3. Factors affecting x-ray Image Quality:
Factors like image contrast and brightness govern how medically useful an x-ray image is. To minimize the exposure of the patient to x-rays, it is important to understand how some of the user controlled factors can be manipulated to generate a medically relevant image in the first attempt.
A. Peak Kilo Voltage (kVp)
- kVp or Peak Kilo Voltage is the maximum potential difference applied across the cathode and anode of the x-ray generator to accelerate the electrons.
- Higher the kVp, higher the kinetic energy of the electrons, which in turn determines the ability of the x-rays to penetrate different tissue densities.
- Therefore, kVp plays an important role in affecting the radiological contrast of the x-ray image.
- As a general rule, denser tissues such as the thigh bone require a higher kVp to achieve an optimal contrast on the image.
B. Current (mA)
- The mA value refers to the current passed through the filament of the vacuum tube for the generation of the electrons.
- Larger is the value of current, more is the intensity of x-rays generated. This means that the brightness of the image is directly proportional to mA.
- If all other factors, such as kVp and time of exposure, are kept constant then the image brightness and the x-ray dosage that the patient is exposed to will increase with increasing mA.
C. Distance (SID)
- The x-ray intensity received by the detector is inversely proportional to the square of the distance from the x-ray source i.e. it follows the inverse square law.
- The distance between the detector and the generator is known as the Source to Image Receptor Distance or SID.
- Almost all the x-ray imaging systems have an adjustable source to image receptor distance and the same can be adjusted by the operator for optimal intensity requirements.
D. Collimation & Filtering
- As mentioned in the previous slides, accurate collimation is crucial to prevent scattering and ensure that the beam is shaped as per the shape of the detector. This helps to maximise the intensity of x-rays received.
- Metal filters, if present on the system, can be used to alter the intensity of x-rays over a certain part of the exposed tissue. This can be used to achieve uniform contrast, as explained earlier.
E. Noise
- Noise can be introduced in the image due to a number of factors, such as patient movement, electrical fluctuations, system vibrations etc. These need to be avoided as much as possible.
These were some of the factors that play a role in impacting the image quality of an x-ray image. Of course, there are other system details that also impact the final image quality, but these factors can be manipulated by the user to achieve best possible performance with the system at hand.
Now we will discuss the applications of x-ray imaging in the medical domain.
4. Applications of X-ray based Medical Imaging:
Radiography –
Radiography is a common diagnostic application for x-ray imaging. A single snapshot of body parts, such as the chest or the joints, is captured using an x-ray film or a digital detector. The difference in x-ray absorption properties of hard tissues and soft tissues makes radiography a very useful technique for capturing high contrast images of bones and other hard tissues. These images are used for detecting fractures and abnormal growths.
Fluoroscopy –
Fluoroscopy is the x-ray imaging technique that is deployed when real-time imaging of the internal body parts is required. The most common uses include orthopaedic surgery and placement of catheters in the body. Fluoroscopy continuously generates x-ray images, as per the desired frame rate, to help guide the surgeon. It provides an exact representation of the placement of the surgical tools with respect to the tissues in question.
Angiography –
Angiography involves the examination of the veins and arteries to detect any blockages or build-ups. A contrast material is injected into the veins so that the blood vessels can be viewed clearly on the x-ray image. Continuous frames are generated to get x-ray snapshots of the entire blood vessel to diagnose any unusual observations.
Mammography –
Mammography is a screening technique for early breast cancer detection, where the breast tissues are exposed to a very low dose of x-rays. Any abnormal growth or lumps can be detected by this imaging technique, thereby making it a very crucial part of women’s healthcare.
CT Scan (Computed Tomography) –
A CT scan is effectively a 3D representation of organs created using projections of multiple x-ray images taken from different angles. These x-ray projections are generated with the help of a rotating x-ray generator and detector pair. CT Scans provide a very clear, visual depiction of internal body parts and are extremely useful for detecting internal injuries. This technology provides quick results in critical scenarios such as head injuries, strokes and heart disease.
X-ray images help with accurate diagnosis as well as surgical procedures for life-threatening diseases. This is why x-ray exposures in limited doses are allowed by medical bodies around the world, for the prompt detection and effective cure of various conditions.
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Comparison of x-ray imaging with other medical imaging techniques
