Quantum, the indispensable ally of modern medicine

This arti­cle is part of our spe­cial issue « Quan­tum: the sec­ond rev­o­lu­tion unfolds ».

Read it here

Quan­tum physics, which explains the behav­iour of atoms and oth­er even small­er par­ti­cles, is the basic struc­ture that enables us to deduce the phys­i­cal behav­iour of mat­ter, not only on a micro­scop­ic scale, but also, in the­o­ry, right up to the human lev­el. After all, aren’t we just a large assem­bly (albeit an extreme­ly com­plex one) of atoms and mol­e­cules, all of which obey the laws of the infin­i­tes­i­mal world?

In real­i­ty, the sit­u­a­tion is dif­fer­ent. Just as you don’t need to know the sub­tleties of flu­id mechan­ics to pour your­self a glass of water, you don’t need to have a detailed under­stand­ing of the inter­ac­tion of the 1028 atoms in your body to start tak­ing care of your­self. As such, quan­tum physics is very much a part of mod­ern med­i­cine: let’s see how the infi­nite­ly small helps us to main­tain good health on a dai­ly basis.

It may seem sur­pris­ing, but one of the most pre­cise tools avail­able to mod­ern med­i­cine is… light. Or, to be more pre­cise, a beam of light that is per­fect­ly cal­i­brat­ed to ensure that all the pho­tons have the same ener­gy and that all the light waves are per­fect­ly coher­ent with each oth­er: the laser (Light Ampli­fi­ca­tion by Stim­u­lat­ed Emis­sion of Radiation).

This extreme­ly pre­cise con­trol of light emis­sion comes from the fact that, accord­ing to quan­tum mechan­ics, atoms have dis­tinct (quan­ti­fied) ener­gy lev­els and that by mak­ing elec­trons jump from one pre­cise orbit to anoth­er, only per­fect­ly iden­ti­cal pho­tons are emitted.

First pre­dict­ed by Albert Ein­stein in 1917 and per­fect­ed in 1960, the laser imme­di­ate­ly found med­ical appli­ca­tions in oph­thal­mol­o­gy (Camp­bell, 1961) and der­ma­tol­ogy (Gold­man, 1963). Today, it is used to treat reti­nal detach­ment, coag­u­late wounds, destroy small can­cer­ous tumours, cut and abrade corneas with extreme pre­ci­sion and, in den­tal surgery, to treat gum disease.

But in addi­tion to its appli­ca­tions in surgery, this tech­nol­o­gy can also be used for lighter treat­ments such as tat­too removal, anti-wrin­kle treat­ments and laser hair removal.

One of the most wide­ly used imag­ing tech­niques is MRI (Mag­net­ic Res­o­nance Imag­ing). It involves observ­ing the behav­iour of the nuclei of hydro­gen atoms immersed in an intense mag­net­ic field. Why hydro­gen? Because it is the main com­po­nent of water (H₂O), which accounts for around 60% of the total mass of a human being, and there are few oth­er bio­log­i­cal mol­e­cules that con­tain no hydro­gen at all.

The prin­ci­ple of MRI is as fol­lows: the hydro­gen nucle­us is made up of a sin­gle pro­ton which, for this pur­pose, can be regard­ed as a tiny mag­net. In a ‘nat­ur­al’ sit­u­a­tion, the human body has no par­tic­u­lar mag­neti­sa­tion, and each hydro­gen nucle­us points in a ran­dom direction.

The first step is to immerse the patient in an extreme­ly intense mag­net­ic field (around 30,000 times the Earth­’s nat­ur­al mag­neti­sa­tion) to ‘arrange’ all the pro­tons in the same direc­tion, all par­al­lel to each oth­er. This bal­ance is then altered by emit­ting a radiofre­quen­cy (RF) wave, and we lis­ten to the RF wave emit­ted back by these pro­tons when they return to their ini­tial state.

Depend­ing on the nature of the medi­um, these pro­tons will not return to their ini­tial state at the same speed. In this way, we can recon­struct a 3D image of the body by dif­fer­en­ti­at­ing between each tis­sue. With­out quan­tum physics and a detailed under­stand­ing of the behav­iour of atom­ic nuclei in an elec­tro­mag­net­ic field, this advanced non-inva­sive imag­ing tech­nique would not be possible.

Even the most unusu­al states of mat­ter, which are still the sub­ject of fun­da­men­tal research in lab­o­ra­to­ries around the world, are essen­tial for med­ical imag­ing. As men­tioned above, MRI requires the patient to be immersed in an extreme­ly intense mag­net­ic field. The high­er the field, the stronger the sig­nal emit­ted when the mag­neti­sa­tion returns to its nor­mal equi­lib­ri­um, and the bet­ter the image quality.

Super­con­duc­tiv­i­ty is one of the rare man­i­fes­ta­tions of mat­ter behav­ing in a pure­ly quan­tum man­ner on our scale.

The prob­lem is that these mag­net­ic fields are so intense that if we were to use a con­ven­tion­al elec­tro­mag­net to gen­er­ate them, the amount of heat caused by the intense elec­tric cur­rent required would melt them in a mat­ter of moments.

To over­come this prob­lem, we use so-called « super­con­duct­ing » mag­nets, which have zero elec­tri­cal resis­tance. With mag­nets of this type, no elec­tri­cal heat­ing occurs. Elec­tric cur­rents can poten­tial­ly be passed through them as intense­ly and for as long as required (with­out any loss of cur­rent, even when the pow­er sup­ply is cut off).

Super­con­duc­tiv­i­ty is one of the rare man­i­fes­ta­tions of mat­ter behav­ing in a pure­ly quan­tum man­ner on our scale. The elec­trons behave like a sin­gle super­flu­id and flow with­out any resis­tance. These super­con­duct­ing ele­ments are also used in mag­ne­toen­cephalog­ra­phy to record the brain’s elec­tri­cal activ­i­ty non-inva­sive­ly and in real time.

How can we find out where can­cer­ous areas are locat­ed in the human body and how they devel­op over time? To do this, we use the hyper­ac­tiv­i­ty of can­cer cells. Can­cer cells divide con­stant­ly and anar­chi­cal­ly, so they expend a lot of ener­gy. Their fuel: sugar.

This is why, dur­ing a PET (Positron Emis­sion Tomog­ra­phy) scan, the sub­ject is made to swal­low sug­ar whose com­po­si­tion has been slight­ly altered. A radioac­tive atom (e.g. Flu­o­rine 18) is attached to each sug­ar mol­e­cule, and when it decays it has the prop­er­ty of emit­ting an anti-mat­ter par­ti­cle: an anti-elec­tron (also known as a positron).

By recon­struct­ing the tra­jec­to­ry of these gam­ma rays, we can find the loca­tion where these mat­ter-anti­mat­ter reac­tions took place, and there­fore the posi­tion of the can­cer­ous tumours.

Sug­ar will accu­mu­late in places that con­sume a lot of ener­gy (tumour areas), and emit anti­elec­trons that, when they come into con­tact with the ‘clas­sic’ elec­trons of the sur­round­ing mat­ter, will anni­hi­late and pro­duce gam­ma pho­tons that pass through the body and are detect­ed out­side. By recon­struct­ing the tra­jec­to­ry of these gam­ma rays, we can find the loca­tion where these mat­ter-anti­mat­ter reac­tions took place, and there­fore the posi­tion of the can­cer­ous tumours.

Inge­nious and, once again, impos­si­ble to achieve with­out under­stand­ing the par­ti­cle physics behind this med­ical imag­ing technique.

Quan­tum physics is an inte­gral part of our dai­ly lives, and as such it has also entered the field of med­i­cine, with­out which a large pro­por­tion of mod­ern treat­ments and imag­ing tech­niques would not be able to func­tion. Far from being con­fined to research lab­o­ra­to­ries, quan­tum physics, par­ti­cle physics and nuclear physics save many lives every day.