Thread by @JayKinariwala

Ever wondered how MR Spectroscopy (MRS) is acquired & how to interpret it? Have you ever come across funky looking zigzag graph with MRI & scratched your head?

If interested in learning BASICS of MRS, follow this 🧵

MRS for non-radiologists
#EmoryNCCTweetorials @MedTweetorials

First available since 1980, 1H-MRS is noninvasive technique uses proton signals to determine relative concentrations of tissue metabolites & thereby acquiring data about chemical composition of a tissue.
13C MRS
23Na MRS(neurocognitive,brain tumor research)
31P MRS(🫀&💪🏻research)

1H-MRS:
-Most widely used
-Performed on 1.5, 3 & 7 T
-Only adds 5-10 min of exam⏰

So why not do it with every MRI?
-Limited value &⬇️specificity
-Most of the time,history & MRI is just enough
-Adding 5-10 mins for every MRI⬇️productivity w/o significant gain in diagnostic yield

That being said, on the verse side, MRS has capability of detecting abnormality even when MRI sequences appear normal - e.g. leukodystrophies / mitochondrial disorders in children, where early WM changes are barely appreciated but MRS can detect it with a reasonable accuracy.

Purpose:
-ID tissue composition in Region of Interest indicated by Voxel (3D;like Pixel for 2D)
-Differentiate between different pathologies based on tissue composition when MRI is similar appearing like High-low grade glioma/metastasis/abscess/radiation necrosis/tumor recurrence

This is what a spectrogram looks like.
Components:
X axis=resonance frequency of metabolite (ppm)
Y Axis=Height of molecule peak depends upon concentration and available 1H
AUC=concentration of metabolite
Right side boxes indicate the Voxel,from which the spectrogram is obtained

MR spectrum=signals from different metabolites ID by unique & highly reproducible frequency distribution

Frequency of metabolite resonance(ppm)=electronic shielding.

Molecules with covalent bonding are coupled and results in doublet peak. E.g.Lactate,Creatinine,Alanine,Lipid.

Acquisition:
Generally 3 RF pulses are used at 90, 180 and 180 degree in orthogonal planes f/b listening to the signals (TE on x axis) due to Free Induction Decay (y axis)👉🏻Fourier transform👉🏻spectrogram

You can👂signals at any time after beginning of decay which determines 📈

TE=Echo time
Shorter TE=⬇️Free induction decay👉🏻⬆️signal heard.

Short TE=20-40 ms-shows more metabolites but has fluctuating baseline.

Intermediate 135-144 & very long 270-288)-better baseline but shows less metabolites - Cho, Cr, NAA but does not show Myoinositol, GLX & lipids

TE matters:

Here is the spectrum form same voxel at different TE.

Short TE=more metabolites but fluctuating baseline, artifactual ⬆️of NAA due to overlap with elevated Glx peak

Longer TE=flatter baseline but less number of metabolites seen, artifactual ⬆️in Cho peak

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Magnet strength matters:
⬆️magnet strength has better defined peaks and low signal to noise ratio, gives better diagnostic yield.

Below is comparison b/w 1.5T & 3T

So if you want to see more metabolites with better signal to noise ratio, go for 3T and short TE.

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Types of MRS:

-Single voxel(PRESS)
-Multi voxel(MRSI/CSI)

Which technique to use?
It depends upon what is the purpose of spectroscopy in that particular patient.

Single-Identify metabolites in small area for diagnosis
Multi-Compare metabolites over larger area for comparison

Single voxel (3 dimensional)
Uses 3 orthogonal slice selective pulses to select a signal from particular voxel where they intersect. The rest of the signals are removed.

Typical voxel size is 4-8 cm3

However, single voxel MRS does not provide spatial variation of metabolites

Multi voxel (2 dimensional)
Signal extraction of restricted region using PRESS sequence in 2 dimensions instead of 3.
Hence,you get a plate of voxels(think of it as flattened cube) 👉🏻 used to extract data of each metabolite to create color coded graphs based upon concentration

Why not get Multi-voxel when you can get more info?

Most of the times single-voxel is sufficient to make diagnosis

Multi-voxel takes longer⏰ to acquire

Multi-voxel MRS is more helpful when one wants to know proliferating part of tumor or differentiating tumor v/s recurrence

E.g. look at the scan below, on T1+C image, one can’t say what is the active growing part of the mass.

But multi-voxel MRS for Choline reveals red areas with ⬆️choline=indicating rapid proliferation.

T1 hypo intense area is probably the least proliferative, necrotic part.

CAUTION⚠️
You must suppress Water to see rest of the metabolites

If not👉🏻significantly dwarf other molecular peaks which we are interested in due to ⬆️⬆️concentration of water

Metabolites you can’t see with MRS-Dopamine,Serotonin & Ach- Conc <0.1 mmol/L & Proteins and enzymes

Spectrogram w/o water suppression:

Water resonates at 4.7 ppm and it is in the highest concentration👉🏻tallest peak👉🏻making other metabolite non-recognizable.

Poor water suppression is also suboptimal.

Look at the spectrogram below.
Source: @Radiopaedia

Now the most important, Learning metabolites:

-ID metabolite based on resonance frequency on x-axis (ppm)
-look at the height & shape of peak
-based on overall appearance of different metabolite peaks, differentiate underlying tissue

Here’s the table to remember for metabolites

Pitfalls:
-Lactate doublet is inverted w/ long TE
-Alanine doublet can be obscured by Lactate👉🏻alanine-specific for meningioma
-Glx can ⬆️NAA falsely at short TE. Compare w/ long TE NAA bcz Glx not seen on long TE
-Unlike Lactate, Lipid doublet doesn’t get inverted at long TE.

Voxel Location matters:

Spectrogram can look slightly different based upon the location.

More gray matter-⬆️ Cho and Cr, ⬇️NAA
More white matter-⬆️NAA

See below, examples of spectrogram from the same patient with different locations.

MRS ratios:
Sometimes, ratios of various metabolites is used rather than absolute concentrations.

Helpful in differentiating from low to high grade glioma

-NAA/Cr(abN <1.6)=viability of the tissue
-Cho/Cr (abN >1.5)= indicates cellular proliferation, used to grade neoplasms.

Cho/NAA(N=0.6) >sensitive and specific than NAA/Cr & Cho/Cr ratio to differentiate gliomas

Hunter’s angle(N~45):straight line drawn through all 3 major peaks (Cho,Cr,NAA) & its angle w/ horizontal plane is called HA

Reversed angle seen in tumors,radiation necrosis, abscess etc.

Take 🏠:
-Identify single / multi voxel
-look at 🧲 strength
-look at TE-short/long
-look at location of voxel & normal spectrogram for that location
-be aware of pitfalls listed before
-ID metabolites & concentration
-look at history, MRI and at last MRS, not reverse.

End of part 1 of this #tweetorial

In the next part, we’ll look at different patterns in different disease process like infection, tumor, demyelination, stroke & arrest & clinical application of MRS on real life scenarios based upon content above.

Till then, stay tuned…

@EmoryNeuroCrit @Capt_Ammonia @caseyalbin @CajalButterfly @sigman_md @JimmySuhMD @mallyaa @EricLawson90 @RyanBPetersonMD @teachplaygrub @a_charidimou @ksuchdev @SrinivasMeghana @rohitmarawar @neuro_intensive @drharunyildiz