01What it is
A tracer goes in
A tiny amount of a radioactive substance, called a tracer, is injected into a vein. It is chosen to travel to one part of the body.
Into a veinTiny amountFollows the blood
NHS 2025 · MedlinePlus 2025
Interactive guide · 16 parts · 2 journeys
Nuclear medicine
From one unstable atom to scans of your heart, brain and bones, and the newest cancer treatments. Scroll, and it happens.
Simple to advancedAbout 20 minutesOfficial sources
Scroll to begin
Part 01 · The big idea
Light from inside
Nuclear medicine puts a tiny glow inside you, then films where it goes.
02The numbers
40 million a year
Around 40 million nuclear medicine procedures happen worldwide each year.
X-ray exams4,200 M
Nuclear medicine40 M
Radiotherapy8.5 M
WHO 2023
03Up close
It sticks where bone is busy
In a bone scan, the tracer gathers where bone is being rebuilt. Busy areas glow brighter. Doctors call them hot spots.
Healthy boneHot spot
MedlinePlus 2025
04Safety check
The glow fades fast
The dose is small, and the radiation is gone from your body within 2 to 3 days. Tell staff first if you are, or might be, pregnant.
Small doseGone in 2–3 daysPregnant? Tell staff
MedlinePlus 2025 · NHS 2025
05In the clinic
A camera that only listens
Hours later, a gamma camera passes slowly over you. It sends nothing in. It only catches the light coming out.
Wait 3–4 hoursScan about 1 hour
MedlinePlus 2025
06Deep dive
Function, not just shape
X-rays and CT show shape. Nuclear medicine shows what tissue is doing. SPECT/CT fuses both.
Know thisYou are the light source. The camera just watches.
MedlinePlus 2025
Interlude
Zoom in
1.7 m
You. Every scan starts here.
1 cm
Bone: a living, rebuilding tissue.
10 µm
A single cell, about a tenth of a hair's width.
2 nm
DNA, the code that radiation can damage.
0.1 nm
An atom: mostly empty space.
~0.000001 nm
Its nucleus. This is where nuclear medicine happens.
Part 2 of 16 unlocked
Myth or fact?
“A nuclear scan makes you radioactive for weeks.”
Keep scrolling to flip itMyth
After a bone scan, the radiation is gone from your body within 2 to 3 days.
Source · MedlinePlus 2025Part 02 · The physics
Why atoms glow
Every atom has a core. Some cores are unstable, and they settle down by throwing out energy.
01What it is
An atom with a restless core
A radioactive atom has an unstable core, or nucleus. To calm down, it releases energy as radiation.
Unstable coreReleases energy
WHO 2023
02The numbers
99 = 43 + 56
Technetium-99m has 43 protons and 56 neutrons. The balance of the two decides whether a core is stable.
StableToo many neutronsToo many protons
NNDC 2017
03Up close
Inside the core
Molybdenum-99 has one neutron too many. A neutron flips into a proton and fires out an electron: beta decay. Now it is technetium.
42 to 43 protonsBeta decay
NNDC 2017
04Safety check
Three kinds of radiation
Paper stops alpha. A few millimetres of plastic stops beta. Gamma needs lead or concrete. Alpha is most harmful inside the body.
Gamma travels farAlpha inside: harmful
Physics · IARC 2012
05In the clinic
See with gamma, treat with particles
Gamma rays escape the body, so cameras can see them. Alpha and beta particles stop inside, so they can treat.
Gamma: imagingAlpha and beta: therapy
NCI 2020
06Deep dive
What the “m” means
Tc-99m is metastable: an excited core that waits, then releases one 140 keV gamma ray. What is left lasts 211,000 years, so it barely glows.
Know thisGamma rays from a scan do not make you, or anything near you, radioactive.
NNDC 2017
Part 3 of 16 unlocked
Myth or fact?
“Gamma rays from a scan make your clothes radioactive.”
Keep scrolling to flip itMyth
Gamma rays pass through. They do not make objects radioactive.
Source · PhysicsPart 03 · Time
The half-life clock
Radioactive atoms do not last. Every half-life, half of them are gone.
01What it is
Half, then half again
A half-life is the time it takes for half of the atoms to decay. Watch 400 atoms become 200.
400 atoms1 half-life
WHO 2023
02The numbers
1 ½ ¼
Each half-life halves what is left. After 10 half-lives, under 0.1% remains: 1 part in 1,024.
50%25%12.5%
Calculated
03Up close
A coin flip for every atom
No one can say which atom goes next. But millions of coin flips add up to a perfectly smooth curve.
Random one by onePredictable in bulk
WHO 2023
04Safety check
Two clocks, one exit
Your body also flushes tracer out, for example in urine. Decay plus flushing means it leaves faster than either alone.
Drink if advisedEffective half-life
CRUK 2026 · NHS CUH 2025
05In the clinic
Seconds to months
Medical isotopes are picked to last just long enough. Long enough to use, short enough to fade.
Rubidium-82: 75 sGermanium-68: 271 days
NNDC 1995–2020
06Deep dive
The maths of fading
Activity A = A0 × e−kt, where k = ln 2 ÷ half-life. On a log scale, decay is a straight line.
Know this1 becquerel (Bq) = one atom decaying per second.
Calculated
Part 4 of 16 unlocked
Myth or fact?
“After two half-lives, a tracer is completely gone.”
Keep scrolling to flip itMyth
A quarter is still left. Each half-life halves what remains.
Source · CalculatedPart 04 · Generators
Parent and daughter
Some atoms decay into another radioactive atom. Hospitals use this to make tracers on the spot.
01What it is
One atom becomes another
Molybdenum-99, the parent, decays into technetium-99m, the daughter. The daughter is the one we scan with.
Parent: Mo-99Daughter: Tc-99m
NNDC 2017
02The numbers
66 hours vs 6 hours
Mo-99 has a half-life of 65.9 h. Tc-99m: 6.0 h. The long-lived parent keeps making fresh daughter.
~11 daughter half-lives per parent
NNDC 2017
03Up close
Milking the cow
Inside a lead-shielded generator, Mo-99 clings to a column. A wash of salt water carries out only the Tc-99m.
Salt water inTc-99m out
Euratom 2014
04Safety check
Shielded and checked
The generator sits inside lead. Every wash is tested so that almost no parent slips through into a patient dose.
Lead shieldLimit: 0.15 kBq Mo-99 per MBq
US NRC 10 CFR 35.204
05In the clinic
Peak at about 23 hours
After each wash, Tc-99m builds back up and peaks after about 23 hours. So the generator can be milked every day.
Daily washWeekly delivery
NNDC (calculated)
06Deep dive
Equilibrium
Parent lives ~11× longer: the daughter rises, then fades with it. Parent lives far longer (Ge-68, Sr-82): the daughter keeps pace exactly.
Know thisA generator keeps giving Tc-99m for about 1 to 2 weeks.
NNDC 2012–2019 · Euratom 2014
Part 5 of 16 unlocked
Myth or fact?
“Tc-99m comes straight from a reactor.”
Keep scrolling to flip itMyth
Hospitals milk it from a generator, which keeps working for about 1 to 2 weeks.
Source · Euratom 2014Part 05 · Technetium-99m
The workhorse
One isotope does much of the everyday scanning. Here is why it is nearly perfect.
01What it is
One clean colour of light
Tc-99m gives off a single gamma ray at 140 keV. Strong enough to escape the body. Gentle enough for cameras to catch.
Single gammaSharp peak
NNDC 2017
02The numbers
6 h · 140 keV · 89%
Half-life 6.0 h. The 140.5 keV gamma comes out in 89 of every 100 decays. Used in over 30 million procedures a year.
6.0 h140.5 keV89%
NNDC 2017 · Euratom 2014
03Up close
A key on many keyrings
Tc-99m clips onto different carrier molecules. The carrier decides where it goes.
BoneKidneyHeartLungThyroid
MedlinePlus 2025
04Safety check
Flush it out
Tracer that the body does not use drains out in urine. Drink extra fluids and empty your bladder often.
Extra waterEmpty bladder oftenPregnant? Call first
NHS CUH 2025
05In the clinic
One isotope, many scans
Bones, heart, kidneys, lungs, thyroid. Swap the carrier and the same isotope lights up a different organ.
Bone scanHeart scanKidney scan
MedlinePlus 2025
06Deep dive
How the camera finds a flash
Lead holes, the collimator, let only straight rays through. Each ray makes a crystal flash that light sensors pinpoint.
Know thisAfter 2 days (8 half-lives), under 0.4% of the Tc-99m is left.
NNDC (calculated)
Part 6 of 16 unlocked
Myth or fact?
“Every scan uses the same radioactive drug.”
Keep scrolling to flip itMyth
The carrier molecule decides which organ lights up.
Source · MedlinePlus 2025Part 06 · Production
Made in a reactor
Medical isotopes don't keep, so they are made fresh and raced to hospitals.
01What it is
Splitting uranium
In a research reactor, neutrons split uranium-235 atoms. Some of the pieces are molybdenum-99, the parent of Tc-99m.
NeutronUranium-235Mo-99
Euratom 2014
02The numbers
8 reactors, 10,000 hospitals
In 2014, just 8 reactors made most of the world's Mo-99, for over 10,000 hospitals.
ReactorsProcessorsGenerator makersHospitals
Euratom 2014
03Up close
The blue glow
In a reactor pool, fast charged particles moving through water give off an eerie blue light called Cherenkov light.
Water shieldsBlue glow
Physics
04Safety check
Hot cells
Radioactive targets are opened behind thick shielding, using remote arms, in specialised processing facilities.
Thick shieldingRemote arms
Euratom 2014
05In the clinic
A race against decay
Mo-99 loses about 22% every day. Reactor, processor, generator maker, hospital: every hour counts.
~22% lost per dayCan't be stockpiled
NNDC (calculated) · Euratom 2014
06Deep dive
Three ways to make tracers
Reactors make Mo-99. Cyclotrons make fluorine-18. Generators make Tc-99m and Ga-68 right in the hospital.
Know thisAfter one week, under a fifth of the Mo-99 is left.
Euratom 2014 · NNDC
Part 7 of 16 unlocked
Myth or fact?
“Medical isotopes can be stockpiled like other medicines.”
Keep scrolling to flip itMyth
Mo-99 loses about 22% a day, so supply has to keep flowing.
Source · NNDC · Euratom 2014Part 07 · Thyroid
The iodine magnet
Your thyroid soaks up iodine. Nuclear medicine uses that to see it, and to treat it.
01What it is
A butterfly in your neck
The thyroid sits low at the front of your neck. Iodine naturally collects in its cells, which use it to make hormones.
Front of the neckCollects iodine
NCI 2020
02The numbers
5–15% in 6 hours
In an uptake test you swallow a radioactive iodine pill. A normal thyroid takes up about 5–15% by 6 hours and 10–30% by 24 hours.
At 6 hours5–15%
At 24 hours10–30%
MedlinePlus 2026
03Up close
Tiny iodine traps
Thyroid cells pull iodine out of the blood and hold on to it, so a radioactive version collects there too.
Thyroid cellsIodine
NCI 2020
04Safety check
A room of your own
After radioiodine treatment you usually stay a few days in a single room. Sweat, urine and saliva are radioactive for a few days.
Single roomFew daysNo child visitors
CRUK 2026
05In the clinic
Hot or cold?
On a scan, hot nodules take up extra iodine and can make the thyroid overactive. Cold nodules take up less.
Hot noduleCold nodule
MedlinePlus 2026
06Deep dive
Two iodines, two jobs
Iodine-123 (13 hours) gives gamma rays for pictures. Iodine-131 (8 days) also fires beta particles that treat an overactive thyroid or thyroid cancer.
Know thisRadioiodine has treated thyroid cancer since the 1940s.
NNDC 2006–2021 · NHS 2023 · NCI 2020
Part 8 of 16 unlocked
Myth or fact?
“Radioactive iodine is only used for cancer.”
Keep scrolling to flip itMyth
It also treats an overactive thyroid.
Source · NHS 2023Part 08 · Heart
The stress test
Run, then rest. Two sets of pictures show whether every part of the heart muscle gets enough blood.
01What it is
Pictures of blood flow
A nuclear stress test shows how well blood flows into the heart muscle, at rest and under stress.
RestStress
MedlinePlus 2024
02The numbers
15 to 45 minutes
After each injection there is a wait of 15 to 45 minutes before the camera takes pictures.
Rest picturesStress pictures
MedlinePlus 2024
03Up close
Muscle drinks the tracer
Thallium or sestamibi rides in the blood into the heart muscle. Well-supplied muscle glows. Starved muscle stays dim.
Good flowPoor flow
MedlinePlus 2024
04Safety check
Treadmill or medicine
If you can't exercise, a medicine can widen the heart arteries or make the heart work harder. Heart rhythm and blood pressure are watched throughout.
MonitoredMedicine option
MedlinePlus 2024
05In the clinic
Rest vs stress
A computer compares the two sets. Dim areas can mean a narrowed artery, or scarring from an old heart attack.
Narrowed arteryOld scar
MedlinePlus 2024
06Deep dive
The bull's-eye
Software unrolls the heart's wall into one round map, so the rest and stress pictures can be compared side by side.
Know thisThallium-201 lasts about 3 days. Tc-99m lasts just 6 hours.
MedlinePlus 2024 · NNDC 2017–2022
Part 9 of 16 unlocked
Myth or fact?
“You must run on a treadmill for a heart scan.”
Keep scrolling to flip itMyth
If you can't exercise, a medicine can stress the heart instead.
Source · MedlinePlus 2024Part 09 · Lungs
Air in, blood through
Two scans in one: one follows the air you breathe, the other the blood flowing past it.
01What it is
The V/Q scan
V is ventilation, the air. Q is perfusion, the blood flow. Together they map how well each part of the lungs works.
Ventilation: airPerfusion: blood
MedlinePlus 2024
02The numbers
10 to 90 micrometres
The blood-flow tracer is tiny protein clumps, mostly 10 to 90 micrometres across. Over 90% lodge in the lungs within minutes.
10–90 µmLodged in 1–5 min
FDA 2020
03Up close
Stuck, then gone
The clumps catch in the smallest lung vessels, then break up. Half has left the lungs in about 11 hours.
Tiny vessels~11 h to halve
FDA 2020
04Safety check
Breathe the gas
For the air part you breathe a radioactive gas through a mask. Tell the department first if you might be pregnant.
MaskPregnant? Call first
MedlinePlus 2024 · NHS CUH 2025
05In the clinic
Finding a clot
A clot blocks blood but not air. A patch that gets air but no tracer points to a pulmonary embolism.
Air: yesBlood: no
MedlinePlus 2024
06Deep dive
Matching the maps
Doctors compare the air map and the blood map side by side, area by area.
Know thisThe tracer is albumin, a protein found in blood, made into tiny clumps.
MedlinePlus 2024 · FDA 2020
Part 10 of 16 unlocked
Myth or fact?
“A lung scan means breathing radioactive smoke.”
Keep scrolling to flip itMyth
You breathe a radioactive gas through a mask, and a tracer goes into a vein.
Source · MedlinePlus 2024Part 10 · Kidneys
The kidney race
Watch tracer race into each kidney and drain away, and see how much work each one does.
01What it is
A scan of filtering
A renal scan measures how well your kidneys work. It also shows their size, position and shape.
FunctionSize and shape
MedlinePlus 2025
02The numbers
30 minutes to an hour
Many pictures are taken, each lasting 1 or 2 seconds. The whole scan takes about 30 minutes to an hour.
1–2 s per frame30–60 min
MedlinePlus 2025
03Up close
A million filters
Each kidney has about a million tiny filters called nephrons. Together your kidneys filter about 150 quarts (140 litres) of blood a day.
~1 million each~140 L a day
NIDDK 2018
04Safety check
Drink, then empty
Water helps flush the tracer through. Drink extra and empty your bladder often.
Extra waterEmpty often
NHS CUH 2025
05In the clinic
Reading the curves
A graph shows tracer rising into each kidney, then draining out. A slow drain can point to a blockage.
RiseDrainBlockage?
MedlinePlus 2025
06Deep dive
Left vs right
Software compares the two curves to show how much of the work each kidney is doing.
Know thisFrames last 1 to 2 seconds each, like a flip-book of your kidneys.
MedlinePlus 2025
Interlude
Into the scanner
Lie back
The bed is narrow and comfortable. You keep still.
Slide in
The bed glides into a ring full of detectors.
All around you
Detectors on every side catch the rays leaving your body.
Slice by slice
A computer turns millions of hits into thin slices.
Stack them up
Slices stack into a 3D picture you can turn and explore.
Two in one
CT adds the shape. Nuclear medicine adds what tissue is doing.
Part 11 of 16 unlocked
Myth or fact?
“A kidney scan takes all day.”
Keep scrolling to flip itMyth
It usually takes about 30 minutes to an hour.
Source · MedlinePlus 2025Part 11 · PET
Antimatter on purpose
PET uses atoms that send out positrons, tiny bits of antimatter. Each one becomes two flashes of light.
01What it is
Matter meets antimatter
A positron travels a tiny distance and meets an electron. Both vanish. Two gamma rays shoot out, back to back.
PositronElectronTwo gamma rays
NCI · NNDC
02The numbers
511 keV · 180°
Each gamma ray carries 511 keV, flying in opposite directions. Fluorine-18 lasts 110 min, gallium-68 68 min, rubidium-82 just 75 s.
F-18: 110 minGa-68: 68 minRb-82: 75 s
NNDC 1995–2019
03Up close
Cancer cells crave sugar
A common PET tracer is radioactive glucose. Cancer cells usually take up more glucose than normal cells, so they glow brighter.
Normal cellCancer cell
NCI dictionary
04Safety check
Fast first, distance after
You usually stop eating 6 hours before. Afterwards, avoid long close contact with young children or anyone pregnant, as staff advise.
No food for 6 hChildren and pregnancy
NHS 2025
05In the clinic
The ring catches twins
A ring of detectors waits for two hits at the same instant. Each pair draws a line. Where lines cross, the tracer is.
CoincidenceScan 30–60 min
NHS 2025
06Deep dive
Where PET atoms come from
F-18 is made in a cyclotron: protons spiral up to speed and hit oxygen-18 water. Ga-68 and Rb-82 come from generators.
Know thisF-18 halves every 110 minutes, so it is made nearby and used fast.
Physics · NNDC 1995
Part 12 of 16 unlocked
Myth or fact?
“PET scans really use antimatter.”
Keep scrolling to flip itFact
Each positron is antimatter. It meets an electron and becomes two gamma rays.
Source · PhysicsPart 12 · Brain
Thinking in colour
Scans can show the brain's fuel use, unusual proteins, and dopamine nerve endings.
01What it is
The brain runs on sugar
Glucose is the brain cells' main fuel. FDG PET shows where the brain is using it, and where use has dropped.
GlucoseFDG PET
NIA
02The numbers
3 to 6 hours
For a DaTscan, SPECT pictures start 3 to 6 hours after the injection. Its iodine-123 lasts about 13.2 hours.
Wait 3–6 hI-123: 13.2 h
FDA 2011 · NNDC 2021
03Up close
Amyloid plaques
Amyloid PET shows abnormal deposits of a protein called beta-amyloid, a hallmark of Alzheimer's disease.
PlaquesHealthy cells
NIA
04Safety check
What a clear scan means
Few or no plaques usually means Alzheimer's is not causing the symptoms. Scans can also rule out other causes.
Rule outPart of the picture
NIA · NHS 2022
05In the clinic
Comma or oval?
A DaTscan lights up dopamine nerve endings. Healthy areas look like two commas; in Parkinsonian syndromes they can shrink to ovals.
Commas: typicalOvals: reduced
FDA 2011
06Deep dive
The next frontier
Tau PET detects tangles of another protein, tau, inside nerve cells. For now it is used mainly in research.
Know thisFDG PET can help when doctors suspect frontotemporal dementia.
NIA
Part 13 of 16 unlocked
Myth or fact?
“Brain scans can read your thoughts.”
Keep scrolling to flip itMyth
They show fuel use, proteins and dopamine nerve endings, not thoughts.
Source · NIA · FDA 2011Part 13 · Surgery
The listening probe
A drop of tracer near a tumour shows surgeons the first lymph node cancer would reach.
01What it is
The sentinel node
It is the first lymph node that cancer cells are most likely to spread to from the main tumour.
First stopLymph node
NCI
02The numbers
From 20–40 to a few
In breast cancer, surgeons once removed 20 to 40 armpit nodes. Now they can often take just the sentinel node or nodes.
Before20–40
Sentinel only1+
NCI
03Up close
Riding the lymph
Tracer injected near the tumour drains along lymph vessels, the same route cancer cells might take.
InjectionLymph flow
NCI
04Safety check
Tracer, dye, or both
The surgeon may use a radioactive tracer, a blue dye, or both, to find the node.
Radioactive tracerBlue dye
NCI
05In the clinic
Follow the counts
A handheld detector counts gamma rays. The number climbs as it nears the glowing node.
Handheld detectorCounts climb
NCI
06Deep dive
Where it's used
Most often in breast cancer and melanoma. Sometimes in penile and endometrial cancers, and it is being studied for others.
Know thisIt can spare many people from having lots of lymph nodes removed.
NCI
Part 14 of 16 unlocked
Myth or fact?
“Surgeons find the sentinel node by guesswork.”
Keep scrolling to flip itMyth
A handheld detector counts gamma rays from the tracer.
Source · NCIPart 14 · Theranostics
See it, then treat it
Use the same homing molecule twice: first to find the cancer, then to treat it.
01What it is
Same key, two payloads
A molecule that seeks a cancer target can carry an imaging atom, gallium-68, or a treatment atom, lutetium-177. Find first, then treat.
Ga-68: seeLu-177: treat
NCI 2020 · NCI 2024
02The numbers
15.3 vs 11.3 months
In the VISION trial of 831 men with advanced prostate cancer, median survival was 15.3 months with Lu-177-PSMA plus usual care, vs 11.3 months with usual care alone.
+ Lu-177-PSMA15.3 mo
Usual care11.3 mo
FDA 2022
03Up close
Docking on the target
Many prostate cancer cells carry a surface protein called PSMA. The drug locks onto it and delivers radiation right there.
PSMA receptorHealthy cellDrug
FDA 2022 · NCI 2020
04Safety check
Side effects to know
Most common: tiredness, dry mouth, nausea, anaemia. Doctors watch blood counts and kidneys closely.
TirednessDry mouthBlood countsKidneys
FDA 2022
05In the clinic
Beta rays break DNA
Lu-177 fires beta particles that can reach about 10 to 30 cells around it, breaking DNA. Its gamma rays show where the drug went.
Beta: treatGamma: image
NCI 2020
06Deep dive
Six doses, six weeks apart
Lu-177-PSMA is given up to 6 times, 6 weeks apart. Lu-177-dotatate (approved 2018) treats some neuroendocrine tumours the same way.
Know thisRadioiodine has done this since the 1940s: thyroid cells soak up iodine.
FDA 2022 · NCI 2020
Part 15 of 16 unlocked
Myth or fact?
“Radiation can be aimed at cancer from inside the body.”
Keep scrolling to flip itFact
Radiopharmaceuticals deliver radiation directly to cancer cells.
Source · NCI 2020Part 15 · Alpha therapy
Short range, big punch
The newest treatments use alpha particles: heavy, short-range and very destructive to the cells they hit.
01What it is
A heavy shot
An alpha particle is 2 protons and 2 neutrons. It is about 7,000 times heavier than a beta electron, so it hits hard and stops fast.
2 protons2 neutrons
Physics · JRC
02The numbers
< 0.1 mm · 5–9 MeV
In tissue, alpha particles travel less than 0.1 mm, under 10 cell widths, carrying 5 to 9 MeV each.
Alpha: < 10 cellsBeta: reaches further
JRC
03Up close
Radium hides in bone
Radium behaves like calcium. It collects where bone is being rebuilt, often where cancer has spread. Radium-223 was approved in 2013.
Acts like calciumBone spread
NCI 2020 · NCI 2024
04Safety check
Harmless outside, harmful inside
Alpha particles cannot get through skin. Inside the body they are powerful. IARC classes internalised alpha emitters as causing cancer.
Skin stops alphaInside: strict control
IARC 2012
05In the clinic
One atom, four alphas
Actinium-225 decays through francium, astatine, bismuth, polonium and lead. Along the way it fires 4 alpha particles.
Ac-225: 9.9 daysPo-213: 3.7 µs
NNDC 2019–2022
06Deep dive
The recoil problem
Each alpha kicks the new atom backwards, hard enough to break it free of its carrier. Keeping daughters in place is a key research goal.
Know thisActinium-225 is highly promising, but in limited supply.
Physics · JRC
Part 16 of 16 unlocked
Myth or fact?
“Alpha particles are harmless because paper stops them.”
Keep scrolling to flip itMyth
Outside the body, yes. Inside, IARC lists them as causing cancer.
Source · IARC 2012Part 16 · Safety
How much radiation?
Every scan has a dose. Here is how it compares with everyday life.
01What it is
Measured in millisieverts
Dose is measured in millisieverts (mSv). We all get some every day from the sky, the ground, food and the air.
CosmicRadon gasFood
UKHSA 2026 · WHO 2023
02The numbers
2.6 mSv a year from life
The UK average is 2.6 mSv a year. A chest X-ray is 0.014 mSv. A bone scan is about 4 mSv.
Chest X-ray 0.014Bone scan ~4CT chest 6.6
UKHSA 2026 · NHS RBFT 2025
03Up close
Cells can repair
Radiation can damage DNA, and cells have ways to repair it. WHO says cancer risk rises clearly at doses above 100 mSv.
DNA breakRepair
WHO 2023
04Safety check
Time · distance · shielding
Spend less time near a source, keep your distance, put a shield between. Double the distance and the dose falls to a quarter.
TimeDistanceShielding
Physics
05In the clinic
After radioiodine
After radioiodine for an overactive thyroid, avoid long close contact with children and pregnant women for about 3 weeks. Avoid pregnancy for at least 6 months.
~3 weeks: distance6 months: no pregnancy
NHS 2023
06Deep dive
Putting risk in numbers
A bone scan (about 4 mSv) adds an estimated lifetime cancer risk of about 1 in 5,000. Scans are done only when the benefit is greater.
Know thisRadon gives people in Cornwall about 6.9 mSv a year.
NHS RBFT 2025 · UKHSA 2026
All 16 parts unlocked
Myth or fact?
“Every scan carries a big cancer risk.”
Keep scrolling to flip itMyth
A bone scan adds an estimated lifetime risk of about 1 in 5,000.
Source · NHS RBFT 2025Recap
The one-minute version
01
Light from inside
A tracer goes in, travels to an organ, and a camera films the light coming out.
02
Why atoms glow
An unstable core releases energy. Gamma is for seeing. Alpha and beta are for treating.
03
Half-life
Half gone each half-life. After ten, less than 0.1% is left.
04
Parent and daughter
Mo-99 (66 h) makes Tc-99m (6 h). Hospitals milk the generator every day.
05
The workhorse
Tc-99m: one 140 keV gamma, 6 hours, and a carrier that picks the organ.
06
Made in a reactor
Neutrons split uranium. Mo-99 loses ~22% a day, so it is raced to hospitals.
07
Thyroid
Iodine collects in the thyroid: 5–15% by 6 hours. I-123 to see, I-131 to treat.
08
Heart
Stress and rest pictures show where heart muscle gets enough blood.
09
Lungs
Air map plus blood map. Air but no blood can mean a clot.
10
Kidneys
Curves show tracer rising in and draining out of each kidney.
11
PET
Positron meets electron. Two 511 keV rays fly apart. The ring connects the pairs.
12
Brain
FDG shows fuel use, amyloid PET shows plaques, DaTscan shows dopamine endings.
13
Surgery
Tracer finds the first lymph node; a handheld detector counts its rays.
14
Theranostics
Find it with gallium-68, treat it with lutetium-177. Same homing molecule.
15
Alpha therapy
Under 0.1 mm, 5 to 9 MeV. Actinium-225 fires four alphas down its chain.
16
Dose
Small, compared with daily life, and only used when the benefit is bigger.
References
Where every number comes from
Every figure was checked against the source below, and its year is shown on the card that uses it. Half-lives and gamma energies come from the US National Nuclear Data Center (NNDC), the official evaluated nuclear data library, because the health agencies do not publish decay data. “Calculated” means worked out from those NNDC values; “Physics” marks textbook physics (such as the inverse-square law) rather than a statistic.
- WHO · 2023Ionizing radiation and health effects (fact sheet)Part 1: 40 M proceduresPart 3: half-lifePart 16: risk above 100 mSv
- IARC · 2012IARC Monographs Vol. 100D: RadiationPart 2: radiation typesPart 15: internalised alpha
- NCI · 2020Radiopharmaceuticals: radiation therapy enters the molecular agePart 7: iodine in thyroid cellsPart 14: 10–30 cellsPart 15: radium in bone
- NCI · 2024Radiopharmaceutical therapy (approved agents and dates)Part 14: Lutathera 2018Part 15: radium-223 2013
- NCIDictionary of Cancer Terms: PET scanPart 11: radioactive glucose
- NCI · 2019Sentinel lymph node biopsy (fact sheet)Part 13: every card
- NHS · 2025PET scanPart 1Part 11: fasting, distance, scan time
- NHS · 2023Overactive thyroid: treatment (radioiodine)Part 7Part 16: 3 weeks, 6 months
- NHS · 2022Parkinson's disease: diagnosisPart 12: SPECT to rule out other causes
- NHS · CUH · 2025Radionuclide whole body bone scan (patient leaflet)Part 5, 9, 10: fluids, bladder, pregnancy
- NHS · RBFT · 2025Common nuclear medicine procedures: radiation risksPart 16: bone scan ~4 mSv, 1 in 5,000
- Cancer Research UK · 2026Having radioactive iodine treatmentPart 3: leaving the bodyPart 7: single room
- Cancer Research UK · 2025PET-CT scanPart 11
- MedlinePlus · 2025Bone scanPart 1: hot spots, timing, 2–3 daysPart 5
- MedlinePlus · 2025Nuclear scanPart 1: tracer into a veinPart 5
- MedlinePlus · 2026Thyroid scanPart 7: hot and cold nodules
- MedlinePlus · 2026Radioactive iodine uptakePart 7: 5–15% at 6 h, 10–30% at 24 h
- MedlinePlus · 2024Nuclear stress testPart 8: every card
- MedlinePlus · 2024Pulmonary ventilation/perfusion scanPart 9: gas, albumin, clots
- MedlinePlus · 2025Renal scanPart 10: function, 1–2 s frames, 30–60 min
- MedlinePlus · 2024PET scanPart 11
- NIA (NIH)How biomarkers help diagnose dementiaPart 12: FDG, amyloid, tau PET
- NIDDK (NIH) · 2018Your kidneys and how they workPart 10: a million nephrons, 150 quarts a day
- UKHSA · 2026Ionising radiation: dose comparisonsPart 16: 2.6 mSv, X-ray, flight, CT, Cornwall radon
- US FDA · 2022FDA approves Pluvicto for metastatic castration-resistant prostate cancerPart 14: VISION, 831 men, 15.3 vs 11.3 mo, doses, side effects
- US FDA · 2011DaTscan (ioflupane I-123) prescribing informationPart 12: 3–6 h, comma vs oval
- US FDA · 2020Pulmotech MAA: cross-discipline reviewPart 9: 10–90 µm, >90% in 1–5 min, 10.8 h
- US NRC10 CFR 35.204: permissible molybdenum-99 concentrationPart 4: every eluate tested, 0.15 kBq per MBq limit
- NNDC / ENSDF · 1995–2022Evaluated nuclear data: half-lives and gamma energies (Mo-99, Tc-99m, F-18, Ga-68, Ge-68, Rb-82, Sr-82, Y-90, I-123, I-131, Tl-201, Lu-177, Ra-223, Ac-225 chain)Parts 2–15: every half-life and energy
- EU Joint Research CentreMedical applications of radionuclides and targeted alpha therapyPart 15: < 0.1 mm, 5–9 MeV, limited supply
- Euratom Supply Agency · 2014Supply of medical radioisotopes (WG4 report)Part 4: generators, 1–2 weeksPart 5: 30 M+ proceduresPart 6: fission, 8 reactors, 10,000 hospitals
This guide explains how nuclear medicine works. It is not medical advice. Your own care team will tell you what applies to your scan or treatment. Figures checked October 2026.