In this post I do my best to decipher and understand some MRI images of my shoulder. In my next post I’ll discuss the actual diagnosis and prognosis of the damage to my shoulder.
The MRI looks like a large capsule with one end open. I was asked to lie down on a cot on rails that was pushed into the capsule for the imaging. We’re all familiar with this from TV shows like House.
The MRI specifically targeted my right shoulder, so it was equipped with a device that looked like a large shoulder pad, like a football player might wear. It was hard on the outside, and I was asked to push my shoulder into the interior padding as far as possible. This is the only area that was scanned.
The device is very loud. In addition to the expected hums and groans, there was a loud and intermittent hammering, like someone outside the capsule was chiseling his way in. I was given a cheap over-ear headset on which they played some Norah Jones (I had asked for jazz). I had to lie as still as possible to ensure image clarity (I was told to avoid even heavy breathing), and the session lasted about 40 minutes. This was pretty easy for me as I’m not claustrophobic; I’d put it down as the most pleasurable experience so far in this long process of finding out what’s wrong with my shoulder.
There were 6 image sequences taken of my right shoulder during the MRI session. The following video is composed of proton density and fat saturation MRI images of the shoulder in the Sagittal plane, which is one of the six sequences.
I could make videos of the other sequences — and even combine them into a single video — but I doubt anyone is that interested. In my next post I’ll try to put down what the MRI images actually reveal about my shoulder. Read on to learn about how MRI works and what the indicated imaging techniques are.
Explanation of MRI Imaging Techniques
“The body is largely composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner, the magnetic moments of some of these protons align with the direction of the field.
“A radio frequency transmitter is then briefly turned on, producing an electromagnetic field. In simple terms, the photons of this field have just the right energy, known as the resonance frequency, to flip the spin of the aligned protons. As the intensity and duration of the field increases, more aligned spins are affected. After the field is turned off, the protons decay to the original spin-down state and the difference in energy between the two states is released as a photon. It is these photons that produce the signal which can be detected by the scanner. The frequency at which the protons resonate depends on the strength of the magnetic field. As a result of conservation of energy, this also dictates the frequency of the released photons.”
[From the Wikipedia Article.]
Protein Density, or Spin Density, is an MRI technique in which some effects are minimized in order to reveal the signal change coming from differences in the amount of available spins (hydrogen nuclei in water).
Fat Saturation (FatSat) is another technique. I found a great explanation:
“Since the hydrogen nuclei of fat and water resonate at slightly different frequencies, it is possible to excite just the fat with a special RF pulse, and then destroy (“spoil”) the resulting signal with a gradient pulse. This loss of fat signal is referred to as “saturation,“ a loose reference to a term from classical MR spectroscopy. The net magnetization of the water is preserved, so the normal pulse sequence that follows the FatSat pulse “sees” just the water signal.”
The Sagittal Plane
“The term is derived from the Latin word Sagitta, meaning “arrow”. An image of an arrow piercing a body and passing from front (anterior) to back (posterior) on a parabolic trajectory would be one way to demonstrate the derivation of the term. Another explanation would be the notching of the sagittal suture posteriorly by the lambdoidal suture—similar to feathers on an arrow.
“Thus, the Sagittal axis is the axis perpendicular to the sagittal plane, i.e. the sagittal axis is formed by the intersection of the coronal and the transversal planes. Abduction and adduction are terms for movements of limbs relative to the coronal plane.”
From the Wikipedia article.