Imaging techniques with radionuclides
A radionuclide is a radioactive isotope. For imaging it is joined to a chemical the body uses (a radiopharmaceutical), so it goes to the organ we want to see: for example iodine to the thyroid, or a bone-seeking phosphate to bones. The image shows function (how the organ is working), not just shape.
A good tracer
- Gamma emitter only: gamma rays leave the body to be detected; alpha or beta would just damage cells.
- Right energy: about 100–200 keV gets out of the body but is still stopped by the detector crystal.
- Half-life of a few hours: long enough to do the scan, short enough to keep the dose low.
- Easy to make on site and to attach to many chemicals.
Technetium-99m fits all of these: 140 keV gamma, Tp = 6 h. It is made in hospitals from molybdenum-99 (half-life 66 h) in a "technetium generator". The m means metastable: an excited nucleus that loses energy by gamma emission.
PET (positron emission tomography)
A tracer such as fluorine-18 in a sugar (FDG) emits positrons. Each positron meets an electron within a millimetre and they annihilate, giving two 511 keV gamma photons going in opposite directions. A ring of detectors picks up both at the same time; the source lies on the line between them. Busy tissue such as many tumours takes up more sugar, so it shows up bright.
Half-life: physical, biological and effective
- Physical half-life TP: time for half the radioactive nuclei to decay.
- Biological half-life TB: time for the body to remove half the substance (urine, sweat, breath).
- Effective half-life TE: time for the activity in the body to halve, with both happening together.
1/TE = 1/TP + 1/TB, which gives TE = TPTB ÷ (TP + TB). TE is always smaller than both. Activity in the body: A = A₀ (½)^(t / TE).
The gamma camera
- Collimator: a thick lead plate with many long parallel holes. Only gamma rays travelling along the holes get through, so each point on the crystal gets rays only from directly below it. Without it the image would be a blur.
- Scintillator: a large sodium iodide crystal. Each gamma photon that is absorbed makes a tiny flash of many visible-light photons.
- Photomultiplier tubes (PMTs): an array behind the crystal. Each turns light into an electrical pulse, multiplied many times by a chain of dynodes. The tube nearest the flash gets the most light.
- Computer: compares the pulse sizes from the tubes to work out where each flash happened and builds up the image over many minutes. Pulse height also checks the energy, rejecting scattered rays.
Longer holes give a sharper image but fewer counts; the trade-off is between resolution and sensitivity.
Therapy: high-energy X-rays and radioactive implants
High-energy X-rays
A linear accelerator (linac) speeds electrons up to several MeV and fires them at a heavy-metal target to make MeV X-rays. They go deep and spare the skin more than low-energy X-rays. The machine rotates round the patient so many beams cross at the tumour: the tumour gets the full dose, while each part of healthy tissue gets only a small part. Shaped beams (multi-leaf collimators) follow the tumour outline. Doses are split into many small sessions so healthy cells can recover.
Radioactive implants (brachytherapy)
Small sealed sources are placed inside or next to the tumour, for example iodine-125 seeds in the prostate. Beta or low-energy gamma emitters give a big dose to a small volume and little elsewhere. Iodine-131 taken by mouth is absorbed by the thyroid and treats it from inside.
Imaging comparisons
| Method | Ionising? | Best for | Limits |
|---|---|---|---|
| X-ray | Yes (low dose) | Bones, chest; quick, cheap | Poor soft-tissue contrast; 2D |
| CT | Yes (higher dose) | 3D slices, injuries, tumours | Higher dose; costly |
| Ultrasound | No | Pregnancy, soft tissue, real time | Not through bone or air; lower detail |
| MRI | No | Brain, joints, soft tissue detail | Expensive, slow, no metal |
| Gamma camera / PET | Yes (tracer inside) | Function: blood flow, thyroid, spread of cancer | Low resolution; patient is radioactive for a while |
Doctors weigh the information gained against the risk and cost; for children and pregnant women non-ionising methods are chosen when they can do the job.
Try it: two taps, one tank
Fill a bottle with water and make two small holes. Open one hole and time how long the level takes to halve. Then the other hole alone. Then both together. The "both" time is shorter than either, just like TE. Check with the 3D free play: set TP = 6 h and TB = 24 h and read TE.
Key formulas and definitions
- 1/T_E = 1/T_P + 1/T_B, so T_E = T_P T_B ÷ (T_P + T_B)
- Activity A = A₀ (½)^(t / T)
- Positron annihilation: two photons of 511 keV each, in opposite directions
- Number of half-lives n = t ÷ T; fraction left = (½)ⁿ
Worked examples
1. Tc-99m has T_P = 6.0 h. In a patient T_B = 24 h. Find T_E.
T_E = 6 × 24 ÷ (6 + 24) = 144 ÷ 30 = 4.8 h.
2. A tracer has T_E = 4.0 h. What fraction is left after 12 h?
n = 12 ÷ 4 = 3. Fraction = (½)³ = 1/8 = 12.5%.
3. Iodine-131: T_P = 8.0 days, T_B = 24 days in the thyroid. Find T_E.
T_E = 8 × 24 ÷ 32 = 6.0 days.
4. Why is Tc-99m better than an alpha emitter for imaging?
Alpha particles are stopped within a few cells, so they never reach the camera and only cause damage. Gamma rays from Tc-99m leave the body and are detected.
5. An injection has activity 400 MBq. T_E = 5.0 h. Find the activity after 10 h.
Two effective half-lives: 400 → 200 → 100 MBq.
6. T_E = 3.0 h and T_P = 6.0 h. Find T_B.
1/T_B = 1/3 − 1/6 = 1/6, so T_B = 6.0 h.
Common mistakes
- Adding half-lives (T_E = T_P + T_B). You add the reciprocals, and T_E is smaller than both.
- Thinking the collimator focuses the rays like a lens. It only absorbs rays that are not parallel to the holes.
- Saying the gamma camera sends out radiation. The patient is the source; the camera only detects.
- Choosing a tracer with a very long half-life: it gives the patient an unnecessary dose for days.