Medical imaging has long been used to detect all kinds of conditions like cancers, osteoporosis, heart disease and more. However, is doing the tests more harmful than safe?

BY: Eleanor Yap

 

Medical imaging gives doctors a glimpse of the interior of our bodies. This can help them detect the extent of a disease or trauma. However, is doing it safe?

Ageless Online talks to Dr Lim Tze Chwan, consultant, Department of Diagnostic Radiology, Khoo Teck Puat Hospital, to put our minds at ease. We find out more about the different medical imaging and how they help, and the extent of radiation they emit:

 

What are the different types of medical imaging?

Medical diagnostic imaging comes in various modalities, which can be broadly divided into two main groups:

a) Imaging requiring the use of ionising radiation (such as radiography, computed tomography, fluoroscopy, angiography, bone density scanning and nuclear medicine) and

b) Imaging that does not involve ionising radiation (ultrasonography and magnetic resonance imaging).

As a general principle, medical diagnostic imaging aims to produce an image (or a series of images) that is representative of a particular part of the human body. A trained specialist, whose role is to differentiate between normal and abnormal features, can then interpret these images. The information derived from these studies is useful in directing the management or treatment of many conditions. 

Below is a brief description of the imaging modalities:

Imaging using ionising radiation:

• Radiography (X-rays) – More commonly known as “X-rays”, radiographs are produced as a result of the interaction between X-ray photons and various detectors. An X-ray photon is a type of electromagnetic radiation that carries sufficient energy to pass through our body, and depending on the type of tissue that the photons traverse, varying levels of energy transfer occur. This leads to a resultant image that is in various shades of black, grey and white. Tissue that readily allows X-ray photons to cross without much loss in energy will appear “darker”. In contrast, tissue that absorbs more energy from the photons will appear “lighter”. Radiographs are readily available in most parts of the world and are relatively inexpensive. Hence, radiography tends to be the main imaging modality when it comes to screening programmes.   

• Computed tomography (CT) – Computed tomography makes use of X-ray photons and computer processing to create a series of cross-sectional images. This technique involves considerably more exposure to radiation compared to projection radiography. However, CT resolves the problem of overlapping structures that tend to confound the interpretation of conventional radiographs. This modality also allows for better differentiation of various body structures and detection of abnormalities, especially when contrast media are administered concurrently.

• Fluoroscopy – Fluoroscopy is a technique of utilising projection radiography to view dynamic movement of contrast media, instruments or parts of the body. Apart from its application as a useful diagnostic tool, fluoroscopy is also widely used in various interventional procedures such as bone/joint surgery, angioplasty and advanced endoscopic procedures. 

• Angiography – Angiography is a technique of utilising fluoroscopy to evaluate the circulatory system for bleeding, blockage, structural abnormality and malformation. This usually requires an iodinated contrast medium to be administered into the circulation via either the venous system or the arterial system. Appropriate treatment can be instituted once the abnormality is localised on an angiogram.

• Bone density scanning or Dual-energy X-ray absorptiometry (DEXA) – The primary use for DEXA is in assessment of bone density. Osteoporosis is a common problem of ageing and DEXA allows for the amount of calcium (bone density) to be determined non-invasively. The amount of radiation exposure is lower than for radiographs. However, the images produced are only useful in determining bone density and are not of sufficient resolution to diagnose other bone abnormalities such as fractures, tumours and infections. There is also a limited role in DEXA for determination of body composition. 

• Nuclear medicine – Nuclear medicine involves the administration of radioactive substances for diagnostic as well as therapeutic purposes. As opposed to the afore-described imaging modalities, ionising radiation is emitted from within the body rather than from an external source (such as an X-ray tube). The radioactive substances are tagged to various chemical or pharmaceutical compounds, such that the resultant radiopharmaceutical can be transported to specific organs. As a result, nuclear medicine allows one to evaluate the extent of disease involvement based on cellular function rather than structural distortion. In some conditions, the radiation that is emitted by the radiopharmaceuticals at the intended site can be used to treat the underlying pathology. Imaging techniques combining both functional and structural imaging, such as positron emission tomography – computed tomography (PET-CT) and positron emission tomography – magnetic resonance imaging (PET-MRI), have various applications in oncology, neurology and cardiology.     

Imaging not using ionising radiation:

• Ultrasonography – Ultrasonography is an imaging technique using the transmission and reflection of high-frequency sound waves to visualise internal body structures. High-frequency sound waves are emitted by the ultrasound probe and transmitted through tissues with varying degree of reflection, depending on the type as well as condition of the tissues. The ultrasound probe also serves as the receiver of the reflected sound waves. Flow of blood can also be investigated using Doppler effect (Doppler ultrasound). Ultrasonography produces cross-sectional images but does not involve ionising radiation. In addition, the physical footprint of an ultrasound scanner, as well as the cost, is significantly smaller compared to other modes of cross-sectional imaging (e.g. CT or magnetic resonance imaging), making this modality relatively available.

• Magnetic resonance imaging (MRI) – Another form of cross-sectional imaging that does not involve ionising radiation is magnetic resonance imaging. This technique relies on strong magnetic fields and radiofrequency signals. There is a considerable overlap in clinical applications of MRI with that of CT. One of the advantages of MRI is that the patient is not subjected to radiation exposure. Secondly, imaging of certain tissues or organs is better with MRI (relative to CT) due to better image quality and diagnostic information, as a result of the intrinsic characteristics of these tissues/organs. The intrinsic characteristics of other structures may also favour CT over MRI, as the imaging modality of choice. Hence, if both MRI and CT yield the same level of diagnostic information for a particular study, MRI is preferred over CT due to the lack of radiation exposure. However, there are contraindications in MRI, such as most cardiac pacemakers, most cochlear implants, metallic foreign bodies, some metallic surgical implants and claustrophobia, etc. Recent introduction of the extremity MRI scanner, and to a certain extent, the wide-bore MRI scanner, allows claustrophobic patients to be scanned without significant penalty in imaging quality. 

 

Is X-ray harmful?

X-ray photons can cause damage to DNA because of their ability to ionise atoms and disrupt molecular bonds, with the resultant effects manifested at the cellular level. Damage to the cellular DNA may lead to cancers. Risk projection studies have indicated that a single exposure of 100 millisievert (mSv) would increase the chances of cancer development by one percent. In the context of medical diagnostic imaging, the radiation dose of a chest radiograph is about 0.02 mSv, while that of a CT scan is about 10 to 20 mSv. The chance of a diagnostic study with radiation exposure exceeding 100 mSv is very small and hence the cancer risk from ionising radiation through medical imaging is very low. In principle, healthcare professionals will have to balance the benefits and potential risks of performing diagnostic studies involving ionising radiation, and minimise the radiation dose to as low a level as reasonably achievable.   

 

How can medical imaging save lives?

Medical imaging can save lives by:

• Timely disease and injury detection – Early disease detection allows for early intervention and invariably leads to a better outcome. This is the basis of health screening programmes around the world. Medical imaging plays an important role in most of these programmes, especially cancer screening. Screening of at-risk groups of the population, via chest radiographs and mammograms, aims to detect cancers at an early stage.

Medical imaging has an important role in the management of acute trauma, by detecting or confirming the site and extent of injury. 

As some empirical therapies may cause potential adverse effects or complications, medical imaging is increasingly utilised to confirm the diagnosis before therapeutic procedures or medications are administered.

• Monitoring of treatment response or complications of treatment – Apart from detection of the primary pathology or injury, medical imaging can also be used to detect the complications arising as a result of therapy. Imaging follow-up is common in cancer management to monitor the disease response to chemotherapeutic agents. Patients who are in cancer remission can also be monitored via medical imaging to exclude progression or recurrence of disease.

• Prognostic tool – Information derived from medical imaging is often helpful for prognostication of eventual outcome, rate of disease progression and treatment response. Some cancer staging systems take into account the findings derived from imaging studies to determine the stage of disease.  

• Therapeutic option – Interventional radiology is a subspecialty of radiology that uses image-guided procedures to diagnose and treat various conditions. Such procedures tend to be minimally invasive and can be life-saving in some instances, e.g. catheter embolisation of a bleeding vessel and transcatheter arterial chemoembolisation.

As mentioned above, nuclear medicine can also be therapeutic when radiation from radiopharmaceutical is utilised to treat the underlying condition. An example is that of radioactive iodine (RAI) therapy. As the thyroid gland absorbs iodine, radioactive iodine isotope (¹³¹I) is concentrated in the thyroid gland and the resultant emitted radiation causes ablation of the thyroid tissue. RAI therapy is used to treat some forms of thyroid cancer and hyperthyroidism.

 

(** PHOTO CREDIT: Khoo Teck Puat Hospital)

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