What is the difference between CT and radiography?

When you hear “X-ray imaging,” what do you picture? You might think of medical images showing bones that people take at hospitals. Those are certainly X-ray images, but are they radiography or CT (computed tomography)?

The terminology is confusing. Even with years of experience using different X-ray techniques, I also had similar questions at the beginning. If you wonder about the difference between radiography and CT, this article is for you. It will help you discover which X-ray imaging technique is best for your needs.

Let’s review these key points:

1.     What is X-ray imaging?
2.     What is the difference between radiography and CT?
3.     When should you choose radiography?
4.     When should you choose CT?

1. What is X-ray imaging?

X-rays have a wide range of applications because they interact with materials in many ways. When we are discussing X-ray imaging, we are specifically talking about non-destructive techniques that use X-rays as a light source to reveal and capture detailed images of the internal structure of objects.

A common example of such non-destructive X-ray application is the medical images most people think of: black-and-white pictures where bones appear as the brightest areas while softer tissues appear darker. These iconic grayscale images differentiate materials based on different X-ray absorption rates, which are caused by varying density, composition, and thickness. These differences lead to varying levels of grayscale contrast. Both radiography and the majority of CTs rely on this absorption contrast principle to generate images.

The figure below shows two major factors affecting X-ray absorption. Generally, X-rays are absorbed more when they pass through heavier elements or denser materials, as well as thicker objects. The higher absorption results in fewer X-ray photons reaching the detector. 

You might expect to see denser material showing as the darker area on the detector. However, we usually artificially flip black and white for digital detectors to be consistent with classic medical radiography collected using film, where areas with fewer photon signals appear brighter. This is why bones appear bright while soft tissues look dark in medical X-ray images. Bones absorb more X-rays due to their greater density compared to soft tissues. Here’s a good YouTube video by The National Institute of Biomedical Imaging and Bioengineering explaining this topic.

2. What is different between radiography and CT?

Now we know that X-ray imaging techniques like radiography and CT use different absorption rates to generate contrast when looking at structures, but what’s the difference between the two techniques?

Let’s start by discussing radiography. The term "radiograph" refers to an image created using radiation. In this process, X-rays are directed at a sample object. Some of the X-rays are absorbed by the object, while others pass through and create contrast based on the differences in absorption. These transmitted X-rays then reach a detector on the other side.

Similar to taking a regular photograph, where you can adjust brightness and exposure time, you can also modify the energy, intensity, and duration of the X-ray exposure.  However, the outcome is always a two-dimensional image, known as a radiograph, which shows all the internal structures layered on top of one another. This lack of "depth" can sometimes make it challenging to fully understand the object's internal features. The medical X-ray image we’ve been discussing so far is radiography.

Now let’s talk about CT scans. To begin, you still take radiographs of the object you want to image. However, instead of capturing just one image from a single viewing angle of the sample, multiple radiographs are taken from various angles, covering a range of 0° to 180° or 360°. These radiographs are then computationally reconstructed into a 3D volume using advanced calculations, resulting in what is known as a tomograph.

A tomograph is a set of virtual cross-sections of an object, like what you would see when you cut things open slice by slice. But instead of physically cutting the sample, you are cutting it virtually.  This technique allows X-ray CT to show cross-sections non-destructively, enabling you to view them atany desired depth. Instead of a superimposed 2D representation, you can now access a virtual copy of the 3D structure to dissect and investigate.

The main difference between radiography and CT is the availability of depth information. For example, look at the picture below showing an object, its 2D radiograph, and 3D CT reconstruction. You can’t really tell which pillar you are looking at using only radiography, while you can clearly differentiate the pillars in the foreground and background with depth information provided by CT.

You might be wondering why we don’t just use CT for everything, since it provides more depth information. In the next section, I will explain why radiography is still the preferred choice in many cases.

3. When should you choose radiography?

Despite the vast amount of information CT provides over radiography, radiography remains the go-to choice for many X-ray imaging applications for several major reasons.

The first reason is overall lower X-ray dosage. X-rays are one of the most common forms of ionizing radiation, known to pose health risks with overexposure. Since CT takes numerous radiographs per scan, the X-ray exposure is a lot higher than taking a single radiograph. This concern is especially true for medical applications when patients or fragile organisms are being imaged, which is why medical X-rays are predominantly radiographs.

The second reason is higher speed. While CT can complete a full scan in a short duration—typically within tens of seconds—taking a single radiograph will always be faster than capturing multiple images for a CT scan. Therefore, if high throughput is the most critical factor for your application, radiography is the better choice.

The third reason is lower cost. Radiography systems typically have fewer components, making them more affordable than CT systems. This simplicity also allows for increased portability, which is a significant advantage for field inspections. In contrast, CT systems require either a goniometer stage to rotate the object being imaged or a gantry system to rotate both the X-ray source and detector around the sample.

The more intricate mechanical setup and the need for a high-spec computer for complex image processing both increase the overall price of CT systems. Additionally, the detailed structural information provided by CT often requires specialized analysis software to fully explore 3D images. These extra requirements further contribute to the higher cost of CT systems compared to radiography systems.

The final reason is that there are fewer sample dimension limitations. If the sample being imaged can fit between the X-ray source and detector, a radiograph can be taken. In comparison, CT requires sufficient space for the sample to rotate 180° or 360° to fit inside the field of view (FOV). This can be troublesome when imaging irregularly shaped or large objects.

The picture below shows the setup difference between radiography and CT. Radiography works as long as the sample fits in the device, but CT needs additional space for the sample to rotate, risking collisions if clearance is tight.

4. When should you choose CT?

Despite all the reasons that favor radiography, CT is growing rapidly in both academic and industrial applications, thanks to the advancement of computational power that enables 3D data reconstruction and faster AI-based analysis techniques. Here are a few critical advantages CT has over radiography.

The first clear advantage is a greater amount of information. A CT scan captures every structural detail within the resolution limit from the surface to the core of your sample. This scan offers a non-destructive way to explore both external and internal features by creating a digital twin model with comprehensive structural information ready for various analyses.

The second clear advantage is the 3D perspective. In radiography, all the information is layered onto a single 2D plane, which limits the amount of detail available due to the absence of depth. For example, as illustrated below, a small cylindrical void inside a cube can appear identical to a cylindrical hole running through the entire cube in a radiograph. In contrast, a CT scan offers a 3D view to help avoid this type of ambiguity.

You may think taking a couple more radiographs will provide enough additional information to reach the same conclusion a CT offers. That applies to simple cases like the one shown above, but more complex structures often require a CT scan to study properly.

Below is a 3D video from a CT scan of a smart doorbell, showing the intricate layering of chipsets, joints, and wiring. Even with multiple radiographs, you would not be able to tell if one chip is missing or detached, while a CT scan can show you visually and intuitively.

(Video rendered with Dragonfly. For more information regarding CT data analysis, check out our blog articles on 3D video rendering and Software selection.)

3D perspective also allows intuitive data interpretation. Since we live in a 3D space, visualizing objects in three dimensions is the most natural way to understand them. CT scans provide direct 3D views of structure elements, allowing you to see any feature of interest exactly the way it is inside a volumetric space. This eliminates the need for guessing or piecing together information, making interpretation straightforward and intuitive.

The third clear advantage is the excellent synergy with other analysis techniques. CT scans provide comprehensive structural data by creating a digital twin of the scanned object, making it an ideal centerpiece for correlative analysis. Additionally, mechanical property simulation can use a CT scan as an accurate representation of the structure, avoiding the need to create a 3D model separately.

Finally, one of the biggest advantages of CT is its capability for volume-based quantitative analysis. Few techniques offer direct 3D quantification of dimensions, volume fractions, sizes, and shapes, and even fewer do so non-destructively as CT does. While CT is by no means perfect—with flaws such as artifacts that can affect the quality and precision of reconstructed 3D data, like beam hardening—many uncertainties caused by these artifacts can be minimized by selecting appropriate parameters, using image processing, or correlating information from other analysis techniques. This volume quantification capability makes CT an essential tool for structural characterization in both routine inspection and novel research.

Takeaway

Let’s review some takeaway points:

1.     X-ray imaging is a non-destructive technique that captures the internal structures of objects.
2.     The key difference between radiography and CT is the availability of depth perspective.
3.     Choose radiography for lower cost, higher speed, and lower X-ray exposure dosage.
4.     Choose CT for a greater amount of information, 3D qualitative and quantitative analysis.

Both radiography and CT are powerful X-ray imaging techniques with respective advantages that make them the ideal choice for various scenarios. I love CT more as a material scientist, since it is technically an upgrade to radiography in providing structural information in every way. However, it is understandable that not every application requires CT.

With the additional complexity that comes with additional structural analysis capability, CT is likely an X-ray imaging technique most people are less familiar with. If you are interested in getting into CT analysis, here are a few virtual workshops that will give you a good starting point to explore the fascinating world of X-ray imaging using CT:

X-ray Computed Tomography Virtual Workshop

X-ray Computed Tomography for Materials & Life Sciences

Decoding Defects: Failure Analysis Using X-ray CT

I hope I have clarified the difference between radiography and CT and helped you decide the right X-ray imaging technique for your application. Are you more interested in radiography or CT? Have you used either of these techniques for inspection or research? We’d love to hear your thoughts! Feel free to share your feedback via email. We are always eager to help answer any questions you may have regarding X-ray imaging.

If you have any questions or need help choosing the right X-ray imaging tool, you can talk to one of our CT experts by clicking the “Talk to an expert” button at the top right of the page or send us a message at imaging@rigaku.com.