Practical MR Protocols Abdomen: The Basics

Introduction

Good morning everyone.

My name is Ivan Pedroza from UT Southwestern in Dallas, and I'm gonna be speaking about MR protocols for abdominal imaging.

UT Southwestern, many other institutions, we have multiple MRI scanners from different vendors and different fields.

So managing abdominal protocols becomes a challenge, as you can imagine.

And the key to manage those protocols is really to standardize your clinical protocols.

And that's what I'm gonna try to emphasize today.

Types of Studies and Scanners

The first question is what type of studies we do and what scanners?

And if you standardize your protocols, you'll be able to do pretty much everything everywhere like we do, perhaps with exception of studies that require a very large field of view, like Mr. Enterography, which we do only at 1.5, and we prefer the three T for end rectal prostate, mr, although you can do it at 1.5 as well.

Coverage and Resolution Considerations

The second question you may wanna ask is, how much coverage do I need to answer the clinical question?

What's the anatomic coverage?

What spatial resolution do I need to answer those questions?

And what type of contrast to know is ratio?

We need to be able to detect the pathology.

And it's important recognize that all these affect acquisition time and because for the abdomen breath hole imaging is the best approach to reduce respiratory motion.

In general, we think of acquisition time as being breath hole time capacity.

So it's important also to recognize that that fact when you're dealing with patients, obviously.

Abdominal Protocol Overview

So the abdominal protocol, as you can imagine, will be a combination of T one weighted imaging, T two weighted imaging, diffusion weighted imaging, and dynamic contrast enhanced mri.

T1 Weighted Imaging

So let's start with the T one weighted images, and you can start with the classic two dase and opposed phase imaging.

This is a gradient echo acquisition that takes advantage of the fact that the proteins of fat and water rotate in the magnetic field at a slightly different velocity.

And because of this, we can image these proteins when they are pointing in the same direction to have the same phase by choosing the optimal TE or echo time, or we can image those protons when they're pointing in opposite direction.

And what happens is when they're pointing in the same direction in phase, the signal in the image will be the added signal from fat and water pros.

Whereas when we are in the opposed phase, those boxes that contain both fat and water will have a cancellation of the signal because of the opposite direction of the vectors.

And you can recognize the post phase images with a classic India ink artifact or edge artifact that you see at the interface between water and fat.

For example, between the abdomen abdominal intraabdominal fat and the liver and swing.

You can also take advantage of those characteristics.

For example, in a adrenal nodule that drops in Cigna intensity on a post phase image compared to in phase image, you know that there is fat in this lesion and then you can characterize it as an adrenal adenoma.

Now it's important to recognize the critical importance of selecting the optimal tes for your endphase and other phase imaging at 1.5, the optimal oppose phase te is at 2.2 milliseconds, and the endphase is 4.4 milliseconds.

And if we choose these two tes for our images in that order, we know that if it is you see dropping signal intensity and opposed phase images, like in this case in the liver, you can tell with certainty that there is fat and water in the liver.

There is a fatty liver in the early days of three T.

This was a challenge with the early MR scanners.

First of all, three T things happen twice as fast.

So the first opposed phase D is at 1.1 millisecond.

The first enphase D is at 2.2 milliseconds, and the single decay happens much faster at three T than at 1.5.

The manufacturers had a challenge being able to collect the first echo at 1.1 milliseconds, and then some of them decide, well, let's choose to first acquire the enphase echo at 2.2.

Then they also had a challenge selecting the 3.3 opposed phase at t because it was only 1.1 milliseconds after.

And they said, well, let's go for the third echo of post face.

And what happens with this approach is that if you see signal drop on the post face image, you don't know if it's because there is fat in the liver or because there is single decay due to T two star artifacts.

So here you have an example where there's no question that the liver looks darker on the post face image compared to the endphase image.

However, if you look at the tes, this is not the optimal TE selection.

The opposed phase te is very long compared to the face.

And what happens is, actually this was iron, the position in the liver, you can see the liver is very dark on T two compared to the skeletal muscle, not fat.

So with the current MR scanners better grade in performance, higher bandwidth utilization, we can now achieve the same te order that we use at one five and collect first the post phase at 1.1 and the endphase after at 2.2.

So now there's no uncertainty.

If you see signal drop on the post phase image, you know that this is because there is fat in the liver.

But be aware that that can happen, particularly if you're using one of the all three t mr scanners.

Dixon-Based Acquisition

There's another approach based on Dixon based acquisition.

And what we're gonna do here is collect two echos and separate the signal fat and water mathematically.

And basically we look at the mathematical water only reconstruction, you're effectively looking at a fat suppressed image.

And each vendor has a version of this type of acquisition as you can see here.

And the advantage here is that in a single breath hole, one can acquire a 3D gradient echo acquisition and get these four reconstructions a water only reconstructions, which again is a fat suppressed image, a fat only reconstruction.

And then if you add those two, one gets an endphase image, and if you subtract this two, you get an out of phase image.

So one question is, can we use this in phase and a post phase imaging from A 3D acquisition instead of the two dase out of phase?

Well, if you compare this two in general, the 2D classic t two DT one enphase auto phase tends to have less artifacts in my opinion.

But the 3D gives you thinner slices because the 3D acquisition and no inter slice gap and also potentially a better tissue contrast between liver and spleen, particularly A three T.

So when do we use this T one in a in and out of phase from the 3D acquisition?

Basically, when we're looking for small lesions like for example, adrenal nodules, where the space over resolution can be an advantage, so we use in our adrenal protocol, but also we use the water only acquisition for all our dynamic serious now.

These Dixon based techniques provide more homogeneous fat suppression than the old fashioned 3D SPGR acquisition.

Finally, if one wants to quantify fat or iron, you can have modifications of these multi TE acquisitions, although typically you're gonna have to acquire these images with more echoes.

And some of these are now available in the market for fat iron is still a work in progress.

So here you have an example of the same patient image with the Dixon based technique and a standard 3D SPGR sequence with fat suppression.

And as you can see, first of all, there's more artifacts on the standard SPGR, but also fat will fail every now and then on these acquisitions, you can see that the fat is bright compared to the M Dixon acquisition, which provides very homogeneous fat suppression.

T2 Weighted Imaging

So moving along to the T two weighted imaging, the first question is what type of fat suppression you wanna perform since fat suppress T two weighted images represent a standard acquisition in our protocol.

Fat Suppression Techniques

So you can go for frequency selective fat suppression and this or fat saturation.

These techniques effectively destroy the signal in a range of frequencies that corresponds to the fat signal.

As you can see here, when we destroy that fat peak, the image becomes dark and the areas where there it's fat.

This approach is very simple to implement, but it has some challenges.

As you know, the frequency of these peaks will depend on the magnetic field that the protons experience in each location as dictated by the lot mower equation.

If you have an inhomogeneous magnetic fill, those frequencies are gonna shift in, and you may end up having either partial fat saturation or potentially even water saturation with this type of technique.

So this is sensitive to B zero in homogeneity, but also because we have to apply a flip angle typically in the order of a hundred, 110 degrees, is because of in homogeneity in the B one field, we may also end up not crossing the no point at the optimal time, and we don't get good fat suppression.

So an alternative to this is to use a frequency selective inversion recovery technique instead of saturation, in which case we apply a 180 degree instead of this other flip angle.

And this tend to be more robust, less B one sensitive, although there's still sensitivity to B zero heterogeneity.

A third approach is to do a non-selective fat inversion.

Instead of using frequency selective fat suppression, we use an IR that inverts the signal of both fat and water.

And then after that inversion, we wait for the fat to cross the null point and perform the imaging.

One of the problems with this approach is that this is not fat selective.

Any tissue that recovers at the same rate as the fat and it crosses the null point more or less at the same time, is gonna be saturated.

And I'm gonna show you an example of this in a minute.

It's also important that you need to adjust in version time because of the magnetic field change in T one, as the magnetic field gets lo larger, the T one gets longer.

So let's see how this affect our tissue contrast.

So here we have a standard single sofa ache on the left and with fat saturation you can see.

And a fat, this is actually a multi-shot technique on the right, which affect also the contrast.

You can see that the lesion is better seen with fat saturation than without, but if I saw you a stir image, an inversion recovery, and the same patient done at the same time, you can see that the image is better seen on the stir acquisition.

You can also appreciate that the fat suppression is more homogeneous on the stir.

Again, you don't have P zero problems, which you have when you apply the frequency selective fat suppression near error, for example.

So you could conclude that the story is better in this case, however, this is the problem that I referred to before.

This is a patient with a known cirrhosis, and there is a mass next to the gallbladder, which is bright on T one relative to the liver.

So that means it has a sore T one, so it's gonna be crossing the null point closer to fat than to liver probably.

And the stir shows dark signal on this image.

So you will erroneously conclude that the T two in this lesion is dark, because what's happening is there is actually a competing effect between the T two of the lesion and the T one saturation caused by the inversion pulse.

When we perform a standard fast spin echo T two with frequency selective fat suppression in this patient, you can see that the lesion is indeed brighter than the liver.

So the third signal is really not real.

This was indeed an hepatocellular carcinoma.

Single-Shot vs. Multi-Shot Techniques

The second question about the T two weighted imaging is, do we wanna use single-shot fast spinco techniques or multi-shot techniques?

Well, all these acquisitions, first of all, are multi slice acquisitions, and it's important to just review how these are acquired.

In the ideal world, the RF pulses will look like this square perfectly tight.

In reality, they look more like this, like guian curves, and there's overlap of the RF pulses which results in crosstalk.

And this is why we like to apply an inters slice gap to avoid signal saturation.

Of course, this will decrease the through plane resolution.

You can acquire two separate breath holes of interleave acquisitions or slices, and then put them together into a single acquisition.

This is one approach to avoid that problem to get contiguous slices, but of course depends on the repro reproducibility of the breath hole from the patient.

With a single shot technique, which you have two names here from two manufacturers, there's basically the same kind of sequence.

We can apply a 90 degree excitation pulse, a single pulse, followed by an echo train of 180 degree pulses.

And this is a very fast acquisition because it's a single shot.

And also because we're gonna take advantage of the symmetry of case space, we're gonna acquire only slightly over 50% of case space data and reconstruct the other half based on this symmetry.

So we're basically cutting the acquisition time by a factor of two.

One thing to remember is that with echo train imaging, in other words, whenever you acquire multiple one eighties after a single RF pulse, the longer the echo train, the the shorter the echo train, the better the signal to noise and the sharper is your image.

So in that way, the single shot technique is the worst case scenario.

You have the longest echo train, so you're gonna have lower single to noise and blur images compared to multi-shot acquisitions.

However, there are other considerations to be made when you acquire multi-shot to these slices.

You can acquire these images in an in leaf fashion, as I'm showing you in this diagram where you acquire all the images basically simultaneously.

So if the patient moves or if there's any kind of respiratory motion or physiologic motion, it's gonna affect all the images in your acquisition where single shot techniques not only are acquired very fast at about a image per second, but they're also acquired sequentially.

So this provides a motion insensitive approach in many ways for abdominal imaging.

When the patient moves, you still get pretty optimal image quality.

So again, this is the challenge, right?

Do we go for better contrast the noise.

So you can see this lesion better on the multi, multi-shot technique than in the single shot, but you can see artifacts across the image, and those artifacts are actually sometimes as severe as st in a patient with ASEs even when they're doing a good job with their breath hole.

So in our experience, the single shot provides a more robust technique and we use those with face saturation, because we rely more on diffusion and contrast enhanced techniques to detect the lesion, we wanna characterize the T two signal on the single shot.

Again, even with severe motion, the single shot techniques will still provide diagnostic images, which you cannot do with a multi-shot technique in the axial plane.

You may need to pay attention to the abdominal viscera to recognize motion because again, the image quality will be preserved, but you can see how the anatomy is jumping a little bit.

So in effect, there are skipping areas of anatomy because the patient is moving while those axial images are acquired.

But this has an issue solution.

You can use respiratory ga triggering.

It only prolongs a slightly the acquisition time, and you basically ensure that the slides that acquire contiguously and you don't skip any areas of anatomy.

So in conclusion for the fat saturation, if you have a spare technique, particular three t that provides more robust fat suppression, if you have spare technique, a spectral inversion recovery, that's also an option.

Stir in general is not used in our case unless we have a patient with metal in the body or something that we're trying to really get a homogeneous fat suppression.

In that case, stir may do a better job.

So for the type of acquisition, again, we rely on the single shot technique with fast situation, not on the multis slice acquisition.

MRCP Techniques

What about MRCP?

You can acquire these images with the same type of sequence with a single shot, fast pin echo, and you can acquire this with a relatively short te about 60 to 80 milliseconds, and that will give you great anatomy and you still see the bile ducts very well, or you can perform the same sequence with a very long te about 6, 7, 800 milliseconds.

And effectively what's gonna happen is that everything that has relatively short to two is gonna get saturated and only fluid fill structures that had a very long tude will remain bright on your image.

And that's how we see the bile ducts.

So I refer to those as the 2D thin slice technique.

And this is the 2D thick slab technique, although some people call the ones on the right radial acquisition.

And the reason is, the way these are typically acquired, we center in the common bile duct in the mid common bile duct just before it enters the pancreas.

We rotate this to get a coronal and a couple of obliques, and then these are acquired sequentially, as you can see here with providing different projections of the biliary system.

And eventually you can overlap some of the CSF and kidney, et cetera.

It is important also to recognize that you have to acquire these in separate breath holes if these are very fast.

So the tendencies for the technology sometimes to say, okay, I can acquire all these five in a 15 second breath hole.

The problem is, if you do that, you don't give time for the single to recover, and then your first image will look fine.

But then the second, third, fourth and fifth will be saturated and you won't get good image quality.

So please remember, do a separate breath hole for each one of these acquisitions.

You can also acquire MRCP images with a 3D technique.

These are true 3D FFSE acquisitions with the slices in the AP direction.

We prefer the non-real approach because it gives you better SNR and better coverage.

And you can reconstruct these using volume rendering meep or any other reconstruction that you may need for diagnostic purposes.

And there's one scenario where I find this specifically helpful, which is in patients with ASEs, when there's a lot of fluid in the abdomen, it's extremely hard to find a good 2D fixed lab projection MRCP as one on the left because of the fluid overlapping the signal in the bile ducts.

With a 3D acquisition, if you reconstruct the entire acquisition, you will have the same problem, but because it's a 3D, again, you can reconstruct the data, eliminating all the surrounding tissues and just selecting the area of interest like you do for an MRA, for example.

And in this case, you can appreciate now that there is an aran right pad duct draining into the common paddock duct, which only in retrospect I think you can recognize on the 2D projection image.

But the question is, is this worth doing all the time?

The 3D acquisition is about five minutes.

The 2D projection is about two seconds.

So you have to really select what indications you wanna do the 3D acquisition because it takes magnet time and not always provides additional information.

Diffusion Weighted Imaging

Moving to the fusion weighted imaging, there's gonna be a full talk on this later, so I'm not gonna spend a lot of time, but just very briefly from the technical point of view.

DWI relies on the application of two diffusion sensitized in gradients of the same amplitude, but different sign.

So pros that are that are a static will get defaced by the first gradient and then fully rephrased by the second gradient.

And you see no signal loss after the application of both gradients.

In contrast, proteins that move locally will get defaced by the first gradient, and they don't get fully rephrased by the second gradient, and you see a signal loss.

So that signal loss is proportional to how much those water proteins move.

And we can plot that signal over time as a function of the strength of the diffusion gradients.

And you can see that the tumors will decay slower in signal intensity compared to normal tissues because there's more membranes, more cellularity, more barriers for those those water molecules to move.

So the decaying signal is lower.

So that's the contrast that we achieve in diffusion wave imaging between cancer and non-cancer tissue.

In my experience, one has to be careful when using diffusing in the abdomen for characterization.

It is good to do first a quality check to be sure that all your B values were acquired exec exactly at the same anatomic level, because even with respiratory gating techniques or even with navigator techniques, sometimes those images, those different B values are not exactly at the same level.

So you're relying on a mathematical calculation of images were not acquired on the same level.

You can go to get to erroneous characterization of the lesions.

However, each individual B value will provide you contrast for lesion characterization for lesion detection, sorry.

And that's why division where imaging works so great for detecting lesions.

This is a patient with breast cancer and we look carefully, you can start to see some tumors in the liver.

There are metastatic sites, another one posteriorly, another one segment four, maybe two of them together, another one on the left.

So it takes some effort to find in some of the post contrast images, actually quite difficult to find those lesions.

However, on the diffusion with imaging, you can see these lesions from across the room.

So again, that's the value of diffusion.

You can see these lesions very, very well.

The contrast is phenomenal.

And even a per metastasis that is also present on the ttu.

Dynamic Contrast Enhanced MR Techniques

Now I'll finish with the dynamic contrast enhanced MR techniques.

And for these we're gonna use 3D fat saturated spoiled grading echo images.

These acquisitions can be reconstructing virtual, any plane with multiplanar reformations, you can also do vascular reconstruction.

So this is very helpful when you're looking for vascular pathology.

In the same exam, We use a single dose of gadolinium.

We use macrocyclic agents to reduce the risk of NSF.

We weight the dose.

We use a single dose, which is based on the patient weight.

And we floss with 26 of saline at the same rate, and we inject the two per second.

Our contrast.

Now, one important QA issue as well is how do you get a consistent arterial phase in every patient that has no respiratory motion?

And it seems like something we should be doing well by now, but I found that so things have trouble sometimes getting this.

And that's because we don't again, we don't have a standardized protocols to do this routinely.

So I'll show you the approach we've taken.

So what we do is we ask the text to really include the heart, always in the Mr Fluoro acquisition.

When the contrast reaches the left ventricle, as you can see here, at that point, we ask the text to start the breathing instructions, and we give two sets of breathing instructions.

It's been shown that more than one set of breathing instructions will increase the breath of capacity up to 30%, which is huge if you're talking about the pacing.

They can only do 12, 15 to 15 seconds.

And after those two sets of breathing instructions, then we acquire the arterial phase.

And this provides a very consistent arterial phase with no respiratory motion in my experience.

So we acquired pre contrast images, arterial phase, portal phase, the late venous phase, and then an equilibrium phase.

Two minutes later,

Hepatobiliary Agents

I'll just touch on the bil agents.

Again, there's been there's gonna be more discussion of this later, but of the two that we have available, these are the indications we use for metastatic disease characterize FNH versus adenoma.

And to do functional quote unquote MRCP acquisitions, we use vis.

And the reason is because you can achieve the pbil phase in 20 minutes with BTA at least 40 minutes, frequently over an hour.

So it's is more doable in clinical practice.

You can inject your pili agent as you do with the rest of the gadolinium agents dynamically in the arterial portal venous phase.

But then at 20 minutes you get your equitability phase and you can see the contrast screening in the bill system as well as in the collecting system in the kidneys.

Also important to remember, you wanna, you may wanna switch the order of acquisitions to be more effective with the scan time.

Instead of using first, instead of acquiring first T one and T two and deficient wave images followed by contrast, which is what you do for an abdominal scan, we change the order, and we acquire T one in that phase.

If we have to do an MRCP, we do that too.

And then right after that we do the dynamic contrast enhanced techniques, and we follow that with the T two and the diffusion, and then the para bilar is, so by doing this, you're basically acquiring the T twos and diffusions while you're waiting for the paraia phase.

And you can reduce the scan time.

Total scan time within 40 minutes is very doable.

The biggest advantage of these agents is that you don't have a time constraint.

So with arterial andal venous phase, you have a very narrow window to acquire those images and acquire the entire liver.

And you're gonna see this kind of case.

If you do enough blood imaging, you're gonna have patients that cannot do a good breath hole despite all your good efforts.

And the advantage with EV is once you get the para billary phase, you have all the time in the world to actually acquire these images.

So we actually decided to split the para Billary acquisition.

Instead of doing a whole whole lever acquisition, we split into top and bottom with higher resolution per shorter of breath hold time.

So instead of your typical 18 to 20 seconds, we do about 12 second acquisition with high resolution of the top of the liver and then at the bottom.

And then we do the same for a coronal front and back.

And as you can see here, this is the same patients, same exam.

The dynamic series was completely non-diagnostic.

The probability phase, you can actually see very small metastasis in this patient.

They were completely missed in the rest of the dynamic series.

Conclusion

So in conclusion, remember you have different approaches for chemical safety imaging.

The 2D, the classic 2D in phase opposed phase images still are helpful in my opinion that 3D with thinner slices can have also a role to detect the small lesions.

The T two weight images, in my opinion, the single shot fat pin echo with fat saturation may have less contrast than the multi-shot, but they're much more robust and it work very well in every vendor, every scanner.

So we rely on those, CPS again with the same kind of technique, single-shot technique, and 3D part, even when you have ASEs in the abdomen is very, very helpful.

And then really the three DS pgr is the main way to go for the dynamic contrast enhanced techniques.

And we have switched to Dixon based approach because of the improved fat saturation.

And if you're gonna do e of these, remember to swap the order of the acquisition.

So the Ts and diffusions are acquired after the dynamics.

So you can actually save some time in your scanner.

And with this, I would like to thank you very much for your attention.