Paediatric chest phantom for CT: Keeping infant dose ALARA

Summary

Introduction: Paediatric imaging protocols should be carefully optimised to maintain the desired image quality for the delivered radiation dose. To do this, a specially designed paediatric chest phantom has been constructed to optimise CT chest examinations for infants.

Methods: A novel chest phantom was fabricated using tissue equivalent materials to mimic thoracic structures of a child aged 0-1 year old. The phantom was designed to provide dosimetry and image quality measurements within the anthropomorphic structure.

Results: The phantom materials were validated across a spectrum of tube voltages to be equivalent to paediatric tissues observed in a clinical study. The phantom provides a useful means of optimising paediatric chest CT imaging protocols, keeping patient dose in accordance with the as-low-as-reasonably-achievable (ALARA) principle and ultimately minimising radiation risks for infants receiving chest CT examinations.

Introduction 

Radiation protection standards in diagnostic imaging aim to utilize the medical benefits of radiation while keeping the associated risks as-low-as-reasonably-achievable (ALARA Principle). Children’s growing bodies and rapidly dividing cells, make them more susceptible to the harmful effects of radiation.

Computed tomography (CT) x-ray imaging is an invaluable tool for diagnosing the patient’s medical complaint. However, the medical needs and benefits should outweigh risks associated with the level of radiation dose the patient receives from the CT examination. The ALARA principle requires that the radiation dose should be minimised to a level that still provides an adequate image quality for the diagnostic purpose. 

Children should not have a CT scan using the normal adult X-ray exposure factors. The X-ray tube current and tube voltage should be reduced to allow for the child’s smaller size (as recommended by the Image Gently campaign). Lowering the X-ray exposure factors reduces patient dose, but it also reduces the quality of the CT images. There needs to be a careful balance to ensure that, in reducing the X-ray dose the patient receives,  an image of sufficient quality is produced for the diagnostic purpose. 

With the use of suitable physics test tools, the CT image quality can be optimised against radiation dose for the diagnostic imaging procedure. To do this, an anthropomorphic phantom was specially designed to simulate the shape and physical properties of an infant. X-ray dosimetry measurements can be obtained by exposing the phantom in the CT scanner. For accurate measurements, the phantom needs to have structures similar to the anatomy of a child. It should be composed of materials that absorb the X-rays equivalent to human tissue absorption. By using a phantom, it avoids having to unnecessarily expose patients to obtain this information. Instead, the phantom offers the major advantage of providing a safe method for repeatedly scanning it many times to obtain the most optimum X-ray exposure factors for the best possible image quality and lowest possible radiation dose. 

What are the phantom materials?

The phantom was constructed from materials that have similar physical properties as body tissue (referred to as ‘tissue-equivalent-material’). The materials must mimic how X-rays are attenuated in different body tissues. For example, absorption of X-rays in a child’s lung is very different to in muscle tissue. For this special project, the properties of the thoracic structure in a 0-1 year-old infant had to be developed.

X-ray attenuation depends on the density and atomic number (Z) of the absorbing material. Higher Z elements will absorb more x-rays to give a higher radiation dose. For example, bone is a high-density material. It attenuates more x-rays, which shows up on the CT as a white or bright image. The brightness level (called CT number or Hounsfield unit) is different for different anatomical tissues. The inherent differences in physical properties, and CT numbers between tissues is the fundamental basis of medical x-ray imaging.    

Paediatric bones and lung tissue are physically different to adult’s since children are still growing and developing. That makes it difficult to choose suitable materials to use to construct the phantom’s bones comparable to a 0-1 year-old child. As just explained, the CT number of an adult bone cannot be used to estimate paediatric bone. To establish more accurate CT numbers for an infant’s anatomy, a study was carried out via a clinical survey. The average CT numbers for the various tissues were obtained from previous CT examined children of 0-2 years old. From the result of this study, CT numbers for the various tissues were used to select the most appropriate tissue-equivalent-materials to use in the construction of the phantom. The soft tissue is modelled using an acrylic material; Polymethyl methacrylate (PMMA). The spine and ribs are made out of a silicone based rubber material and the lungs are made of cork.  

What does the phantom look like? 

The phantom was constructed in slices, similar to a loaf of bread. The slices are all held together by a central acrylic rod (figure 1). There are a total of 10 phantom slices but only a maximum of 8 slices can fit on the rod at once. The phantom slices can be rearranged to suit the desired imaging task. For example, if the tools used for measuring image quality are not required, the dosimetry slices can be used on their own. The phantom construction can be changed to suit whether image quality or dose measurements are required.

When the phantom is assembled, it does not look like a child. There are no arms or legs. But the internal structures of the phantom closely resemble the anatomical structures of a child’s chest. When imaged in the CT scanner, the phantom looks very similar to a small child aged between 0 and 1 year. Figure 2 shows an example of an axial CT slice of an 8 month-old child. It is compared to an anthropomorphic slice of the phantom. 

How was the phantom constructed?

The body of the phantom was fabricated using computed numerical control (CNC) milling. CNC is the inverse of 3D printing. Instead of building the desired shape from scratch, CNC milling starts with a block of material and precisely cuts it away until you are left with the desired shape (Figure 3.). Each phantom slice, started from a 20 mm thick sheet of PMMA. Cavities for the lungs, ribs, spine and central rod were machined using the CNC mill. 

After the slices were machined the cavities needed to be filled with the bone and lung equivalent materials. The cork and foam materials used in the lung cavities were precisely cut to the correct shape and pushed into the slices without any gaps.  The bone materials were mixed together to form a solution that could be poured carefully into the cavities and left overnight to solidify (Figure 4.).

What information can you get from the phantom?

The phantom is a unique tool that can be used on any CT scanner, to find the optimal settings for an infant’s chest CT scan. The phantom can be scanned many times – scanning a patient many times is not justified. Ideally when a patient comes for a CT, the first scan will produce a good set of images. If the image quality is not good enough, the extra scan adds to the patient’s whole of life radiation burden. For this reason, there is a tendency to over-expose, in fear of under-exposing, which may lead to a repeat examination. The phantom can also provide dose estimates to ensure that the best image quality can be obtained with the lowest possible radiation dose. 

It’s always useful to obtain a measurement of image quality and dose at the same time. Half of the phantom measures different aspects of image quality, such as:

• image noise;
• contrast resolution; and
• spatial resolution.

The other half is specifically anthropomorphic. It’s designed for thermo-luminescent dosimeter (TLD) measurements of radiation dose. These slices have specially chosen holes to insert the small TLD capsules used to measure the radiation dose deposited in the different tissue sites of the phantom. The TLD measurements are taken in phantom locations simulating the child’s skin, breast, thyroid, thymus, ribs, liver, lungs and spine (Figure 5.). These phantom slices have been used to compare the dose delivered in paediatric organs during different protocols on the same CT scanner, within a paediatric hospital. 

Validation of the phantom

All materials used to construct the phantom were validated as tissue equivalent using the measured CT number and comparing it to the results from the clinical survey of paediatric tissues. This comparison of phantom materials and clinical tissues was performed over a range of x-ray tube voltages; included here are results for 70 kVp and 100 kVp. 70 kVp – 100 kVp are typically child sized x-ray tube voltages, not appropriate for scanning large adults.  

Note that there is a very wide range of CT numbers observed within the clinical survey due to such large variation between children most of whom have had a chest CT  for lung pathologies. Children can have vastly different levels of bone mineral density which would cause such a large variation in measured CT number in both the ribs and spine tissues. Similarly, there is a wide range of measured CT numbers for lung tissue with an overall average CT number higher than the expected -1000 HU measured in air. This could be caused by many factors including increased liquid/mucosa, incorrect breath hold or poor lung pathology. 

Despite large variations in clinical data, both graphs in Figure 4 demonstrate that the materials used to construct the phantom can closely approximate the CT numbers clinically observed in infants. 

Future Work

Having established that the phantom is constructed using tissue equivalent materials approximating the anatomy of an infant’s chest, it can be used as a special quality assurance tool on any CT scanner intended for paediatrics. It is a valuable accessory for initialising or optimising paediatric chest CT protocols. This phantom has the potential to enable image quality and dose optimisation of chest CT examinations, ensuring the dose to infants stay as low as reasonably achievable.

There is potential for the phantom to be further developed to include other common radiological procedures – abdominal and pelvic CT examinations for infants.

Reference

Image Gently Organisation (2014). Image gently and CT scans. https://www.imagegently.org/Procedures/computed-Tomography#2018475-publications.

Seonaid Rodgers BSc MSc, 7 June 2021