Medical Imaging

In last week’s post I promised to write another this week to look into other types of medical imaging and what risks are associated with them. There is lots more detail for those interested in searching the internet. 

An ultrasound is a very straightforward and safe way to test for everything from blood circulation to gall stones. It involves sending high frequency sound waves into your body and when they bounce off the walls of the organ or vessels being examined, the echo is recorded. To take the test you will be asked to lie down and a clear conducting gel is put on your skin over the area being examined. It helps with the transmission of the sound waves. A handheld flat ‘probe’ then gently moves the gel, that both sends and receive signals, over the area in question.  Sometimes you are allowed to see the reflected sound images on a screen. That’s it!   It’s painless and harmless.

An angiogram is used to look mainly at your blood vessels and heart. After giving you a local anesthetic, the procedure usually involves the insertion of a thin hollow tube into your groin from which the radiologist is able to release a contrasting agent that blocks x-rays. X-rays are then sent through your body, recorded and displayed on an overhead screen that you can watch as the test proceeds. It clearly shows up blockages in blood vessels and gives heart surgeons the vital information they need to do by-pass surgery or insert stents etc. As explained in the post on CT Scans when X-rays are absorbed, their energy is released and transferred to electrons that can rip through cells, tearing electrons from all sorts of atoms it meets, creating ions - so x-rays are called ionizing radiation. A second risk involves the contrast agent that contains iodine. It is removed from your blood by your kidneys and liver so it’s very important that you have healthy organs and drink lots of fluids to flush away the iodine solution as quickly as possible - 2 glasses of water before the test and 4 glasses after it.   

An MRI [Magnetic Resonance Imaging] scan, like a CT Scan, produces cross-sectional images that appear to open the body up and allow the doctor to look at it from the inside. MRI scans can give details about soft body tissues like ligaments, cartilages, and nerve systems so that MRI scans are often more versatile than x-rays in that they can image a variety of medical conditions. MRI uses harmless radio waves and a very strong magnetic field so those with metal implants are usually barred from taking the test. MRI machines cost more and scans take longer so they are more expensive.

       A PET [Positron Emission Tomography] scan makes use of a small amount of a radioactive tracer to be able to tell the difference between healthy and diseased tissue.  The usual tracer is a type of sugar called FDG so the test is sometimes called an FDG-PET scan.  Before the exam, a technologist will give you an injection of the radioactive drug and you must wait for an hour or so as it is absorbed in the body. For instance, cancer cells grow faster than healthy ones so they absorb more of the FDG blood sugar. The PET scanner measures the radiation given off by the FDG and produces colored images that show where the cancerous tissue is. It can also detect blood flow to other diseased tissues. The information obtained by a PET scan is not available using any other kind of imaging and provides doctors with crucial information for diagnosis. Many PET scanners also include a CT scanner and a PET-CT Scan will image both body structure and how it is functioning at the same time. The PET machine is like other scanners with a hole in the middle that looks like a donut and a table that slides into the machine. After the test you should drink lots of water to flush out the radioactive FDG and any contrast agents that may have been used.  

The latest test is an MRA, the combination of an MRI and Angiogram. Seems that we are getting closer all the time to ‘Star Wars’ sci-fi technology where diagnosis is determined by just passing a wand over the patient!   Rie 

CT Scans

There are many different imaging techniques that are used by doctors these days to be able to determine what is going on inside our bodies. Available are Ultrasounds, Angiograms, CT (or CAT) scans, MRI’s and PET scans, each with own diagnostic characteristics.  Some are totally non invasive but others involve X-rays and contrasting agents that are injected into the patient to make the images obtained more revealing and useful.

It’s an important subject and one that everyone should be familiar with as more and more use is being made of these very valuable techniques that help in the diagnosis of cancer, blockages in our arteries, sinus problems or any number of ailments. This week a young child in my immediate family had a CT scan of his head and it gave the doctor and his family a very clear picture of a congenital problem and how it could be corrected with surgery. My concern is that in taking the CT scan, multiple X-rays were taken so they could be combined in a computer to give 2-dimensional slices through the head – see picture on left. 

X-rays are just like any other kind of light (called electromagnetic radiation) but our eyes are only sensitive to the visible part of the spectrum and, as shown, x-rays are in the invisible range with short wavelengths and very high energy. Ordinary light doesn’t have enough energy to penetrate flesh but x-rays do.  Notice in the x-ray of the hand, however, that the x-rays are stopped by bone and partly absorbed as they pass through the flesh.

We know that when X-rays are absorbed in the body, their energy is released and it is transferred to an electron that can rip through a cell, tearing electrons from all sorts of atoms it meets, like a billiard ball spreading its energy to other balls on the table. Usually the human body is good at repairing itself but on rare occasions the damage may create the starting point where cancer can develop.  

When digital technology became available it eliminated some of the risk associated with x-rays because a much lower radiation dose could be used and the digital images had sufficient resolution so that they could be manipulated on a computer to obtain detailed information from them.

Thus it is now really quite safe to have the infrequent low dose x-rays taken at the dentists or in hospital? However, studies show that children are more sensitive to radiation, and they should have a CT scan only if it is essential for making a diagnosis and they should not have repeated CTscan unless absolutely necessary.

I plan to write another post next week to look into other types of medical imaging and what risks are associated with them.   Rie