Radiopharmaceuticals consist of a radioactive isotope, which creates the image, and a pharmaceutical, which determines the physiological behaviour of the compound and, therefore, where the signal accumulates to form the image.
There are several properties of the ideal radioisotope for diagnostic purposes (i.e. not therapeutic):
- Half life which is short enough to limit radiation dose to patient but long enough to allow good signal during imaging (ideally 1.5 x length of imaging)
- Emits gamma rays which are of high enough energy to leave the body, reach the camera and contribute to the image. The low energy of alpha or beta particles means they are absorbed by the body which increases the radiation dose to the patient and limits the radiation that reaches the camera to produce the image
- Mono-energetic gamma emitter (i.e. gamma rays of one energy). The ideal energy range is 100 to 250 keV for optimal imaging
- Decays to stable daughter isotopes that will not cause significant radiation dose to patient
- Easy to bind to different pharmaceuticals
- Doesn’t change behaviour of pharmaceutical
And there are several properties of the ideal pharmaceutical:
- High target:non-target uptake ratio
- Easy and cheap to produce
- Non-toxic
- Does not alter physiology in order to give accurate depiction of patient’s physiology
Clinical radiopharmaceuticals
There are many combinations of radioisotopes and pharmaceuticals that are used in medicine and in imaging. Some of these will produce an image and other will just produce a measurement but no image.
Classification by system
The most common imaging tests for different systems have been outlined below.
Cardiac imaging
- Thallium-201: For myocardial perfusion. Injected while the patient is in peak exercise or shortly after the pharmacological stress agent (adenosine) is administered. Patient is imaged immediately to see muscle that is non-perfused and then 3-4 hours later to see muscle that is persistently non-perfused i.e. irreversible infarct vs. poorly perfused i.e. ischaemia.
- Technetium-99m (99mTc) sestamibi or tetrofosmin: For myocardial perfusion. This has a shorter half-life than thallium-201. It also requires a second injection on the delayed study.
- MUGA: For ventriculography. The patient’s RBCs are radiolabelled and injected back into the patient. This study is used to assess regional and global wall motion, ventricular function and cardiac chamber size but not myocardial perfusion.
Endocrine imaging
- 99mTc-pertechnetate: For thyroid function. Patient is imaged within 15-30 minutes after injection.
- 99mTc-sestamibi or tetrofosmin: for parathyroid function. Patient scanned at 20 minutes and 2 hours after injection.
- Iodine-131 MIBG: For neuroendocrine imaging. Thyroid blockade administered 5 days before scan. Patient imaged 1-2 days after injection of I131-MIBG.
- 99mTc-MDP (methylene diphosphonate): For bone scan. Imaged 2-5 hours after injection. Can be performed as SPECT.
Renal imaging
- Tc99m-DTPA and Tc99m-MAG3: For GFR estimation. Dynamic images acquired for 25-30 minutes after injection. Can give diuretics.
- Tc99m-DMSA: For cortical function e.g. scarring. Patient imaged 3 hours after injection. Static study, not functional.
CNS imaging
- Brain SPECT with technetium-99m HMPOA: Start imaging patient 20 minutes to 2-3 hours after injection.
- Iodine 123 Ioflupane (aka DaTscan): SPECT. Used in imaging Parkinson’s disease. Thyroid blockade administered. Patient imaged 3-6 hours after injection.
Lung imaging
- 99mTc-DTPA aerosol: For ventilation. Static image immediately after inhalation.
- 99mTc-MAA injection: For perfusion. Imaged immediately after injection then static images taken from different angles.
Infection / inflammation imaging
- Gallium-67 citrate: Performed for regional or whole body imaging, planar or SPECT. Patient imaged at 48 hours and 72 hours.
Oncology imaging
- Gallium-67 citrate: For non-Hodgkin lymphoma, melanoma and hepatocellular carcinoma. Image on day 2 and 3.
- 99mTc-octreotide: imaged at 2-4 hours and at 4 hours
- 18F-FDG PET: Imaged at 30-60 minutes after injection for 5-60 minutes.
GI imaging
- 99mTc-mebrofenin: For hepatobiliary function. Continuous dynamic imaging up to 60 minutes. Delayed imaging at 3-4 hours if needed.
- Radiolabeled test meal using 99mTc-sulfur colloid: For gastric emptying. Planar images taken 1 minute immediately after then repeated for 1 minute every hour.
Classification by radioisotope
Below is a summary of each radioisotope and the common uses. In the radioisotope header is the radioisotope, type of radiation emitted, method of production and half-life. For each radiopharmaceutical is the method of administration, whether it is used in vivo or in vitro, clinical use and whether it produces images or not. Despite the length of this table it is not exhaustive!
| Carbon-11 | Positron | Cyclotron | 20.3 m | |
| C11-Choline | IV | In vivo | PET: suspected prostate recurrence | Imaging |
| C11-L-Methyl-methionine | IV | In vivo | Brain tumour imaging Parathyroid imaging | Imaging |
| Carbon-14 | Beta | Reactor | 5730 y | |
| C14-Glycocholic acid | Oral | In vitro | Breath test for small intestine bacterial overgrowth | Non-imaging |
| C14-Urea | Oral | In vitro | Breath test for H. pylori | Non-imaging |
| Chromium-51 | Gamma | Reactor | 28 d | |
| Cr51 | IV | In vitro | RBC volume heart scan, RBC sequestration, GI blood loss | Non-imaging |
| Cr51-EDTA | IV | In vitro | GFR | Non-imaging |
| Cobalt-57 | Gamma | Cyclotron | 279 d | |
| Co57-Cyanocobalamin | Oral | In vitro | GI absorption | Non-imaging |
| Fluorine-18 | Positron | Cyclotron | 109 m | |
| F18-FDG | IV | In vivo | Tumour imaging Myocardial imaging | Imaging |
| F18-Sodium fluoride | IV | In vivo | Bone imaging | Imaging |
| F18-Fluorocholine | IV | In vivo | Prostate cancer | Imaging |
| F18-Desmothoxyfallypride | IV | In vivo | Dopamine receptor imaging | Imaging |
| Gallium-67 | Gamma | Cyclotron | 78.3 h | |
| Ga67-Ga3+ | IV | In vivo | Tumour imaging Infection / inflammation | Imaging |
| Ga67-Citrate | IV | In vivo | Hodgkin’s disease, lymphoma, bronchogenic carcinoma Acute inflammation | Imaging |
| Gallium-68 | Positron | Generator | 68 m | |
| Ga68-Dotatoc | IV | In vivo | Neuroendocrine tumour | Imaging |
| Ga68-PSMA | IV | In vivo | Prostate cancer | Imaging |
| Indium-111 | Gamma | Cyclotron | 1.81 d | |
| In111-DTPA | Intraperitoneal | In vivo | Ventriculoperitoneal shunt patency | Imaging |
| In111-DTPA | Intra-cisternal | In vivo | Cisternography | Imaging |
| In111-leucocytes | IV | In vivo | Infection / inflammation | Imaging |
| In111-platelets | IV | In vivo | Thrombus imaging | Imaging |
| In111-Pentetreotide or Octreotide | IV | In vivo | Neuroendocrine tumour | Imaging |
| Iodine-123 | Gamma | Cyclotron | 13.2 h | |
| I123-Iodide | IV or oral | In vivo | Thyroid function Thyroid cancer metastases | Imaging |
| I123-MIBG | IV | In vivo | Neuroectodermal tumour imaging | Imaging |
| I123-ioflupane aka DaTscan | IV | In vivo | SPECT: Parkinson’s disease | Imaging |
| Iodine-131 | Gamma and beta | Reactor | 8.06 d | |
| I131-Iodide | Oral | In vivo | Hyperthyroidism / thyroid cancer treatment | Therapeutic |
| I131-Iodide | IV or oral | In vivo | Thyroid metastases | Imaging |
| I131-MIBG | IV | In vivo | Neuroectodermal tumour imaging | Imaging |
| Krypton-81m | Gamma | Cyclotron | 13 s | |
| K81m-gas | Inhalation | In vivo | Lung ventilation imaging | Imaging |
| Kr81m-aqueous solution | IV | In vivo | Lung perfusion imaging | Imaging |
| Oxygen-15 | Positron | Cyclotron | 2.04 s | |
| O15-water | IV bolus | In vivo | Cerebral blood flow Myocardial blood flow | Imaging |
| Strontium-89 | Beta | Reactor | 50.5 d | |
| Sr89-Chloride | IV | In vivo | Bone metastases treatment | Therapeutic |
| Technetium-99m | Gamma | Generator | 6.02 h | |
| Tc99m-Pertechnetate | IV | In vivo | Thyroid uptake and imaging Stomach and salivary glands Meckel’s diverticulum Brain imaging | Imaging |
| Tc99m-human albumin | IV | In vivo | Cardiac blood pool / peripheral vascular imaging Lung perfusion imaging | Imaging |
| Tc99m-Phosphonates and phosphates | IV | In vivo | Bone imaging Myocardial imaging | Imaging |
| Tc99m-DTPA | IV | In vivo | Renal imaging Brain imaging | Imaging |
| Tc99m-DTPA | Inhalation | In vivo | Lung ventilation | Imaging |
| Tc99m-DMSA | IV | In vivo | Tumour imaging Renal function | Imaging |
| Tc99m-Colloid | IV | In vivo | Bone marrow GI bleeding | Imaging |
| Tc99m-Colloid | Interstitial | In vivo | Lymph node drainage | Imaging |
| Tc99m-Colloid | Oral | In vivo | Oesophageal transit and reflux Gastric emptying | Imaging |
| Tc99m-HIDA | IV | In vivo | Functional biliary system | Imaging |
| Tc99m-denatured RBCs | IV | In vivo | RBC volume Spleen imaging | Imaging |
| Tc99m-whole RBCs | IV | In vivo | GI bleeding Cardiac blood pool | Imaging |
| Tc99m-MAG3 | IV | In vivo | Renal imaging | Imaging |
| Tc99m-HMPOA | IV | In vivo | Cerebral blood flow | Imaging |
| Tc99m-examatazime labelled leucocytes | IV | In vivo | Infection / inflammation | Imaging |
| Tc99m-Sestamibi | IV | In vivo | Parathyroid / thyroid Myocardial | Imaging |
| Tc99m-Tetrosfosmin | IV | In vivo | Parathyroid Myocardial | Imaging |
| Tc99m-Tilmanocept | Interstitial | Lymphatic mapping with handheld gamma counter | Non-imaging | |
| Thallium-201 | Gamma | Cyclotron | 73.5 h | |
| Tl201-Tl+ | IV | In vivo | Thyroid tumour / parathyroid adenoma Myocardial | Imaging |
| Xenon-133 | Gamma | Reactor | 2.26 d | |
| Xe133-gas | Inhalation | In vivo | Lung ventilation | Imaging |
| Xe133 in isotonic sodium chloride solution | IV | In vivo | Cerebral blood flow | Imaging |
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
