Medical imaging – The science of seeing inside your body
Dr Michael Wilson, University of Birmingham
Find out how physicists build machines that do what our eyes cannot – see inside the human body.
This lecture revealed how:
Over the past hundred years physicists have developed increasingly sophisticated techniques to see inside the body;
These techniques use x-ray, radioactive molecules and magnetic fields to produce images of the body
These images allow doctors to better diagnose and treat illness and disease.
The lecture can be viewed/downloaded from:
Visible light can pass through the body but it is not very useful for imaging (yet).
The very “interesting” pictures below are of my (Helen’s) finger taken with my camera with the flash on, at different distances. The red and IR part of the spectrum interact with the oxygen in my blood to give the red colour.
The video below shows how the different hues of red actually allow you to calculate the pulse if you have the right equipment.
The amplitude of the pulse can also give you an idea about oxygen saturation of the blood. This is the basis of a Pulse Oximeter.
The reason why visible light can’t be used for direct imaging can be demonstrated by shining light through a tank of water with a little milk added.
This is the type of scattering which plays the major role in medical applications. In tissues the light is scattered at cells or their components. In milk it is scattered mostly at the tiny fat droplets.
The above picture is actually a cross of light viewed through milky water. It appears foggy.
Electron beams are known as cathode rays and are the basis of old fashioned television sets. Beams of electrons can be affected by magnets. As seen in the picture below.
Electron beams are used to produce X-rays. In an X-ray tube electrons from the cathode collide with the anode material, usually tungsten, molybdenum or copper, and accelerate other electrons, ions and nuclei within the anode material. About 1% of the energy generated is emitted/radiated, usually perpendicular to the path of the electron beam, as X-rays.
Wilhelm Conrad Röntgen (27 March 1845 – 10 February 1923) was a German physicist, who is usually credited as the discoverer of X-rays in 1895, because he was the first to systematically study them, although he was not the first to have observed their effects. He is also the one who gave them the name “X-rays”, though many referred to these as “Röntgen rays” (and the associated X-ray radiograms as, “Röntgenograms”) for several decades after their discovery and even to this day in some languages, including Röntgen’s native German. For his discovery he was awarded the first Nobel Prize in Physics in 1901.
Above right is a print of Wilhelm Röntgen’s first “medical” X-ray. It is of his wife’s hand, taken on 22 December 1895 and presented to Ludwig Zehnder of the Physik Institut, University of Freiburg, on 1 January 1896.
The use of X-rays went global within weeks.
The above left image show an X-ray of lungs damaged my pneumonia. The above right image shows an X-ray of teeth.
X-rays are brilliant at imaging hard material like bones and teeth but they need a bit of help if you want to image something like the digestive system.
X-ray of the stomach with both positive (barium sulphate) and negative (carbon dioxide) radiocontrast.
Radioactivity was discovered in 1896 by the French scientist Henri Becquerel (15 December 1852 – 25 August 1908), while working on phosphorescent materials. These materials glow in the dark after exposure to light, and he suspected that the glow produced in cathode ray tubes by X-rays might be associated with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent salts on it. All results were negative until he used uranium salts. The result with these compounds was a blackening of the plate. These radiations were called Becquerel Rays.
Below left is a photograph of Becquerel.
Above right is a picture of the image of Becquerel’s photographic plate which has been fogged by exposure to radiation from a uranium salt. The shadow of a metal Maltese Cross placed between the plate and the uranium salt is clearly visible.
It soon became clear that the blackening of the plate had nothing to do with phosphorescence, because the plate blackened when the mineral was in the dark. Non-phosphorescent salts of uranium and metallic uranium also blackened the plate. It was clear that there is a form of radiation that could pass through paper that was causing the plate to become black.
At first it seemed that the new radiation was similar to the then recently discovered X-rays. Further research by Becquerel, Ernest Rutherford, Paul Villard, Pierre Curie, Marie Curie, and others discovered that this form of radioactivity was significantly more complicated.
Pierre and Marie Curie in their Paris laboratory, before 1907.
Radioactive quackery refers to various products sold during the early 20th century containing radioactive ingredients, after the discovery in 1896 of radioactive decay. The products promised radioactivity as a cure for various illnesses. Many brands of toothpaste were laced with radioactive substances that were claimed to make teeth shine whiter, such as Doramad Radioactive Toothpaste.
“Tho-radia” powder, based on radium and thorium, according to the formula of Dr. Alfred Curie (not related to Pierre and Marie Curie) was claimed to have curative properties.
Unlike X-rays radioactivity cures were not such a hit.
Rutherford discovered the concept of radioactive half-life, proved that radioactivity involved the transmutation of one chemical element to another, and also differentiated and named alpha and beta radiation. In 1907 he proved that alpha particles were helium nuclei and in 1911 he was awarded a Nobel Prize for the interpretation of the Rutherford scattering experiment.
Under his leadership the neutron was discovered by James Chadwick in 1932 and in the same year the first experiment to split the nucleus in a fully controlled manner, performed by students working under his direction, John Cockcroft and Ernest Walton.
Splitting the atom releases a great deal of energy but trying to add a proton to a nucleus requires a great deal of energy to overcome great forces.
In order to probe the nucleus still further more energetic particles than that delivered by radioactivity had to be found. And the cyclotron was one of the methods used to produce them.
The cyclotron was invented and patented by Ernest Lawrence of the University of California, Berkeley, where it was first operated in 1932. A graduate student, M. Stanley Livingston, did much of the work of translating the idea into working hardware.
A cyclotron is also used to produced different isotopes of the elements needed in nuclear medicine.
There are three types of radioactivity, alpha, beta and gamma. Alpha radiation is very dangerous if it gets inside the body.
Alexander Valterovich Litvinenko (4 December 1962 to 23 November 2006) was an officer of Russian FSB fugitive secret service who specialized in tackling organized crime. In 2000 he fled with his family to London and was granted asylum in the United Kingdom, where he worked as a journalist, writer and consultant for British intelligence services.
On 1 November 2006, he suddenly fell ill and was hospitalised. His illness was later attributed to poisoning with radionuclide polonium-210 after the Health Protection Agency found significant amounts of the rare and highly toxic element in his body. However, the London coroner’s inquest is yet to be completed.
Po 210 is an alpha emitter that has a half-life of 138.376 days; it decays directly to its daughter isotope Pb 206. A milligram of Po 210 emits as many alpha particles per second as 5 grams of Ra 226. Very little is required to cause radiation sickness.
Gamma rays have a much greater use in medicine.
Nuclear medicine is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease.
In nuclear medicine procedures, radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs or cellular receptors. This property of radiopharmaceuticals allows nuclear medicine the ability to image the extent of a disease process in the body, based on the cellular function and physiology, rather than relying on physical changes in the tissue anatomy. In some diseases, nuclear medicine studies can identify medical problems at an earlier stage than other diagnostic tests. Nuclear medicine, in a sense, is “radiology done inside out”, or “endo-radiology”, because it records radiation emitting from within the body rather than radiation that is generated by external sources like X-rays.
Before the widespread application of technetium-99m in nuclear medicine, the radioactive isotope thallium-201, with a half-life of 73 hours, was the main substance for nuclear cardiography. The nuclide is still used for stress tests for risk stratification in patients with coronary artery disease (CAD). Care needs to be taken with it because a trace of it is incredibly poisonous
Krypton-83 has application in magnetic resonance imaging (MRI) for imaging airways. In particular, it may be used to distinguish between hydrophobic and hydrophilic surfaces containing an airway
Gamma emission from the radioisotope 133 Xe of xenon can be used to image the heart, lungs, and brain, for example, by means of single photon emission computed tomography. 133Xe has also been used to measure blood flow.
Xenon, particularly hyperpolarized 129Xe, is a useful contrast agent for magnetic resonance imaging (MRI).
Iodine, as an element with high electron density and atomic number, absorbs X-rays well. Therefore, it may be used as a radiocontrast agent by filtering out imaging X-rays weaker than 33.3 keV. Also used in angiography and CT scanning.
Although gallium has no known role in biology, it mimics iron (III), the gallium ion localizes to and interacts with many processes in the body in which iron (III) is manipulated. As these processes include inflammation, which is a marker for many disease states, several gallium salts are used, or are in development, as both pharmaceuticals and radiopharmaceuticals in medicine.
111In emits gamma radiation and is used in indium leukocyte imaging, or indium scintigraphy, a technique of medical imaging that is particularly helpful in differentiating conditions such as osteomyelitis from decubitus ulcers for assessment of route and duration of antibiotic therapy. Indium leukocyte scintigraphy has many applications, including early phase drug development, and the monitoring of activity of white blood cells. For the test, blood is taken from the patient, white cells removed, labelled with the radioactive 111In, and then re-injected back into the patient. Gamma imaging will then reveal any areas of on-going white cell localization such as new and developing areas of infection.
Technetium-99m (“m” indicates that this is a metastable nuclear isomer) is used in radioactive isotope medical tests, for example as the radioactive part of a radioactive tracer that medical equipment can detect in the human body. It is well suited to the role because it emits readily detectable 140 keV gamma rays, and its half-life is 6.01 hours (meaning that about 94% of it decays to technetium-99 in 24 hours).
Hal Oscar Anger (May 20, 1920 – October 31, 2005) was an American electrical engineer and biophysicist at Donner Laboratory, University of California, Berkeley, known for his invention of the gamma camera (also known as a scintillation or Anger camera).
A gamma camera is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy. The applications of scintigraphy include early drug development and nuclear medical imaging to view and analyse images of the human body or the distribution of medically injected, inhaled, or ingested radionuclides emitting gamma rays.
Below is an example of lung a scintigraphy examination.
In order to produce images of the desired organs (e.g. heart) the patient needs to have absorbed an intravenous injection of radioactive material, usually thallium-201 or technetium-99m.
Gamma cameras contain sodium iodide crystals activated with thallium, NaI(Tl), and when subjected to ionizing radiation they emit photons (i.e., scintillate). They are used in scintillation detectors, traditionally in nuclear medicine, geophysics, nuclear physics, and environmental measurements. NaI(Tl) is the most widely used scintillation material. The crystals are usually coupled with a photomultiplier tube, in a hermetically sealed assembly, as sodium iodide is hygroscopic (it absorbs water). The crystals give out blue light when hit by gamma photons.
Photomultiplier tubes (photomultipliers or PMTs for short), are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the current produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages, enabling (for example) individual photons to be detected when the incident flux of light is very low. They are therefore very useful in producing images of organs when the crystals have not given out much light. In order to have as much detail in the image as possible you need as many pixels as possible. Unfortunately this comes at a price. 250000 pixels would cost £25000000.
Visible light does have an advantage over X-rays and gamma rays in that they can be focused. This is important in the human eye. The lens and cornea focus light on to the retina.
The above images are of the same object but you can distinguish the cross on the right because there is a lens present. You can also get the cross image with a pin hole camera with many pin holes. This is the sort of arrangement that is necessary with gamma and X-rays.
It is now possible to use flashes of light to see individual molecules inside the body. This is how it is now possible to image the epiphyseal plates producing bones and the effect of cancer on the bone.
Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. A device used in tomography is called a tomograph, while the image produced is a tomogram. Tomography as the computed tomographic (CT) scanner was invented by Sir Godfrey Hounsfield, and thereby made an exceptional contribution to medicine. The method is used in radiology, archaeology, biology, geophysics, oceanography, materials science, astrophysics, quantum Information, and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction.
Normal pictures are two dimensional but medical diagnosis needs three dimensional images. Bunches of X rays are needed.
More modern variations of tomography involve gathering projection data from multiple directions and feeding the data into a tomographic reconstruction software algorithm processed by computer. Different types of signal acquisition can be used in similar calculation algorithms in order to create a tomographic image. Tomograms are derived using several different physical phenomena. Safe level of radiation is up to 10 scans per second.
|physical phenomenon||Type of tomogram|
|electrons||Electron tomography or 3D TEM|
|magnetic particles||magnetic particle imaging|
The Radon transform is widely applicable to tomography, the creation of an image from the scattering data associated with cross-sectional scans of an object.
Dr Wilson demonstrated how tomography worked by showing how lots of scans can build up a picture. Each beam contains information. There were lots of mini images of a stick of rock and when they were put together it showed a picture of Angelina Jolie.
The body is transparent to radio waves but radio frequencies are very useful. All materials have a magnetic susceptibility (the X – factor). In electromagnetism, the magnetic susceptibility is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field.
The above picture shows Dr Wilson demonstrating this with a grape. The grape was actually being affected by the magnet. Every material has the X factor, although it may be tiny.
Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize internal structures of the body in detail. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body. MRI can create more detailed images of the human body than are possible with X-rays. It uses magnetic fields and radio waves to produce an image that is dependent on the distribution of hydrogen (protons) in the body.
When a person is inside the powerful magnetic field of the scanner, the average magnetic moment of many protons (found in the water inside the body) becomes aligned with the direction of the field. A radio frequency current is briefly turned on, producing a varying electromagnetic field. This electromagnetic field has just the right frequency, known as the resonance frequency, to be absorbed and flip the spin of the protons in the magnetic field. After the electromagnetic field is turned off, the spins of the protons return to thermodynamic equilibrium and the bulk magnetization becomes realigned with the static magnetic field. During this relaxation, a radio frequency signal (electromagnetic radiation in the RF range) is generated, which can be measured with receiver coils.
Dr Wilson demonstrating this spinning and flipping with a magnetic top. The flip occurs when the top briefly stops spinning.
The MRI scanner messes with the “magnets” in us.
In physics, resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response amplitude is a relative maximum are known as the system’s resonant frequencies, or resonance frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy. Resonance usually happens when the object is forced to oscillate at its natural frequency. Dr Wilson demonstrated by producing a note when blowing across a partially filled bottle of water.
Functional magnetic resonance imaging or functional MRI (fMRI) is an MRI procedure that measures brain activity by detecting associated changes in blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases.
These fMRI images above are from a study showing parts of the brain lighting up on seeing houses and other parts on seeing faces. The ‘r’ values are correlations, with higher positive or negative values indicating a better match.
Cardiological contraction is caused by changes in electrical potential in the hearts muscle cells; electrical activity that the body conducts to its surface.
Although it is altered by the intervening tissue, the resulting signal at the skin accurately reflects the cardiological cycle and can be used to identify any anatomical and physiological anomalies in a completely non-invasive manner.
Ultrasound imaging systems uses piezoelectric transducers as source and detector.
Piezoelectric crystals vibrate in response to an alternating voltage, and when placed against a patient’s skin and driven at high frequencies produce ultrasound pulses that travel through the body.
As they travel outwards and encounter different layers within the body the ultrasound waves are reflected back towards the source.
The returning signal drives the crystals in reverse and produces an electronic signal that is processed to construct the image. Compared to MRI, ultrasound has the advantages of low cost and portability.
It is also preferred over x-ray imaging for procedures in which ionising radiation poses a significant risk, such as checking foetal development during pre-natal care.
Kidney problems are investigated using many imaging techniques including ultrasound, IVP (intravenous pyelogram), CAT scan (computed tomography) and MRI (magnetic resonance imaging).
Above left picture is an ultrasound image showing a kidney and the above right picture is an IVP image of the kidney.
The above pictures are CAT scans of kidneys.
The above picture is an MRI of the kidneys.