Understanding Brain Imaging

Daniel Lende and Greg Downey run the though-pro­vok­ing Neu­roan­thro­pol­o­gy blog. Daniel also teach­es a class at Uni­ver­si­ty of Notre Dame, and he asked his stu­dents to sub­mit group-based blog posts in lieu of the tra­di­tion­al final essays. He explains more on Why A Final Essay When We Can Do This?.

Below you have a spec­tac­u­lar post writ­ten by 4 of his stu­dents. They show how brain imag­ing is start­ing to pro­vide a win­dow into the plas­tic­i­ty (glos­sary here) of our brains, and how our very own actions impact them. For good and for bad.

Under­stand­ing Brain Imaging

— By Chris Dud­ley, Matt Gasperetti, Mikey Nar­vaez, and Sarah Walorski

Do you remem­ber the anti-drug pub­lic ser­vice announce­ment from the 1980s that showed an egg fry­ing in a hot pan which rep­re­sent­ed your brain on drugs?

Dur­ing the 1990s, brain imag­ing moved beyond fried eggs as com­put­er tech­nol­o­gy allowed researchers to process large amounts of data required for func­tion­al imag­ing approach­es. As a result, the PSA men­tioned above no longer pro­vides the most accu­rate anal­o­gy illus­trat­ing what hap­pens to the brain when exposed to drugs.

Today, brain imag­ing research has helped cre­ate a sophis­ti­cat­ed “dis­ease mod­el of chem­i­cal depen­dence relat­ed to changes in the func­tion of neu­ro­trans­mit­ters and recep­tors in the brain. These cir­cuits are respon­si­ble for reward pro­cess­ing, mem­o­ry and learn­ing, moti­va­tion and dri­ve, in addi­tion to con­trol (Nora Volkow describes these cir­cuits in a 2004 lit­er­a­ture review).

This par­tic­u­lar post focus­es on the tech­niques used most com­mon­ly to study the brain’s role in addic­tion and oth­er men­tal health prob­lems. We will cov­er the prin­ci­ple behind each method, advan­tages and lim­i­ta­tions of each, and pro­vide an exam­ple of the results that can be obtained.

Beyond the Fry­ing Pan: EEG and CT

Elec­troen­cephalog­ra­phy (EEG) and Com­put­ed tomog­ra­phy (CT) were two of the first meth­ods used to study the brain. EEG uti­lizes elec­trodes placed on the scalp that mea­sure elec­tri­cal impuls­es, where­as CT cre­ates a three-dimen­sion­al image of the brain with two-dimen­sion­al x‑rays.

EEG is a non-inva­sive pro­ce­dure with high tem­po­ral res­o­lu­tion; it is often used to record the brain’s response to a stim­u­lus (e.g. an indi­vid­ual ingests a drug and the change in brain activ­i­ty is recorded).

EEG is lim­it­ed because it can only record data from the sur­face of the brain. In addi­tion, EEG does not pro­duce images of the brain it only mea­sures elec­tri­cal impulses.

CT is used to cre­ate three-dimen­sion­al images. Unfor­tu­nate­ly, CT can­not pro­duce high-res­o­lu­tion images of soft tis­sue (i.e. the brain) and requires high lev­els of radi­a­tion. While CT is still used, pre­dom­i­nate­ly to cre­ate images of the body, it has to be used infre­quent­ly to avoid exces­sive radi­a­tion expo­sure. Although EEG and CT did not teach us much about addic­tion, these meth­ods were the tech­no­log­i­cal pre­cur­sors to more egg-cel­lent brain imag­ing methods.

Unscram­bling the Mys­ter­ies of the Brain with MRI and fMRI

Ever won­der what it’s like to have radio waves sent through your brain? Well, Mag­net­ic Res­o­nance Imag­ing (MRI) may be for you! MRI pro­duces high qual­i­ty images of the brain by using a large, cylin­dri­cal mag­net to cre­ate a mag­net­ic field around the head. Radio waves are sent through this field and alter hydro­gen nuclei in the brain. These detectable changes are sub­se­quent­ly trans­mit­ted to a com­put­er and used to gen­er­ate a series of images.

Using these images, sci­en­tists are able to deter­mine minute changes in the brain that occur over time by com­par­ing dif­fer­ent MRI scans.

MRI is use­ful because it can pro­duce high­er res­o­lu­tion images than CT scans and does not expose patients to exces­sive radi­a­tion. On the con­trary, MRI is lim­it­ed because there is no way to pro­duce high-res­o­lu­tion images mea­sur­ing tem­po­ral change.

This leads to the recent devel­op­ment of Func­tion­al Mag­net­ic Res­o­nance Imag­ing (fMRI). This tech­nique, devel­oped only recent­ly, has allowed sci­en­tist to use MRI tech­nol­o­gy to cap­ture images of var­i­ous brain func­tions.

Func­tion­al MRI focus­es on the flow of oxy­genat­ed blood with­in the brain. To sim­pli­fy things, when an area of the brain is stim­u­lat­ed, oxy­genat­ed blood rush­es to that area. Func­tion­al MRI is able to cap­ture this flow of blood because of the slight dif­fer­ence in mag­net­ism between oxy­genat­ed and deoxy­genat­ed blood.

This method is advan­ta­geous because it allows researchers to cap­ture a series of images every sec­ond. These images can be used to cre­ate “movies” mon­i­tor­ing changes in brain activity.

By pro­duc­ing sequen­tial images, fMRI records the areas of the brain that are acti­vat­ed. In addi­tion to detect­ing sub­stance use, fMRI is also a good lie-detect­ing device, as it sens­es activ­i­ty in cer­tain regions of the brain asso­ci­at­ed with spe­cif­ic behaviors.

Like all good things, includ­ing eggs, which con­tain a great deal of cho­les­terol, there is a down­side to fMRI: blood flow is only an indi­rect mea­sure of neu­ronal activ­i­ty and fMRI only shows where activ­i­ty takes place not exact­ly what is going on.

PET: Great Acronym, Great Images

Positron Emis­sion Tomog­ra­phy (PET) is anoth­er com­mon­ly used brain imag­ing tech­nique. PET, which is derived from CT, was the first func­tion­al imag­ing technique.

This method uti­lizes small amounts of radio­trac­ers (i.e. mol­e­cules with a short-lived radioac­tive con­stituent atom such as car­bon-11 or oxy­gen-15), which are local­ized by sen­sors that cre­ate com­put­er-com­piled images. These images depict the rel­a­tive amount of radio­trac­er present by using a col­or gra­di­ent red being the high­est con­cen­tra­tion and blue the lowest.

Like fMRI, PET can be used to study region­al cere­bral blood flow (rCBF) via radio-labeled water in the blood­stream. Addi­tion­al­ly, PET can look at glu­cose metab­o­lism that shows region­al neu­ronal activation.

Unfor­tu­nate­ly, PET can­not achieve the tem­po­ral or spa­tial res­o­lu­tion pos­si­ble with fMRI, and uses trace amounts of radi­a­tion: only one PET scan is allowed per year due to con­cerns regard­ing radi­a­tion exposure.

PET is most use­ful when study­ing neu­ro­trans­mit­ter func­tion and holds an advan­tage in tem­po­ral and spa­tial res­o­lu­tion over SPECT (dis­cussed below).

By using a radio-labeled neu­ro­trans­mit­ter, it is pos­si­ble to study the loca­tion a neu­ro­trans­mit­ter’s action, the amount of neu­ro­trans­mit­ter release, and abun­dance of recep­tor lev­els. This has rel­e­vance in addic­tion stud­ies, which have shown that dopamine recep­tor lev­els decrease with long-term cocaine use, lead­ing to tol­er­ance to low drug doses.

The only down­side is that it takes time to devel­op appro­pri­ate radio­trac­ers. Cur­rent­ly, the dopamine, GABA, and cannabi­noid cir­cuits can be exam­ined, but suit­able radio­trac­ers for oth­er neu­ro­trans­mit­ters are still lack­ing. It is like­ly that future research will solve this problem.

Egg SPECT to Be Amazed

The name Sin­gle Pho­ton Emis­sion Com­put­er­ized Tomog­ra­phy (SPECT) sounds impres­sive because it is. This method uti­lizes trac­ers that are direct­ly inject­ed into the body’s blood flow to high­light the lev­el of neu­ronal activ­i­ty in the brain.

After trac­ers are inject­ed, a “gam­ma” cam­era rotates around the head to record data, and a com­put­er uses the data to con­struct 2D or 3D images of active brain regions inac­tive areas of the brain show up as dark voids.

SPECT con­firms areas of the brain that cor­re­spond with a per­son­’s neur­al activ­i­ty, and can be used to iden­ti­fy symp­toms asso­ci­at­ed with drug use or men­tal ill­ness. It can track the effects of coun­sel­ing and med­ica­tions: as an indi­vid­ual gets bet­ter, brain areas will change in activ­i­ty lev­el.

Although SPECT can’t cre­ate images as detailed as a PET scan, images can be viewed in both 2D and 3D. This method is not very expen­sive, and the pro­ce­dure does­n’t need as many tech­ni­cal and med­ical staff to com­plete. The fol­low­ing SPECT images illus­trate the effect of drug use on brain function.

SPECT not only shows dam­age, but also shows improve­ment when sub­stance use is discontinued.

To see more exam­ples of how the brain is affect­ed by drug and alco­hol use using SPECT, click here: http://www.amenclinics.com/bp/atlas/ch15.php

Beyond Brain Imaging

We have come a long way since the fry­ing pan days. Clear­ly, the results of brain imag­ing stud­ies are very use­ful in that they help researchers bet­ter under­stand what hap­pens to a brain that has been fried by drugs.

Addi­tion­al­ly, poten­tial drug ther­a­py treat­ments have been sug­gest­ed based on the “cir­cuits” mod­el. The gen­er­al aim of this approach includes decreas­ing a drug’s reward val­ue, dis­so­ci­at­ing drug use from plea­sur­able mem­o­ries, and restor­ing nor­mal brain activity.

While brain imag­ing is a very use­ful tool, it does not pro­vide a com­plete under­stand­ing of addic­tion. Each tech­nique has lim­i­ta­tions, as described above, but future devel­op­ments are sure to strength­en these tech­nolo­gies. In addi­tion, treat­ing addic­tion pure­ly as a brain dis­ease has its own lim­i­ta­tions, in that it ignores the pow­er­ful socio-cul­tur­al fac­tors that con­tribute to drug use.

And just like any­thing else with­in the sci­en­tif­ic realm, it is cru­cial that all hypothe­ses be rig­or­ous­ly test­ed: qual­i­ty of research meth­ods, and restraint when inter­pret­ing results, must not be sac­ri­ficed in order to draw excit­ing conclusions.

Final­ly, it is impor­tant to note that brain imag­ing is only able to show cor­re­la­tions in data, not cau­sa­tions: as a wise pro­fes­sor once said, “lines drawn on a map can­not show you why coun­tries wage war.”  How­ev­er, in the future, increased sci­en­tif­ic under­stand­ing of brain addic­tion, as well as the results of future imag­ing stud­ies, may be able to show how bor­ders change as a result of war.

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Remem­ber, this blog post was writ­ten as part of a Neu­roan­thro­pol­o­gy class. You can find the oth­er sev­en blog posts by click­ing on Why A Final Essay When We Can Do This?. Enjoy!