Here is a basic summary of what DTI is all about, and what some of those DTI parametric maps represent.
A one-page cheat sheet is at the end.
What is diffusion weighting?
We use magnetic field gradients to do useful things like encoding. But they also cause dephasing of signal, which we don’t want to happen (this is because when a gradient is on, there is a range of precessional frequencies along the gradient, even within a voxel). Diffusion weighting uses this dephasing effect to our advantage, to show where diffusion occurs.
How does diffusion weighting work?
Simple. A gradient is switched on—a big one to cause lots of dephasing. Then another gradient is used to completely undo all of the dephasing caused by the first gradient. We should end up with no effect, right? Right—but only if tissues and fluids are stationary. If there is motion—including microscopic diffusion of water molecules—then the dephasing caused by the first gradient is not “undone” by the second because the water molecules experienced different a gradient strength from the first gradient to the second, because they moved. As a result, the dephasing stays and we get signal loss on a diffusion weighted image.
So what are b values?
b values are actually a neat way of summing up into one parameter how much dephasing we are going to allow (size of diffusion weighting gradients etc). We need images with different b values to be able to work out diffusion-related parameters such as the ADC (see below). Note that we can’t say what diffusion is from the signal intensity alone, but from the signal intensity loss, which is why we usually acquire a b=0 (baseline) image to compare with.
So what is DTI?
We can only see that good diffusion exists (because of signal loss) along the direction in which a gradient is applied. So if we want to know the diffusion in all directions, we have to get many diffusion weighted images with diffusion weighting gradients in different directions. Ideally “all directions” would mean every possible direction on a sphere, but in practice we do, say, 12, 16, or 32 gradient directions (or more). The actual choice is up to the you. The minimum number of directions we can get away with is six—for example, one diffusion weighting anteriorly, one posteriorly, one superiorly, one inferiorly, one to the right and one to the left. That just about covers 3D space. Of course the diffusion in the brain is not always going to be exactly along one of these directions, so that’s why more directions are often used.
Showing DTI Data
Diffusion Tensor Imaging (DTI) collects information from all the diffusion weighted images (in however many directions was chosen) and tries to sum up all that information about where water can “diffuse to” in each voxel. DTI uses an ellipsoid (a stretched-out sphere, if you like) to represent where water can go. A long thin ellipsoid means very good diffusion for water along the long axis of that ellipsoid. A sphere (not an ellipsoid any more) means the same diffusivity in all directions. The mathematical way of describing the ellipsoid for each voxel is the tensor.
Sorry, what? A tensor?
The tensor is the maths part of DTI. If you like, just think of the diffusion ellipsoid when someone refers to the diffusion tensor. However, it is useful to know that the parametric maps which we produce in DTI (which are images where the pixel values represent some parameter other than signal intensity) are derived from the maths that are used to describe the tensor/ellipsoid at each voxel.
DTI Parametric Maps
It would be nice to draw little 3D ellipsoids at each pixel location, right? Unfortunately that wouldn’t make clinically readable images! So a number of parameters are used which relate to diffusion. There is a choice because the best one for every clinical situation isn’t yet determined. So you can choose. Some examples of parametric maps are now discussed.
Isotropic Image:
If we simply average all the diffusion weighted images which were acquired in all the directions, an image is produced which gives some sense of the total diffusion taking into account all directions. But this Isotropic Image is not generally used clinically, because the ADC is a better map of average diffusion, because the Isotropic Image is affected by T2 shine-through.
ADC: Apparent Diffusion Coefficient (or Constant)
The ADC map shows the average diffusion-freedom water molecules have in each voxel. This parameter can be calculated from all the diffusion weighted images which are acquired in DTI. Note that it is an average of all directions acquired, when performing DTI. The “A” for Apparent is there because the ADC is affected by partial volume averaging, perfusion, and some measurement errors. The average ADC map is sometimes called the trace map, which is to do with the mathematics of how it is calculated. It is not the same as the Isotropic Image; the ADC uses the mathematics of the tensor (the sum of the scalar values of the eigenvalues of the tensor, divided by three (which is the trace/3), sometimes called simply the “trace” image, but let’s not get into the maths now). The ADC is a useful DTI map.
On an ADC map, good diffusion is bright. This is opposite to the diffusion weighted images, where good diffusion is dark. The ADC removes the effect of T2 shine-through.
eADC: enhanced (or exponential) Apparent Diffusion Coefficient (or Constant)
The eADC shows the attenuation of the signal due to diffusion. On an eADC map, good diffusion is dark, just like the diffusion weighted source images. This is opposite to the ADC. Like the ADC, the eADC also removes the effect of T2 shine-through. Clinicians can choose to use ADC or eADC maps depending on whether they want contrast to match (or be opposite to) the diffusion weighted source images.
FA: Fractional Anisotropy
“Anisotropy” refers to how restricted diffusion is. An = not; iso = the same; tropic = direction (from Greek tropos “turn”). So anisotropy means “not the same in all directions”, which is what we are trying to find out about the diffusion of water molecules in each voxel. The ADC and eADC just communicate information about the diffusion in a voxel, whereas anisotropy maps go one step further and communicate information about the orientation of the underlying structure of the fiber tracts in the brain.
There are a number of ways of calculating (and thus, describing) anisotropy. FA is the main one. FA (and RA and VR, below) are rotationally invariant, which is important, because it means that the FA values produced wouldn’t be different if all your diffusion weighting gradients were rotated a bit, or if the patient was in a different position.
RA: Relative Anisotropy
RA is similar to FA, but it is a slightly different calculation (like FA it uses the scalar values from the tensor eigenvectors, but never mind about that now).
FA gives better detail. Use FA.
VR: Volume Ratio
VR is another calculated measure of anisotropy. The SNR and detail of VR is lower than FA and RA. The one thing VR has going for it is that the contrast between regions of low and high anisotropy is stronger than FA or RA.
Dave,
June 27th, 2008, at 11:41 pm #I must admit you have lost me with the technicalities but I am really interested in DTI. My daughter (10) has Global Developmental Delay (mental disability) and cannot talk, in nappies but mobile. She had MRI at 2 -3 years with negative results (all clear) but we know that there must be a problem in her skull / brain. I could carry on for hours about this but to summarise I would really appreciate some info on the main differences between MRI and DTI in terms of what results DTI can throw up that MRI cannot? I have actually contacted University of York who have a DTI scanner ..they were keen to help but life gets difficult because we need an anaethestist etc. Anyway, thanks for your time.
Neil Wood
Hi Neil,
DTI is a way of processing carefully acquired MRI images, to produce (calculate) new images. These calculated images give clinicians information about which parts of the brain are connected up, which can be compared with “normal”.
Another type of MRI is fMRI (functional MRI) in which the patient performs a task (e.g. looking at colours on a screen, or finger-tapping, etc) and from the MRI images it is possible to see which parts of the brain “light up” (are active). fMRI is not suitable for anaesthetised patients (they can’t perform the task) but I thought I would mention it so that you don’t confuse fMRI with DTI.
Finally MRI provides traditional structural information (e.g. grey matter and white matter distribution), and can be used to provide more information about blood supplies in the head (a contrast agent is injected in the arm and images are taken when it arrives in brain tissue).
Hope this helps.
June 28th, 2008, at 9:34 am #Dave
I hope you may be able to help, after a lot of research I have come by DTI on the net and there after have found this site. My partner is 45yrs of age and for the past 20yrs roughly has suffered what consultants feel is a form of epilepsy. His symptoms present with disorientation and confusion,sweating,panicky disturbance with sleep and twitching to name some. As a early teenager he had been sent to a few children’s centres for a few months at a time for his behaviour. He became addicted to glue around that time and continued sniffing for around 7yrs. His early twenties is when he first started developing his condition. It was apparently either just prior or after that he found his favourite uncle dead. He continued to have these episodes which eventually were becoming more often and lasting longer. He is currently coming out of one that he has been in for just short of 3 weeks. In the past sometimes it has just been a day but that was going back about 2yrs ago. Due to his condition, when his mother died he felt unable to attend the funeral which was about 16yrs ago. Roughly 2yrs ago whilst living at home with his father, he returned home from a evening out and found his father dead. The reason I am giving you this background information,is that when he has an episode he often not knowingly will speak aloud or a incoherent jargon and the understandable dialogue he is remembering father etc and feels his father, uncle are not dead. He has had numerous EEG’s and a couple of MRI’s which have not detected any abnormality. He throughout the years has been prescribed various AED’s and anti-pyschotic medications but continues to suffer he seems resistant to the medication. Do you think suggesting to his consultant a DTI scan,and if so we live in South Wales, UK where would be the nearest DTI centre or have you perhaps heard of similar cases? I would be most grateful for any information as his quality of life and ours as a couple is greatly affected,especially due to the frequency(last one being nine weeks ago till this one)and length of time of his seizure/episode? Many thanks in anticipation.
September 28th, 2008, at 6:40 pm #Hi Ruth,
This website is about the physics of magnetic resonance imaging (MRI), of which diffusion tensor imaging – DTI – is just one part. I doubt readers of this site have any medical advice to offer. I can talk about how DTI works (the physics), but not about its clinical utility for any particular patient.
I do know that the National Society for Epilespy may use MRI scanning on patients to see what is happening during an episode, but I can’t say whether this is clinically relevant to your partner. Most hospitals have MRI scanners these days, but not all do DTI. It would be best to discuss with your partner’s consultant whether DTI is relevant for his condition, before contacting MRI sites directly.
Sorry I can’t be of more help.
Dave
September 28th, 2008, at 7:55 pm #Hi Dave – Nice review site. Is eADC map really just an “inverted” ADC map, or is there more mathematics to it? I find the eADC most helpful, but many colleagues don’t understand it ( neither does Philips Medical Imaging – they don’t know what I’m talking about). The eADC map should be more specific than the b0 diffusion image, I would expect. Thanks! Jan M.
January 9th, 2009, at 1:16 pm #Hi Jan,
January 12th, 2009, at 10:47 am #The ADC and eADC are different calculations. Both remove T2 shine-through, which is where T2 or T2* contrast gets into the DW image because the TE is too long. The T2 shine-through can invert the diffusion contrast (bad!). The problem can be that large diffusion gradients push out the TE.
The ADC map uses an image (S) with large diffusion-weighting and another image (S0) with zero (or close to zero) diffusion weighting. Then using S=S0.e^(-bD) an average ADC can be calculated for each pixel. Alternatively multiple b-values can be used, to acquire multiple images, and a linear regression is used to extract D from S=S0.e^(-bD). (Slight modifications can be made to this method if anisotropy is suspected.)
For the eADC, the ratio of the S image (with the diffusion gradient) and the S0 image (without a diffusion gradient) is calculated. This gives an image which is weighted by e^(-bD). In this way T2 effects are removed without explicity calculating the ADC.
Dave
the eadc can be used in all districts body? or is mainly specific for the brain?
Thank
March 4th, 2009, at 10:48 am #I am curious about why DTI is used so infrquently by clinicians. I have been told that it is fairly difficult to analyze DTI image sets (comparitivly to regular MRI image sets). Is this true?
April 3rd, 2009, at 3:22 pm #Thank you
April 23rd, 2009, at 5:23 pm #Thanks a bunch
April 23rd, 2009, at 5:24 pm #Trying to figure out DTI measurement/coefficient rationale… when would you want to use mean diffusivity (MD) or ADC over or in conjunction with FA? And can you speak more about what axial and radial diffusivity tell us (about white matter degradation?) Thanks!!
July 3rd, 2009, at 12:35 am #Is an image with b=0 DWI? or is it just a T2-W?
thanks
January 10th, 2010, at 5:33 pm #@kostas
January 11th, 2010, at 11:57 am #An image with b=0 has no diffusion weighting, so it is no more “diffusion weighted” than a normal MR image. Whether it’s T2 weighted or not depends on the echo time. (DWI images are often T2 weighted because the diffusion gradients are large and a longer TE is required to fit them in).