**7. Conclusion**

152 Imaging of the Breast – Technical Aspects and Clinical Implication

Where Doseequivalent is the dose for the filter, which gives the same S/N ratio as the no filter case, and Dosefilter is the dose measured when a given filter was applied. (S/N)nofilter is the signal to noise ratio for the no filter case while (S/N)filter is for the filtered case. The dose per frame difference during the measurement was only 2-3 times lower for the filtered cases so we thought that the above approximation would not introduce significant error. This approximation was double checked with measurements and S/N evaluation at two different dose-rates and for 8 times dose difference the error using the above approximation

> � � �� ��������� �

> > � � �� ������

After calculating the equivalent doses for each filter, these dose values were divided by the no filter case dose and dose equivalent S/N dose ratios were obtained. Both the equivalent doses and dose ratios are reported in Table 5. These numbers tell how much absolute and relative dose is needed when using these filters to get the same S/N value as the no filter

Table 5 tells that practically all of the filter materials used in these experiments provide the same S/N ratio at lower dose as the no filter case. From the dose point of view a Cu filter gives nearly the same very low dose as a Eu filter. However, it is important not just to get lower dose but at the same time to provide better contrast resolution and therefore higher

> **S/N Average @ Measured Dose**

No filter (60kVp) 1111 94.0 1111 1.00 Cu filter (100kVp) 569 114.3 385 0.35 Al filter (100kVp) 858 121.3 516 0.46 I filter (60kVp) 598 92.8 614 0.55 Ce filter (60kVp) 404 80.3 830 0.50 Ce filter (70kVp) 598 87.9 685 0.62 Eu filter (70kVp)) 648 121.4 389 0.35 Nd filter (70kVp) 627 98.5 571 0.51

**S/N Equivalent Dose (uR/fr)**

**Dose ratio**

In addition to mammography, Iodine and K edge filters are used for other medical x-ray imaging applications, such as lung tumor imaging and, frequently, angiography. (Nyman U Fau - Elmstahl, Elmstahl B Fau - Leander et al.), (Sato, Tanaka et al. 2004; Sato, Tanaka et al.

� × Dose������ (1)

Dose���������� ≈

was less than 10%.

case.

The equivalent dose from equation (9):

HU numbers for iodine as explained previously.

**Filter Measured** 

**Dose uR/fr**

Table 5. Equivalent dose and dose rates.

2007), (Sato, Hayasi et al. 2006).

**6.4.3 Some other applications of K edge filters** 

This chapter presented different methods used in mammography to improve the contrast between adipose and glandular tissues but especially to find cancerous cells and regions.

First of all the right energy range has to be determined for minimum patient dose with maximum contrast. Dual energy imaging, which uses two images taken at different energies, further enhances the contrast between different tissues and especially when contrast material is used.

Monochromatic X-ray beams are ideal for getting high quality images with optimal dose level and avoiding beam hardening artifacts. Moreover, monochromatic imaging can provide phase contrast images with excellent soft tissue contrast. The major drawbacks are that the synchrotrons, which can provide monochromatic beams with enough flux for medical imaging, are very large and expensive sources of X-rays.

Quasi monochromatic X-rays can be obtained by diffraction of X-rays emitted by an X-ray tube onto a mosaic crystal. The output beam has a limited bandwidth, low output flux, and has a fan beam shape. This limits its application only for scanning type imagers.

Another method of generating quasi monochromatic beams uses K edge filters. These filter materials have K edge electrons with bonding energies in the diagnostic X-ray energy range. The material can absorb an X-ray photon, whose energy is equivalent to or slightly higher than the K edge energy, by releasing an electron. The material's X-ray absorption level dramatically increases at or above this energy. As a result, x-rays higher than the K edge energy are suddenly cut off. Using these materials as X-ray filters, a narrow x-ray transmission energy range below the K edge energy can be obtained. One advantage of the K edge filters is that they provide cone beam shaped X-ray radiation rather than fan beam shaped radiation.

Iodine and Gadolinium are contrast agents injected into the blood flow. They absorb the x-rays better than body tissues providing an x-ray shadow. They accumulate in the cancerous cells and remain there longer than they remain in the blood stream. This enhanced x-ray absorption provides extra contrast for better visibility of tumors. Using K edge filters X-ray image contrast of the I or Gd absorption can be further increased. I and Gd are frequently used as contrast agents for CT imaging. The experimental part of this paper describes evaluation of a few K edge filter materials using Iodine contrast material. These filters were compared to the non filtered case and also to Al and Cu filters, which provided only X-ray beam hardening. CT scans were performed and the HU numbers

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**8. References** 

were calculated for iodine contrast agents. It was found that the beam hardening Al and Cu filters and even the iodine filter decreased the iodine HU number, but the K edge filters, which have K edge energy slightly above the K edge of iodine, increased the HU number (increased iodine contrast). This increase can be as high as 26% for Ce filter at 60 kVp. However, it is seen that using an Iodine filter for an Iodine contrast agent decreases the HU number to the no-filter case. That creates an option to further enhance the contrast by subtracting the Iodine filtered image from the Ce filtered one. This would provide an additional ~17% contrast enhancement of iodine. A drawback of this method is that we need two images (practically dual energy imaging), which increases the dose to the patient and increases noise.

We also found that the increase of HU number is linearly proportional with the iodine content in water.

Furthermore, the doses required for S/N number equivalent to the no filter case are lower for each filter material. However, we need not just lower dose but want to increase the iodine contrast (higher HU number for iodine). From this point of view Eu is a much better choice than Cu even if their dose improvement is nearly the same. The dose improvement is nearly 3 times for both filters but Cu decreased the HU number (the iodine visibility) nearly to half of the no filter case. So both the HU increase and the dose improvement have to be considered in parallel. From these considerations Ce, Eu and Nd are all promising candidates for iodine contrast imaging enhancement. Moreover, the HU contrast enhancement effect depends strongly on the X-ray tube voltage as seen in Table 4 for Ce and also on the object material, which contains the iodine, so careful optimization is required for getting the maximum HU improvement with a significantly decreased dose to the patient.

Mc. Kinley et al investigated the effect of K edge filtering on breast CT and found dose reduction possibility up to 6 times while keeping the same contrast ratio (McKinley Iii, Tornai et al. 2007).

Watanabe et al (Watanabe, Sato et al. 2008) and Roessl et al (Roessl, Brendel et al. 2008) also investigated how K edge imaging improves the signal to noise ratio but they used a photon counting CT method. S/N ratio improvement was demonstrated by using the K edge technique but the photon counting method is much more expensive and a slower process than multislice or cone beam CT with K edge filter. These drawbacks come from the high price of the large area pixellated energy resolution detectors and the counting speed is very limited.

Finally, it is very important that for HU calibration we have to use the same kVp setting and filter that we use for scanning the CT object otherwise huge reconstruction errors could be introduced (Zentai 2011).

We need to add that we used heavy K edge filtering of 200th value layer K edge filters for Ce, Eu and Nd. These thick filters significantly decreased the X-ray intensity even around the K edge. From our measurements and calculations we concluded that thinner filters may provide the same advantages that these thick filters without the large flux reduction. This could ease the tube flux requirement and so decrease the maximum heat load and/or increase the CT scanning speed.

#### **8. References**

154 Imaging of the Breast – Technical Aspects and Clinical Implication

were calculated for iodine contrast agents. It was found that the beam hardening Al and Cu filters and even the iodine filter decreased the iodine HU number, but the K edge filters, which have K edge energy slightly above the K edge of iodine, increased the HU number (increased iodine contrast). This increase can be as high as 26% for Ce filter at 60 kVp. However, it is seen that using an Iodine filter for an Iodine contrast agent decreases the HU number to the no-filter case. That creates an option to further enhance the contrast by subtracting the Iodine filtered image from the Ce filtered one. This would provide an additional ~17% contrast enhancement of iodine. A drawback of this method is that we need two images (practically dual energy imaging), which increases the dose to the

We also found that the increase of HU number is linearly proportional with the iodine

Furthermore, the doses required for S/N number equivalent to the no filter case are lower for each filter material. However, we need not just lower dose but want to increase the iodine contrast (higher HU number for iodine). From this point of view Eu is a much better choice than Cu even if their dose improvement is nearly the same. The dose improvement is nearly 3 times for both filters but Cu decreased the HU number (the iodine visibility) nearly to half of the no filter case. So both the HU increase and the dose improvement have to be considered in parallel. From these considerations Ce, Eu and Nd are all promising candidates for iodine contrast imaging enhancement. Moreover, the HU contrast enhancement effect depends strongly on the X-ray tube voltage as seen in Table 4 for Ce and also on the object material, which contains the iodine, so careful optimization is required for getting the maximum HU improvement with a significantly decreased dose to the patient.

Mc. Kinley et al investigated the effect of K edge filtering on breast CT and found dose reduction possibility up to 6 times while keeping the same contrast ratio (McKinley Iii,

Watanabe et al (Watanabe, Sato et al. 2008) and Roessl et al (Roessl, Brendel et al. 2008) also investigated how K edge imaging improves the signal to noise ratio but they used a photon counting CT method. S/N ratio improvement was demonstrated by using the K edge technique but the photon counting method is much more expensive and a slower process than multislice or cone beam CT with K edge filter. These drawbacks come from the high price of the large area pixellated energy resolution detectors and the counting speed is very

Finally, it is very important that for HU calibration we have to use the same kVp setting and filter that we use for scanning the CT object otherwise huge reconstruction errors could be

We need to add that we used heavy K edge filtering of 200th value layer K edge filters for Ce, Eu and Nd. These thick filters significantly decreased the X-ray intensity even around the K edge. From our measurements and calculations we concluded that thinner filters may provide the same advantages that these thick filters without the large flux reduction. This could ease the tube flux requirement and so decrease the maximum heat load and/or

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