Thursday, January 10, 2008

Why Some MRI Scans Are Better than Others

A Ferrari and a Ford Pinto are both cars, but my guess is that you would much rather drive a Ferrari.

MRI scanners are a lot like cars- there are many kinds of scanners, made by different manufacturers, and some are newer and better than others. As a radiologist, I prefer scans from high-field (closed) scanners, over low-field (open MRI) scanners. I like driving Ferraris more than Pintos. Who doesn't?

You can’t change the laws of physics- there is more signal at high field, and this translates to better images and higher diagnostic confidence. Anybody that tells you anything different is trying to sell you something.

There is a wealth of experience at 1.5 Tesla and for most musculoskeletal imaging, this is a sweet spot, with well-developed coil technology and protocols. Three Tesla (3 T) scanners offer a boost in SNR, but coils are still being developed for this platform, and this is a relatively new technology. It will take some time before we can take full advantage of the inherent boost in SNR that 3 T offers. 3 T will have its biggest impact in the imaging of small joints and the imaging of hyaline cartilage.

Low-field (open) MRI scanners can give good quality images, but like high-field scanners, the hardware should be up-to-date, and the protocols massaged by a knowledgeable radiologist. In the end, however, one cannot compensate for the diminished signal at lower field strength. This becomes most noticeable when imaging small joints such as the wrist, and when imaging articular cartilage.

The biggest secret in MR imaging is that there is wide variation even among 1.5 T scanners. Important hardware parameters that have a large impact on image quality include gradient strength, the number of receiver channels, and coils that are available.

One should also not underestimate the importance of the “soft” side of this equation- the presence and active involvement of a dedicated radiologist that understands how to build and tailor imaging protocols. In the car analogy, you can think of the radiologist as a car driver. You want Mario Andretti driving your car.

An orthopedic surgeon recently told me that the two most important factors he considers when deciding where to send a patient for an MRI scan are: 1) image quality 2) accuracy of radiology report. This one is easy- the patient needs to go to a center with a MRI scanner of recent vintage, staffed by a dedicated, subspecialty-trained radiologist. Done deal.


Anonymous said...


Vic David MD said...

One cannot change physics.

High-field (closed) MRI has more signal, and if the radiologist knows what he is doing, that will translate to higher quality images. Makes a big difference in some cases, especially imaging of articular cartilage and small joint imaging.

There is a reason why the open MRI market is tanking- ask your MRI vendor for the sales figures last year for closed MRI vs. open MRI. Open MRI sales are diving towards the floor.

With that said, a high-quality open MRI read by an excellent radiologist can trump a closed MRI read by a mediocre radiologist. Like many things, depends on who is running the show....

Anonymous said...

I have to agree with anonymous... so let's talk physics... with increase of field strength we get increase of flow. The gradients are max-ed out, no matter what the strength, until we learn to ground the human body so we don't stimulate one to death. I agree we have just hit on 3T technology and yes there is a lot more to do with it, however I want the pinto - a modern day pinto. I can't afford gas prices these days so give me the equivalent of the pinto, please.

Then lets talk coil configuration. This will be where the future will go. Sure parallel imaging and channel config mean a lot. But you sound like an Open hater.. do you know how coils for opens work? Do you realize that scanning in isocenter can have it's advantages? The sweet spot can be a pain to achieve in any bore.

There is one company out there that's Open market is rising fast, faster than any other.

Our patients (customers) like the opens - and here where McDonalds is on every corner, we need to understand how to take care of those patients as well as the NFL players who can't fit in a 60-66cm bore. And honestly, I have seen some opens out perform a 3T when it comes to wrist imaging. True gray and white matter are more differentiated on a 3T. Spec is easier on a 3T. But motion artifact is worse, metal artifact is worse, true T1 weighting takes too long. The 1.5 is here to stay.... just like a ford truck. But sometimes I ride my HD Dresser - wide open spaces, wind blowing thru my hair, good gas mileage... when weather is right, it's great. Then we can look at my ol' 71 Cutlass with the possy rear end and 455 engine with nitro... dang, it's good speed and power and WOW! But can't afford to drive it now... and well, long trips are awful in it... and it doesn't haul anything worth a darn. Do you see my point?

What I am truly saying is there is a magnet that is better for every person, and every part. Opens have their advantages (I'm talking both mid and high field opens) as well do 3T. 1.5T is the standard. Most people buy standard. Some want to always buy the biggest, baddest, everything... I'm a woman... I'd prefer performance, any day! I'm also a realist... what I like isn't for everyone and I'm ok with that. I'm not the best or the fastest or maybe even the most efficient, but I do know where strength lie and my strength is knowing that currently with what the market is showing I can't afford to rely on just one field strength to do the job of many. vic david MD, good luck with that closed mind theory. Anonymous, i'd love to ping some open sequences off you sometime.

Anonymous said...

I would like to comment from a different point of view if I may. Be it driving Pintos or Harleys "in the wind", what I see driving image quality today seems to be simpler than all of the physics you speak of. Simply put, its money, as with everything. Specifically, the never ending battle between output and scan time. Each field strength you speak of has its advantages and disadvantages, but each can produce superb image quality if given the right situation. And lets be honest here, what really has the greatest effect on image quality is the quality of your coils (but that is a whole other story isn't it).
Lets be honest, how much say do most Radiologists have with the purchase of a new magnet? More often than not, very little at best. More often, you are trying to get the most out of what you have to work with already. So lets cut to the chase here.....
Scan quality boils down to Productivity vs. Scan Time. Each institution is struggling with the timeless question, "30 minute or 45 min time slots." Being able to run 5 or 6 minute scans instead of 2 nex 2 minute scans is what drives quality these days. The myth is greater field strength creates better scans in less time. (in the eye of administrators) But the reality is they only create quicker scans of the same quality or better scans of the same time length. Its a fine line creating better AND faster and often impossible.
I think we can all agree that the best images are created by those of you that share the passion and love for your work. Open mindedness and the willingness to continually improve and be involved in the scanning process (yes, that means collaborating with the technologist) will propel your quality to new heights. It is not simply increasing scan time and signal, or handing your tech a protocol you wrote down at some conference, but continuing to experiment and refine your sequences to be as efficient as we can in time limits imposed on us. Gone is the time of sitting in front of the work station all day and only getting up to go to the bathroom or get coffee. Here is the time to be involved. Your staff are not the only ones improving have a responsibility, for your own sake, to be involved as well.

Anonymous said...

Lets face it the open MRI was designed for the obese, hefty or overweight patient and OPEN MRI marketers have spun it to seem like it is for claustrobic patients. It is a diagnostic exam, why would you not want the latest technology in the shortest amount of time. I think most patients would prefer to make the choice themselves. Especially when their radiology may be farmed out to who knows where. 3T is the way to go and I hear from patients everyday especially ms patients and ortho patients who don't have to have a arthrogram because they had a 3T scan that they would never go back to the open mri and spend twice the time in the machine.