Image Resolution Radiology (Modulation Transfer Function)
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- čas přidán 2. 06. 2024
- Image resolution can be directly visualized with images of a bar pattern where the limiting resolution can be determined by the smallest set of line pairs that can be seen.
00:00 Introduction to MTF
00:39 Image Resolution Definition
03:08 Visual Resolution X-ray Radiography
04:05 Visual Resolution Computed Tomography (CT)
04:39 Point Spread Function (PSF)
06:50 Modulation Transfer Function (MTF)
08:44 PSF to MTF (Point spread function to Modulation transfer function)
10:42 MTF in Computed Tomography (CT)
11:05 MTF in X-ray Imaging
Linear Systems for X-ray Technologists
When images are taken on a real x-ray system there will always be some blurring that takes place in the system due to the size of the focal spot and the size of the detector. The idea of linear systems modeling is that we can model the blurring in our x-ray system.
The blurring of the image that occurs when an image is made on an x-ray system can be thought of as starting with a perfect, ‘ideal image’ and then blurring it out more. As the size of the detector elements gets larger the blurring will be increased.
We can think about each point in an ideal image and what happens in reality is that in the measured image each point in the ideal image is blurred by adding contributions from its neighbors. That blurring is called a point spread function (PSF). Each point in the image gets spread to the neighboring points in the image.
As the PSF gets larger there is more blurring into neighboring regions and as the PSF gets smaller there is less blurring.
The blurring is usually modeled in 2D on the images (both up and down and left to right). The blurred shape we see is function of the system and it is called Point Spread Function.
In the figure for the perfect system, we can easily see all of the bar patterns. But in a real system the bar patterns will be blurred and the neighboring objects become hard to tell apart (visually differentiate).
In the next section we will describe how this linear system can be used to make measurements of spatial resolution.
Modulation Transfer Function (MTF) (Frequency Space)
In the real system the contrast is reduced for the higher resolution bars (seen on the bottom of the figure). Next, we will introduce a method to describe the resolution directly in frequency space.
Each of the bar patterns has a given width and this is measured in units of line pairs per mm (lp/mm).
The wider bar patterns have a lower frequency (fewer line pairs per mm) and the narrow bar patterns have a higher frequency (more line pairs per mm).
On the bottom of the figure we have a curve that goes down for higher frequencies. This curve is called the Modulation Transfer Function (MTF). The MTF is used to describe how quickly the ability to see the finer bar patterns goes down (i.e. how does the system effect the higher spatial frequencies).
The modulation transfer function (MTF) describes the frequency behavior of the system and is a curve that has lower values for high frequencies which represent the small image structures.
Small image structures correspond to high frequencies, and large image structures correspond to low frequencies.
We described the point spread function (PSF) and the modulation transfer function (MTF). In this section we will describe the link between the PSF and the MTF.
The point spread function (PSF) is a blurring function that is measured in the image space. The PSF is often assumed to be symmetric. If we take the Fourier Transform of that point spread function, we get the MTF or the Modulation Transfer Function.
This is the same Fourier transform that is used in MRI imaging where the data is collected in Frequency space and the Fourier transform is used to convert to image space (image reconstruction).
The advantage of using the MTF compared with direct visual observation is that MTF provides a very quantifiable number, that is not dependent on a subjective reader.
The different points on the MTF curve describe the system resolution can be compared from system to system. It is standard process to report the frequency (x-axis value) when the MTF curve is at 50% of the maximum value (MTF50), and when the MTF curve is at 10% of the maximum value (MTF10).
We can do this over and over on different systems and get a quantifiable number and all we have to do is scan a wire. If the wire is much smaller than the size of the detector, then all we can ignore the size of the wire.
So all we need to do is to scan a wire and take the Fourier Transform to get the MTF.
That relates to how well you can see the small structures in your image and we can do this in a quantifiable and repeatable way. This gives the same type of information as the direct observation of bar patterns but is less dependent on the reader of the images.
Rad Take-home Point: The modulation transfer function (MTF) is the Fourier transform (frequency space representation) of the point spread function (PSF). - Věda a technologie
Great video. Better description than any Physics text I've ever read on mtf.
Thanks Prof Tim. Much Appreciated!
Thank you, my lecturers just don’t get to the point and waffle without precise definitions. The future of universities should replace crappy lectures with video segments like this
Thanks for the vote 🗳. Pass these videos to your friends. These videos are open to everyone so anyone can use in a lecture or watch ahead and do activities during lecture time
Thank you for this clear explanation👍
You’re welcome ☺️
Nice presentation, thanks!
Glad you liked it! Thanks for leaving the comment
Sir you r just doing a very good job
No one make videos on radiography in India as like u
I am becoming a huge fan of yours ❤❤
Mashallah 😊
Thank you sir for helping us
Lottts of luv from students like me
--From India 🇮🇳
You are welcome 🙏 Thanks for these nice words ☺️
Great video and thanks for sharing! I have one question about the line-pair QA. I wonder why we need to remove the Cu plate when performing line-pair QA? Thank you!
Thanks Mathew, since that test is for the limiting spatial resolution the idea is use a small spot and remove the filtration so more X-rays get to the detector. This test will represent as well as the system could do.
Thank you so much!
Your welcome.
Hi! I was wondering if making the focal spot smaller is the same as collimating an image as they both change the size of the primary beam?
Ayla- changing the focal spot changes the resolution but does not change the field of view and changing the collimation does not in general change the resolution ( there are some exceptions in fluoroscopy).
@@HowRadiologyWorks thank you!!!
It’s very nicely explained. I have a question. What does it mean when MTF exceeds 1? Is it good when it is much larger than 1 or it should be as close as possible to 1?
The MTF is normalize to the value at the zero frequency, so if other values go above one that means that the kernel is doing boosting, ie some frequencies have been artificially enhanced. This is common in lung imaging for instance in CT. These boosted kernels are good for visualization but should not be used for quantitative measurements. Hope that helps 😉
@@HowRadiologyWorks Thanks a lot
Once you have the PSF can you use the FWHM to determine the spatial resolution of the system or should you always need to pass from the MTF ant take R10?
The PSF does contain the information on the spatial resolution and the FWHM is a good metric for overall resolution. Taking the mtf and looking at the 10% or 4% gives a better understanding of the limiting resolution that you can visualize
@@HowRadiologyWorks thank you so much for answering. One more question. Having a wire phantom, once you get the point spread function from it, would you suggest getting the FT of the fit or getting the MTF using a bar resolution phantom? I guess the second one has more sense
@howradiologyworks,
Is there a significant impact to MTF and final image clarity based on the original LSF and Focal Point measurements?
The line spread function is directly related to the MTF, the MTF is the Frequency space representation of the LSF. The blue in the image is due to the detector blur and focal spot blur
Thank you for this remarkable lecture. Which system would have the highest MTF?
What are the choices? In standard radiology mammography has highest resolution
@@HowRadiologyWorks thank you, or as we say in my country 'JazakAllah'
Can MTF exceed 1?!
Some books say yes it ican be in digital radiography system!
Is this true or false?
The MTF can exceed 1 especially when the vendors apply a boost to mid frequencies and the MTF is typically normalized at zero frequency
@@HowRadiologyWorks 🙏
So is it true here that the imaging system is a linear system and that the resultant image is the convolution of the ideal image with the PSF?
Like as always, it depends. In normal x-ray and CT imaging that is true. In newer systems there is more nonlinear behavior with iterative reconstruction in CT or advanced denoising in x-ray. However since these tools are well established they are often applied even when the system is not exactly linear.
@@HowRadiologyWorks
So is it why manufacturers nowadays use the DQE metric to reflect image quality? Because from my understanding, noise and MTF are both embedded in the DQE equation. Do you think you can make a video to clarify that?
@@authman-alshibly that is right and the short answer is because it is still the most accepted option. For a certain task there are more specific methods but nothing that generally covers everything like DQE. I will make one on DQE next year as I have a few other requests ahead.