Properties and limitations of medical computed tomography

Christoph Wetzel, summer semester 2022

Medical computed tomography is a radiographic examination that uses X-rays. The attention must always be paid to the maximum radiation dose delivered to the patient, because the ionising X-rays have a very high penetrating power. In one year, a German citizen is exposed to an average of 2 mSv in connection with medical examinations. A share of 80 % of the radiation dose results from an examination with a CT. However, a patient is only examined with a CT in 15 % of the examinations with a radiation dose. This is the big difference to industrial computed tomography (also called micro CT), where the radiation dose is not so strongly regulated. Furthermore, the resolution of medical computed tomography is not as high as that of comparable micro CTs.

General Introduction

Computed tomography is a radiographic inspection, which enables imaging of a 3-D volume ^[1]. Using specific software many individual images are projected into a three-dimensional image. On this 3D image, inhomogeneities, material defects and thickness changes of the irradiated volume can be displayed. This method uses ionising radiation, which has a very high ^[1]. This radiation is electromagnetic oscillation that propagates in a straight line ^[1]. In comparison, X-rays (10 nm to 100 pm) have a much smaller wavelength than the light visible to humans (380 nm to 780 nm) ^[2].

The ionising radiation or electromagnetic waves used here are X-rays. This radiation is produced by the deceleration of electrons. To do this, a glowing cathode must be heated up, which generates free electrons. A high voltage between the glowing cathode and the anode material accelerates the free electrons towards the anode. Generally, an accelerating voltage of 10 kV to 500 kV is used. As a result of the abrupt deceleration of the electrons at the anode material, radiation is released. This radiation consists of 99 % thermal radiation and only about 1 % X-ray radiation. Based on this fact, the material of the anode must have a high melting point and great thermal conductivity. In addition, the anode is cooled either by air or by water. ^[1]

After the radiation has been generated, the test specimen is transilluminated by a strongly focused X-ray beam, which is detected by one or more detectors. Each detector determines the total absorption along the X-ray beam. This produces a considerably large amount of data, especially when high accuracy is required. In medical CT scanners, the radiation source and the detectors move synchronously around the object. Contrary to the medical CT the device under test rotates and the source and detector remain stationary at the industrial CT devices. This can be seen in the Figure 1 below. Industrial computed tomography is often called micro CT in the literature. ^[3]

Comparison of medical CT and micro CT

General opportunities of computed tomography

Computer tomography is a non-destructive testing method whereby the objects to be examined do not have to be opened or destroyed in order to analyse the interior ^[5]. In addition, the method can show very good spatial and density resolutions ^[5]. It is possible to detect defects down to 1 µm ^[5]. Another advantage of the three-dimensional representation is that no overlapping of objects, such as defects and inhomogeneities, can occur ^[5]. So it can be said that a CT can detect small defects better than a two-dimensional X-ray image ^[4]. Furthermore, computed tomography makes it possible to measure density ^[4]. In contrast to medical CT equipment, industrial or micro CT does not have to take time and radiation dose of the X-ray exposure into account ^[5]. Before the introduction of micro CT, only medical CT was used in industry ^[4].

Technical data

A medical computer tomography (med. CT) device needs only a few seconds for a scan. In addition to the scan time, there is also a cooling process of a few minutes. A medical CT can scan a test object with a diameter of up to 900 mm. Generally, a medical CT is operated with a tube voltage of approximately 120 kV to 130 kV. ^[4]

The micro CT method usually takes much longer for a scan. The average scan time is between 30 and 60 minutes, in some cases even longer ^[6]. The mirco CT can measure samples with a maximum diameter of 650 mm and a maximum height of 1200 mm ^[6]. In contrast to medical CT, micro CT usually uses a higher tube voltage, this value is around 225 kV ^[4]. In addition, a industrial CT has a better spatial resolution ^[4].

Advantages and Disadvantages

The great advantage of the medical CT device is that the scan time is extremely short. This means that the throughput is also very high and an inline inspection (measuring device in the production line) is possible. Another advantage is the very small amount of data per scan, but this is due to the lower resolution. ^[4]

As just pointed out, the image quality of the medical CT scanner is not that good. However, one must always consider whether this high quality resolution is needed in all cases. ^[4]

In contrast to the medical CT, the strengths of the micro CT device are that it can achieve a very high degree of accuracy due to the excellent resolution as a result of the higher tube voltage ^[1]. This means that a high degree of certainty can be provided in material testing regarding the presence or absence of defects ^[1]. As a result, micro CT is used to test other non-destructive tests method for their accuracy ^[1]. In addition, the dimensions of the test specimens is not as limited as with the medical CT and consequently the micro CT is much more flexible than the medical CT ^[4].

A disadvantage of the micro CT is that the purchase price is higher compared to medical devices. It can be assumed that a medical CT costs around 60,000 € and 300,000 € and a micro CT can cost between 300,000 € and 1,300,000 €. ^[4]

Generally, a disadvantage of medical CT and micro CT is that both devices use X-rays, which is ionizing radiation. For this reason, safety precautions must always be taken and the staff, who operates this equipment, must be specially trained ^[1]. In addition, a dose of radiation is administered indirectly to the patient during any diagnostic examination with a CT. Generally, the approved dose is in the order of 1 mSv - 24 mSv for adults. However, these values do not apply to children. Because the ionising radiation and especially the penetrating ability has a negative impact on human health, the maximum permitted dose for children is between 2 mSv - 6.5 mSv. ^[7]

Application in medical field

Critical dose factor

X-rays were discovered in 1895 and it quickly became clear that protection was generally needed when working with this radiation, as damage to health had been observed. The reaction of biological tissue to the absorption of X-rays is very complex. The strength depends on the radiation energy and the absorption effect. Stochastic and deterministic interactions can occur. The deterministic radiation damage is that damage to biological tissue that is specifically caused by administered radiation of known dose. So that above a certain threshold dose a certain damage process occurs. ^[8]

In case of stochastic radiation damage, random damage processes occur. This damage is caused by radiation absorption by carriers of genetic information and chromosomes, which occur with a time lag due to mutations or the formation of carcinomas. ^[8]

In order to calculate the above-mentioned threshold dose of deterministic radiation damage, the absorbed dose $//<![CDATA[ \begin{array}{l}D\end{array} //]]>$ is defined. This unit $//<![CDATA[ \begin{array}{l}D\end{array} //]]>$ is being calculated from the quotient of the radiation energy $//<![CDATA[ \begin{array}{l}dW\end{array} //]]>$ transferred to the tissue or from the difference of the incoming energy $//<![CDATA[ \begin{array}{l}W_0\end{array} //]]>$ and outgoing energy $//<![CDATA[ \begin{array}{l}W\end{array} //]]>$, related to a mass element $//<![CDATA[ \begin{array}{l}dm\end{array} //]]>$. (see equation 2 - 1) The SI unit of this equation is $//<![CDATA[ \begin{array}{l}1 Gy\end{array} //]]>$ (Gray), which is equivalent to $//<![CDATA[ \begin{array}{l}Sv\end{array} //]]>$ (Sievert)$//<![CDATA[ \begin{array}{l}1 Gy = 1 Sv\end{array} //]]>$. ^[8]

$//<![CDATA[ \begin{array}{l}D = \frac{dW}{dm} = \frac{W_0 - W}{dm} = \frac{J}{kg} = Gy = Sv\end{array} //]]>$ (2-1)

Corresponding dose for different tissue/body parts

The following table (Table 2 - 1) lists the typical dose of radiation used in the diagnostic examination with computed tomography of the various body parts/body tissues. In the last four lines of the table, contrast media are indicated in addition to the body part/body tissue. As in X-ray diagnostics, a contrast medium can also be applied for the CT examination. This is completely harmless to the patient. It is either given via the vein or directly into a joint. This technique helps, for example, in the precise diagnosis of joint diseases. ^[9]

An example of a CT of a dislocated shoulder is shown in the following Figure 2, a real CT image of a right shoulder.

Currently available devices (status 2022)

Siemens: Naeotom Alpha

The new product by Siemens Healthineers is called Neaotom Alpha and is a 2 Vectron X-ray tube. This CT has 2 QuantaMax CT detectors with quantum counting and can thus record up to 2 x 144 layers. The temporal resolution is maximum of 66 ms, which corresponds to half the rotation time of the CT scanner. Accordingly, the rotor needs only 0.132 seconds for one rotation. The emission cathode has a current of up to 1,300 mA. The Naeotome works with an accelerating voltage of 70 kV up to 140 kV. The coverage in the z-direction is 144 mm x 0.4 mm or 120 mm x 0.2 mm and can achieve a spatial resolution of 0.16 mm x 0.11 mm x 0.11 mm in the plane in UHR mode (ultra-high-resolution). In this mode, the three-dimensional resolution is doubled. Furthermore, a speed of up to 737 mm/s is possible with the CT device when using the Turbo Flash. Finally, it should be mentioned that the table can support a load of up to 307 kg. ^[10]

General Electric: Revolution Apex Elite Quantix 160

The newly developed Elite Quantix 160 tube from General Electric Healthcare's Apex range uses an emission cathode which has a current of 1,300 mA, giving it a detector coverage of 160 mm. The device has high-definition imaging with a spatial resolution of 0.23 mm. The maximum accelerating voltage is 125 kV. The bore size of this CT device is 800 mm. The rotation time of the detector is 0.23 s/red and can detect at a maximum scan speed of 437 mm/s. ^[11]

Philips healthcare Spectral CT 7500

One of largest CT unit, currently offered by Philips healthcare, is called the Spectral CT 7500. This CT uses a generator power of 120 kV and reaches a range of 80 mm. The Spectral CT7500 has high-definition imaging with a spatial resolution of 0.35 mm and the acceleration voltages can be set between 80 kV and 140 kV. The patient lies in a 900 mm wide tube and can be scanned axially for a maximum of 2000 mm. The CT unit can produce 512 slices and the rotation speed is 0.27 s/red. ^[12]

Literature

1. Grosse C. Einführung in die Zerstörungsfreie Prüfung im Ingenieurwesen

2. https://www.physik.nat.fau.de/files/2018/06/Röntgenstrahlung-Mediziner.pdf (Page accessed on 17.05.2022, 1 pm)

3. Schiebold K. Zerstörungsfreie Werkstoffprüfung - Durchstrahlungsprüfung

4. du Plessis A, le Roux SG, Guelpa A. Comparison of medical and industrial X-ray computed tomography for non-destructive testing

5. van Kaick G, Delorme S. Computed tomography in various fields outside medicine

6. https://www.qa-group.com/de/leistungen/industrielle-computertomographie/ (Page accessed on 12.07.2022, 1 pm)

7. Beart AL, Knauth. M, Sartor K. Radiation Dose from Adult and Pediatric Multidetector Computer Tomograpraphy

8. Spieß L, Teichert G, Schwarzer R, Behnken H, Genzel C, Moderne Röntgenbeugung

9. Dössel O. Bildgebende Verfahren in der Medizin

10. https://www.siemens-healthineers.com/de/computed-tomography/photon-counting-ct-scanner/naeotom-alpha (Page accessed on 23.06.2022, 6 pm)

11. https://www.gehealthcare.com/products/computed-tomography/revolution-family/revolution-apex-platform (Page accessed on 23.06.2022, 6 pm)

12. https://www.philips.de/healthcare/product/728333/spectral-ct-7500-philips-all-new-spectral-detector-ct-750#specifications (Page accessed on 23.06.2022, 6 pm)