Radiological Modalities

In order to understand the nature of foreign objects in the pelvis, clinicians use many radiological modalities that give information about the appearance and the location of these anomalies. The most common modalities are MRI, CT and X-rays. In general, X-ray imaging techniques are based on the fact that the electromagnetic X-radiation could penetrate through the skin and soft tissues to illuminate the inner structure of the body. In an X-ray image, regions such as bones and metal will show up as solid white. Tissues and fat appear as a light white or gray colour, and air (like inside the lungs) shows up black (Withers, 1987).

Computed Tomography (CT).- This modality is a concatenation of X-ray slices at varying depths of the body producing a volumetric image containing both bones and soft tissues. It provides a good bone contrast and uses a standardised Hounsfield Unit (HU), a quantitative scale for describing radio-density. Different tissue types are assigned different HU values, i.e., bone structures have high values and air has

CHAPTER 2. Background

low values. This is useful for orthopaedics for visualising an image, as images can be thresholded to only show bone tissues. In spite of the good quality of CT scanners, they acquire images using harmful ionising radiation. This can be alarming when a patient needs to undergo theCTscan multiple times as the amount of received radiation compounds.

Magnetic Resonance Imaging (MRI).- An MRI scan is harmless and good at capturing the soft tissue contrast. An obvious solution to less harmful medical imaging may seem to replace CT scans with MRI, but it is not that simple. MRI does not provide specific contrast for bony structures as CT does. Therefore, an MRI scan by itself cannot replace a CT scan; the intensity values are not quantitative and it cannot be thresholded for easy bone segmentation like it is possible for CT. This non-quantitative nature ofMRI imaging challenges generalisation of analysis methods over scanners from different vendors, field strengths and imaging centres.

X-ray.- Despite their drawbacks (low resolution, change of contrast) in comparison with the aforementioned modalities, radiographs are the major workhorse used in initial and follow-up imaging of foreign objects (Tseng et al.,2015), trauma diagnoses (Siebenrock et al.,2003;Tannast et al.,2006), and post-operative evaluations (Murray, 1993; Hassan et al., 1995). The standard projection of pelvic radiographs is the plain or AP view projection (see Figure 2.3), which demonstrates the pelvis in the natural anatomical position, and it gives important information for detection, diagnosis or treatment purposes (Jamali et al., 2007). According to Welton et al. (2018), a true anteroposterior view of the pelvis is acquired with the patient in the supine or standing position, with a tube-to-image distance of 120cm and a photon beam centered midway between the pubic symphysis and the top of the iliac crests. The craniocaudal angle of the beam is standardised such that the sacrococcygeal joint is 1cm to 3cmfrom the superior aspect of the pubic symphysis.

Figure 2.3: Pelvic X-ray in anteroposterior view.

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2.1. Biomedical Concepts Radiographs used to evaluate foreign objects have two important concepts to recognise: radiopacity and radiographic visibility. Radiopacity is an intrinsic feature of an object that depends on its ability to absorb (attenuate) or scatter X-ray photons (Halverson and Servaes, 2013). Radiographic visibility depends on the X-ray attenuation features of the object, its surrounding structures, and the overlying and underlying structures that X-ray photons have to pass through to reach the detector.

In other words, a foreign object that is radiographically visible in the airway may not be visible when it is embedded in soft tissue. A foreign object that is radiographically visible in the foot may not be visible when it is embedded in the abdomen where soft tissue thickness is greater. Therefore, radiographic visibility of an object can depend not only on its size and radiopacity but also on its anatomic location, the patient’s body habits, and the surrounding anatomic structures (Tseng et al., 2015).

In clinical practice, an object is described as radiopaque when it is relatively clearer than the surrounding tissue and radiolucently darker than the surrounding tissue (see Figure 2.4). Although plastic and organic foreign bodies (such as wood) are generally radiolucent on radiographs, stone foreign bodies are usually radiopaque.

A common misconception held by physicians about glass foreign bodies is that only leaded glass is radiopaque on radiographs (Lincourt et al., 2007; Kaiser et al., 1997).

In fact, the radiodensity of glass does not depend on lead content, but rather on its density (Jarraya et al., 2014). Therefore, all glass foreign bodies are radiopaque, but with various degree of radiodensity. Metal foreign bodies are almost always radiopaque, with the exception of thin aluminium metal, which has a lower radiodensity and a lower sensitivity for detection on radiographs (Valente et al., 2005).

Figure 2.4: Examples of foreign objects in the pelvis. A) Plain radiograph of the pelvis showing a dense metallic foreign body that was composed of many small magnetic balls in the pelvic region, of which one part was a cloddy body in the bladder and the other part was a long-striped body in the posterior urethra (Li et al.,2018). B) Plain radiograph that shows an ear phone cable within the bladder. This is a clear X-ray example depicting radiopaque foreign body within bladder (Hegde et al., 2018). C) Plain radiograph showing a glass bottle at the beginning of the upper-middle region of the rectum and extending to the anal verge, and its open edge was directed to the bottom, and its base was proximal, on standing (Ozbilgin et al.,2015).

CHAPTER 2. Background

Digitally Reconstructed Radiographs.- Digitally Reconstructed Radiographs DRRs are computed images from CT data. These radiographs play an important role as reference images for image-guided therapy and for 2D−3D image registration. On the other hand, the Beer-Lambert law is designed for monochromatic light and its absorption increases with decrease in radiation wavelength. According to (Sherouse et al., 1990), the features of the DRRs implementation include three main factors:

(1) methods for interslice interpolation; (2) a method for approximating photoelectric and Compton linear attenuation coefficients from Hounsfield units; and (3) selectable pixel size and “film size” of the computed image. Additionally, DRRs have to consider other relevant factors: (a) projection of anatomic contours extracted from CT scans;

(b) projection of collimator edges, custom blocks, and crosshairs; (c) the ability to produce images with an arbitrary ratio of Compton to photoelectric interactions.

The Beer-Lambert law is considered a fundamental principle in the DRRs generation. According to the Beer-Lambert law, absorption of radiation depends on:

(1) intensity of the incident beam; (2) path length; (3) concentration of absorbing species; and (4) extinction coefficient. To compute the absorption of radiation, the following mathematical formulation is considered Equation 2.1:

A =εlc (2.1)

where:

• A is the absorbance

• ε is the molar attenuation coefficient or absorptivity of the attenuating species

• l is the optical path length in cm

• c is the concentration of the attenuating species