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.