Week 6 - Aaron LaViolette

The highlight of this week for me was getting to see the MR machine in action! Dr. Prince was working out an imaging sequence protocol to measure flow in the aorta, inferior vena cava (IVC) and superior vena cava (SVC). The goal is to look at the flow in these vessels and the cardiac output of the heart.

Dr. Prince wants to examine the effect of strenuous breathing on the cardiac output. In particular he wants to compare “normal” patients and chronic obstructive pulmonary disease (COPD) patients. In order to access this, Dr. Prince will have the patient breathe at 40 breaths per minute (BPM) for thirty seconds before imaging. Then he will compare the imaging after this breathing with imaging done before. In order to get the patient to breathe at 40 BPM, Dr. Prince will send the audio from a metronome into the scanning room where the patient is, and the patient breathe with the metronome. Because the “kinks” needed to be worked out of the protocol, we scanned a volunteer (another researcher who works with Dr. Prince) when the magnet was not being used on patients.

One important thing (which was a learning opportunity for me) was how MR safety. Obviously with a large magnetic field metal is problematic. Additionally, the magnetic field is strong enough to wipe the magnetic stripes on credit cards. For this reason, we left keys, wallets, phones, etc. behind when entering the magnet room. I already knew this. What I got to learn is that an MR factify is organized into different zones for public safety. The zones are numbered 1 to 4 with more potential danger the higher the number. Zone 1 is a publicly accessible area typically for reception. Zone 2 is where patients undergo MR screening. It will contain patient “prep” and changing rooms. Zone 3 is an area with potential MR hazards, but not the magnet itself. It includes the control room where the technologists operate the scanner. Zone 4 is the magnet (scanning) room (Wang, 2015).

I was also surprised to learn that the volunteer had to change into a gown. I was surprised by this because the volunteer didn’t have any metal clothing, but it was explained to me that issues in the past have made it so everyone being scanned has to where an MR safe gown. The volunteer was also given ear plugs, because the gradient coils can create a bit of noise.

Once the volunteer was ready we had to get the correct coil for the chest and arrange the posterior part of it on the table, as well as cushions for confront. The volunteer was placed on top of these and the anterior part of the coil on top of him. An ECG and respiration monitor where also hooked up to him. The reason for these is because of the motion caused by heart beats and breathing. If the MR acquisition is not set up to account for this motion, then the final image has what is known as “motion artifact”, or in other words, looks wrong because the patient moved while the picture was being taken. After this was finished we then covered the volunteer with blankets, because there was a lot of air moving through the bore (which would cause convective cooling making the volunteer feel cold).

After this we had to line up the patient with the isocenter of the bore. To do this the table was moved under a light on the edge of the bore that had cross-hairs. The table was lined up so the cross hairs where over the middle of the coil. Once the lineup was complete, a button was pushed on the scanner letting the machine know the table location where the coil was. With the push of a second button the machine moved the table into the bore automatically lining up the coil in the isocenter. At this point Dr. Prince and I went into the control room.

The control room is where the real action happens during the scan. Here the technologist (or Dr. Prince in this case), selects the sequences they want to use. In many cases a protocol with the sequences is already pre-programed. Dr. Prince is also able to adjust parameters such as the field of view, slice thickness, directions of phase and frequency encoding, etc. for the scans form the control room.

We were only able to get so far in Dr. Prince’s planed protocol, before we ran into trouble. With one of the sequences, the control room computer or scanner decided that it would not execute that sequence for whatever reason. We stopped scanning at that point. This to me emphasizes the importance to beta testing and was one of the reasons this was done.

We were able to measure some of the flow curves from the small bit of acquired data. This was done on another computer using a proprietary software. The software let the user trace the edge of the blood vessel in several images over time, and then would compute the flow.

This week also involved getting to see Dr. Prince look at a lot of autosomal dominant polycystic kidney disease (ADPKD) cases. All of these cases need to have to organ volume measured, and how this is done is mentioned in one of my previous blogs. Dr. Prince let me do this for several of the cases (all verified by himself or another MD of course). I was actually getting really close numbers with an MD which shows me it doesn’t take much to train someone. I was reminded that it is easy to train a human, but harder to “train” a computer.

One thing that Dr. Prince mentioned to me is that it is easy to overlook certain aspects of the body when you are so caught up in looking at the kidneys and liver. In particular he said it is easy to forget to look at the pancreas. This is of course the advantage of having a template to dictate the cases, (as he an all the other radiologists I have seen this summer do). Because there is a section that says pancreases, it acts as a reminder. To me this emphasized the importance of quality control.

I also spent time working on my research project that I wrote about last week. This mostly involved running statistical analysis on the data. The good news is there is a strong interclass correlation coefficient (ICC) between the raters. Although the sample size of mice is low, things look good for rapamycin's effectiveness. The timing of these results couldn’t be better as I gave a short presentation on it at the end of this week (the presentation was for all the immersion students, who each gave a brief talk about their project). Next week we plan to meet with a pathologist to discuss how we can match the MR with the histology.


References:

Y. Wang, Principles of Magnetic Resonance Imaging Physics Concepts, Pulse Sequences and Biomedical Applications. New York: CreateSpace Independent Publishing, 2015.