is the reason both series are non-negotiable, and it is the reason CBCT, which images one static bony state, cannot answer the question even in principle. The diagnostic criteria for temporomandibular disorders make this formal: a definitive diagnosis of disc displacement requires MRI, and the clinical exam alone is only a screen.

What else the T2 clock catches

Once you are in the joint, the fluid-sensitive T2 sequence earns its place. Joint effusion, extra fluid in the joint spaces, shows up as bright signal on T2, and it is a useful marker of an inflamed, symptomatic joint. The same T2 sensitivity reads the condylar marrow. Normal marrow is fatty and bright on T1; when it goes dark on T1 and bright on T2 you are seeing marrow edema, an early sign of avascular change or degenerative disease that the bony surface may not yet show.

This generalizes well beyond the joint. Everywhere in the head and neck, MRI’s edge is soft tissue and marrow. It is the modality for the articular disc, for salivary gland masses, for tracking tumor along a nerve, and for catching osteomyelitis or medication-related osteonecrosis in the marrow before the cortex breaks down and CBCT can register it. The one caution worth carrying is that the same dental metal that streaks a CBCT distorts an MRI too, through a different mechanism, so a mouth full of base-metal crowns or implants can throw a signal void right across the region you wanted to read.

The short version for referrals

If the question is about bone, condylar shape, erosion, arthritic change, a fracture, send the CBCT. If the question is about the disc, whether it displaces, whether it reduces, or about soft tissue, effusion, or marrow, send the MRI. And if you genuinely need both the bony detail and the disc, that is not indecision, it is two cameras pointed at two different things, and the joint sometimes requires both to tell the whole story.

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