Root Canal Radiology Refresher
Radiology involves the use of medical imaging to diagnose diseases and guide dental treatment. Radiography is an imaging technique using ionizing electromagnetic radiation (usually x-rays) to view objects inside the body or teeth. In radiographic imaging, a linearly propagating wave forming a divergent beam is both partly transmitted and partly absorbed by the area under investigation showing the structure by varying thickness and density.
A radiographic change will not begin to manifest until demineralization extends through the cortical plate of the bone. More than 30% of bony destruction is required to become visible on a radiograph, meaning the absence of radiographic signs of pathology does not mean the absence of pathology. The pulpal status cannot be determined by radiographic images alone.
The type of radiograph indicated depends on what is sought. A posterior bitewing radiograph is most beneficial for detected coronal caries or other causes of pulpal inflammation. If periapical pathology is suspected, a periapical radiograph is indicated. All radiographs should be taken with a holder to allow for reproducible parallelism and standardization. Specific areas to inspect include:
- Periradicular area – lesions of pulpal origin present as the loss of lamina dura apically. The inflammatory process must spread to the cortical bone to be visible.
- Pulpal area – internal resorption or calcification may indicate long term irritation. Pulp canal obliteration (canal calcification) does not necessarily indicate a need for treatment.
RADIOGRAPHIC EQUIPMENT
The dental x-ray unit tube is surrounded by a glass housing that holds a vacuum and sealed by an aluminum or plastic frame. The glass prevents radiation from escaping. The x-ray source consists of two electrodes, the anode (positive) and cathode (negative). The cathode filament is heated up to emit electrons which are accelerated toward anode by applying a 20–200 kV potential in a vacuumized x-ray tube. A focusing cup magnetically focuses the electrons into a narrow beam directed at a small area on the anode (focal spot). Between the cathode and anode is a tungsten target, which when hit produces x-rays. This process produces an enormous amount of heat. The vacuum prevents combustion and provides a clean environment for electron conduction.
Kilovoltage (kV) refers to the unit of measure of electrical potential between the anode and cathode. Adjusting the kV of a dental x-ray unit will affect both contrast and density. A higher kV will impart more energy to the electrons as they travel between the two points and produce more penetrating beams, with a higher percentage of radiation reaching the film. If the kilovoltage is lower the resulting film will have higher contrast but reduced density. If the kilovoltage is higher the resulting film will have lower contrast but increased density. Most dental units operate at around 70kV.
Milliamperage (mA) is a major factor in determining the quantity (number) of x-rays produced. If the mA is too low the quantity of x-rays reaching the film will be lower, and the film will be under-exposed (low density). If the mA is too high the quantity of x-rays reaching the film will be higher, and the film will be over-exposed (high density). The relationship between milliamperage and exposure time is a direct one. For example, radiographs obtained with a setting of 0.1 sec and 100 mA are identical to those obtained with a setting of 1.0 sec and 10 mA, assuming that all other factors remain constant.
Exposure time is a measure of how long the x-ray tube is “on”. Low quality long-wavelength x-rays are absorbed via aluminum discs in the x-ray tube (filtration). The shape of the beam emitted also reduces radiation exposure. Proper collimation is used to prevent unnecessary exposure outside the area of interest and improves image quality by producing less scatter radiation. Rectangular collimators limit the size and shape of the beam to just larger than the x-ray film to reduce unnecessary patient exposure.
Density is a visual characteristic of x-ray images that describes the overall darkness of the image (degree of silver blackening). An image with a proper density will show black areas (air spaces), white areas (enamel, amalgam) and gray areas (soft tissue, cancellous bone) properly. An increase in density will be noted if:
- kV is increased.
- mA is increased.
- Exposure time is increased.
- Thickness of the subject is decreased.
Contrast refers to the difference in the degree of blackness (densities) between adjacent areas on a dental radiograph. A ‘high contrast’ radiograph has very dark areas and very light areas. A radiograph that instead has many shades of gray is said to have a ‘low contrast’.
The focal spot refers to the area on the anode (tungsten wire) hit by electrons to produce x-rays. The focal spot can also be referred to as the target. The size of the tungsten target ranges from 0.5-1.0mm and is a set parameter of the device (cannot be changed). The smaller the focal spot, the sharper the image.
The target-film distance refers to the distance between the x-ray film and the anode. Decreasing the target-film distance increases the magnification of the object and increases the density. A shorter target-film distance requires less radiation to produce an adequate exposure. If minimizing radiation was the primary aim, an 8 inch x-ray tube would be preferred. However, a shorter tube will be less effective at eliminating divergent x-rays, leading to increased magnification and less diagnostic quality (loss of clarity). The rectangular area projected outwards to produce an x-ray field is known as the effective focus.
Radiographic image quality depends on:
- Secondary radiation – fog and scatter.
- Focal spot size – the smaller the focal spot, the sharper the image produced.
- Object-film distance – decreasing the object-film distance increases sharpness.
- Target-object distance – the larger the target-object distance, the sharper the image.
- Motion – loss of sharpness if the patient moves during the exposure time.
- Exposure time – faster exposure time will result in a sharper image.
In order to take accurate, reproducible radiographs, it is recommended to:
- Use the smallest focal spot size.
- Use the shortest object-film distance to prevent distortion.
- Use the longest target-film distance (longer tube) to reduce divergent x-rays.
- Position the x-ray tube at 90° to the object.
- Keep the object and film parallel.
Possible reasons for an under-exposed film:
- Low kilovoltage.
- Low milliamperage.
- Low exposure time.
- High target-object distance.
Possible reasons for an over-exposed film:
- High kilovoltage.
- High milliamperage.
- High exposure time.
- Low target-object distance.
