FMS stands for Focalspot Measurement System in radiography. It is a quality assurance tool used to measure the focal spot size of an x-ray tube. The focal spot is the area on the anode of the x-ray tube where electrons hit and produce x-rays. Maintaining the proper focal spot size is important for image quality.
What is the focal spot?
The focal spot is the origin point of the x-rays produced in the x-ray tube. It is the region on the angled anode target where the electron beam is focused. The size and shape of the focal spot affects the geometric sharpness and resolution of the resulting x-ray beam. A large focal spot will produce images with more geometric unsharpness compared to a small focal spot.
The focal spot has two dimensions – its length and width. The length is parallel to the direction of electron flow while the width is perpendicular. In radiography, the width of the focal spot is the more important measurement. This width must be small to produce fine and detailed radiographic images.
Why is focal spot size important in radiography?
The size of the focal spot is important because it affects the sharpness and resolution of radiographic images. A large focal spot will result in images with more geometric unsharpness and blurring of fine details. A small focal spot provides sharper images with improved resolution of fine structures and details.
A larger focal spot causes more penetration of the x-ray beam. This reduces subject contrast in the image. A smaller focal spot increases subject contrast and visibility of low contrast objects. It also minimizes objectionable scatter radiation.
Most radiographic applications require a small focal spot for adequate image quality. A smaller focal spot is necessary when imaging small anatomical parts like extremities and for techniques using a fine focus like mammography. A larger focal spot may be acceptable for larger anatomy like the torso and in cases where motion unsharpness overrides focal spot unsharpness.
What is geometric unsharpness?
Geometric unsharpness refers to the blurring of fine details in the radiograph due to the finite size of the focal spot. It is the major factor that limits the sharpness and resolution of a radiographic image.
An ideal point source focal spot would project the patient as a sharp shadow image onto the image receptor. However, the actual focal spot has a measurable size. Rays from the periphery of the focal spot travel at slight angles relative to rays from the center.
This angular divergence causes the edges of structures in the patient to be recorded as blurred shadows on the image receptor. This blurring is geometric unsharpness. The amount of blur increases with focal spot size and distance from the focal spot to the image receptor.
How does focal spot size affect image quality?
The focal spot size has several impacts on radiographic image quality:
- Smaller focal spots improve image resolution and the ability to see fine detail. Large focal spots degrade resolution and small detail visibility.
- Smaller focal spots provide better subject contrast. Large focal spots reduce subject contrast.
- Smaller focal spots minimize scatter radiation reaching the image receptor. Large focal spots increase off-focus radiation.
Mammography requires the smallest focal spots, typically 0.1 mm or less, to produce detailed images of breast tissue structures. General radiography uses 0.5 to 2 mm focal spots. CT and fluoroscopy use larger focal spots due to heat loading on the anode but use collimation to limit the apparent size.
What factors affect focal spot size?
Several technical factors impact the size of the actual focal spot produced in the x-ray tube:
- Tube voltage (kVp) – Higher tube voltages result in a larger focal spot. This is due to a wider distribution of electrons striking the anode at higher energies.
- Tube current (mA) – High tube current settings spread the impacting electrons over a larger target area, enlarging the focal spot.
- Anode angle – The angle of the anode surface affects focal spot size. A smaller angle focuses electrons in a tighter spot.
- Anode rotation – A stationary anode target results in larger focal spots compared to rotating anode designs.
- Focal spot selection – X-ray tubes are designed with small and large focal spot options. Different spot sizes are selected based on the exam.
What is the difference between actual and nominal focal spot sizes?
There are two measurements that characterize the focal spot width:
- The nominal focal spot is the designed focal spot size specified by the x-ray tube manufacturer under standardized conditions.
- The actual focal spot is the measured focal spot size under real-world x-ray machine operating conditions. This varies based on tube settings.
For quality assurance testing, the actual focal spot size must be measured and compared to the nominal size specified by the manufacturer. Actual measurements are also used to determine the impact of focal spot enlargement on image quality at different exposure settings.
How is focal spot size measured?
There are three methods commonly used to measure the actual focal spot size of an x-ray tube:
- Pinhole camera – A pinhole image is projected onto a high resolution detector. The focal spot width is calculated from the pinhole geometry.
- Slit camera – Slit apertures are used to measure focal spot length and width profiles.
- Star pattern device – A metal star test device produces an image that demonstrates focal spot enlargement. The amount of star pattern blur indicates the focal spot size.
These focal spot measurement techniques require specialized devices and phantoms. They are time consuming and difficult to perform accurately. This led to the development of automated focal spot measurement systems.
What is an FMS?
FMS stands for focal spot measurement system. These are automated quality assurance (QA) devices designed specifically to measure the focal spot size in x-ray tubes.
An FMS uses an edge test device rather than a pinhole or slit. This edge is aligned to cast a narrow shadow on a radiation detector. As radiation passes the edge, the penumbra blurring of the shadow is analyzed to derive the focal spot size.
The detector signal in an FMS may be acquired using an image sensor or scintillator/photodiode setup. Custom electronics and software analyze the penumbra to accurately calculate the actual focal spot dimensions under existing tube conditions.
Why are FMS devices used?
Focal spot measurement systems offer major advantages over traditional focal spot measurement techniques:
- FMS devices are highly automated and easy to use for QA testing.
- Measurements can assess focal spot size along both dimensions (length and width).
- Testing can be performed quickly at a range of techniques.
- Results are generated immediately by the system software.
- Minimal radiation exposure is required for FMS measurements.
- Results are more accurate and consistent compared to manual methods.
These benefits make FMS devices ideal for convenient and standardized focal spot quality control. The elimination of human errors in setup and measurement reduces variation in results.
How do you measure focal spot size with an FMS device?
Using a focal spot measurement system involves three basic steps:
- The FMS edge test device is positioned in the beam close to the x-ray tube focal spot. This may require a tube attachment.
- Exposures are made using techniques similar to clinical imaging. The penumbra blurring of the edge is acquired.
- FMS software analyzes the penumbra to calculate the focal spot size at the mAs and kVp settings.
The entire measurement process can be completed in just a few minutes. Results are displayed and can be printed, stored, or transferred electronically.
What are the components of an FMS?
A focal spot measurement system consists of several key components:
- An edge test device, typically tungsten.
- A radiation detector, either scintillator-based or using a CMOS sensor.
- A motion control system to align the edge device in the beam.
- An exposure switch or remote control.
- A computer and software to acquire and analyze data.
- A system enclosure or cart assembly.
These components are integrated by the manufacturer into a complete FMS designed for convenient QC testing. The edge test device, detector positioning, and software are optimized for accurate focal spot measurement.
Why is an edge test device used?
The edge test device is a critical component of a focal spot measurement system. A precisely machined tungsten edge is aligned in the radiation beam to cast a narrow shadow on the detector.
As x-rays pass through the edge, they form a penumbra region of partial attenuation. By analyzing this penumbra, the edge blurring effect of the focal spot can be quantified. The width of the penumbra is directly related to the focal spot size.
The edge must have minimal edge roughness. High precision hole drilling creates two perpendicular edges in a tungsten substrate for measurement of both focal spot dimensions.
What types of detectors are used in FMS devices?
Two basic radiation detectors are used in focal spot measurement systems:
- Scintillator and photodiode – A scintillating screen converts x-rays to light that is measured by a linear photodiode array. Very sensitive but requires careful alignment.
- CMOS sensor – A linear CMOS image sensor directly captures a high resolution digital image of the penumbra region for analysis.
CMOS detectors are gaining popularity due to their simplicity, quantum detection efficiency, inherent linearity, and high spatial resolution. However, scintillator-photodiode detectors can provide very low noise images.
How does the software analyze the penumbra?
Dedicated software is used to acquire and process the penumbra image of the edge. This allows accurate quantification of the focal spot blurring effect.
The software performs several key functions:
- Image acquisition under different exposure conditions.
- Alignment and positioning of the edge and detector.
- Pixel signal averaging to reduce noise.
- Background correction and image linearization.
- Curve fitting analysis of the penumbra region.
- Calculation of the line spread function (LSF).
- Computation of the full width at half max (FWHM) as the focal spot size.
- Display, storage, and output of results.
Dedicated FMS software provides fast, accurate, and automated focal spot measurement independent of human observers.
How are measurements performed?
To measure the focal spot, the edge is precisely aligned in the beam and exposures are made at appropriate techniques. Multiple images may be averaged.
Exposures should be made at clinical kVp and mAs settings relevant to the x-ray tube being tested. Different anode angles, focal spot sizes, and beam filtration should be evaluated.
This allows measurement of focal spot enlargement under normal operating conditions. Any degradation of the focal spot size at high exposures would impact clinical imaging.
The entire procedure takes just a few minutes per exposure condition. The software automatically calculates and displays the measured focal spot dimensions. Results can be saved and compared to baselines.
What are the big advantages of FMS devices?
Focal spot measurement systems offer significant improvements over traditional focal spot testing:
- Highly automated and easy to perform measurements.
- Accurate, consistent results, independent of operator skills.
- Rapid testing of different exposure conditions.
- Two dimensional measurement of focal spot length and width.
- Quantitative focal spot size data, not just qualitative imaging.
- Display and documentation of results.
- Can identify focal spot changes impacting clinical imaging.
These advantages make FMS devices ideal for monitoring x-ray tube performance and regular QA testing of the focal spot.
What are limitations of FMS measurements?
Potential limitations of focal spot measurement systems include:
- Specialized edge phantom and detector hardware is required.
- Only measure under select mAs and kVp settings.
- Difficult to test very small focal spots below 0.1 mm.
- Edge alignment and calibration affect accuracy.
- Measurements do not assess focal spot shape.
However, when used properly by trained personnel, FMS devices provide fast and reliable focal spot quality control on a routine basis.
How often should focal spot size be measured?
Most standards and professional organizations recommend periodic measurement of focal spot size as part of an overall quality control program for diagnostic x-ray systems.
AAPM Report No. 74 recommends baseline acceptance testing and annual Constable measurements thereafter. More frequent 6 month or monthly measurements may be appropriate for older equipment.
Any focal spot measurement should be performed after major x-ray tube service, tube replacement, or system upgrades that could affect the focal spot characteristics.
FMS devices make rapid focal spot QC practical on a monthly or quarterly basis as part of routine performance monitoring.
What are acceptance limits for focal spot size?
Most standards specify that the measured focal spot size should not exceed a percentage of the nominal focal spot size, such as:
- IEC: Measured size no more than 1.5 times nominal size
- AAPM Report 74: Measured size no more than 2 times nominal
However, it is good practice to establish specific baseline values for a particular x-ray unit. Ongoing QC should compare measurements to this baseline, not just broad tolerances.
Any clinically significant increase in focal spot size compared to baseline warrants closer system inspection and potential corrective actions to restore focal spot quality.
What are some key specifications for FMS devices?
When selecting a focal spot measurement system, some key specifications include:
- Focal spot size measurement range
- Reproducibility of measurements
- Detector characteristics – CMOS or scintillator?
- Edge alignment precision and stability
- kVp settings available
- Types of x-ray tubes and anodes compatible
- Speed of measurement process
- Analysis software capabilities
- Size, weight, and portability
- PC interface and data storage
- Price and warranty
Additional features like automated exposure control, reporting, connectivity to QA software, and built-in shielding may also be desirable.
Conclusion
Focal spot measurement systems provide an efficient way to perform routine quality control testing of the x-ray tube focal spot size in radiographic equipment. By automating this important but tedious focal spot measurement procedure, FMS devices eliminate human subjectivity and generate faster, more reliable results.
The use of a precision edge test device and high resolution detector allows accurate quantification of focal spot enlargement under clinical operating conditions. Software analysis of the penumbra region takes the human observer out of the loop.
Overall, FMS helps improve the convenience, standardization, documentation, and understanding of focal spot characteristics in diagnostic x-ray systems. This supports optimal image quality and minimal patient exposure during radiographic examinations.