Basics of Imaging

Introduction

There have been rapid technological advances in medical imaging over the last several decades, generally providing faster, more detailed, & readily available assessments of patients. Clinical work-ups have increasingly incorporated advanced imaging during these years, though not always in an effective or judicious manner. In addition to concerns of financial cost, apprehensions have been increasingly raised about the downsides of several imaging modalities (including the long-term effects of radiation, anesthesia, & contrast agents).

This chapter will serve as an overview of the various imaging modalities available in the work-up & treatment of pediatric patients, discussing basic techniques & terminology that can help the pediatrician better understand the studies they are ordering & the reports they are receiving. Basic applications of each modality will be covered here (though more thoroughly applied in the “Approach to…” chapters introducing each body section). We will also discuss some of the negatives associated with each modality, though the recognized & theorized health risks of these modalities will be covered in a separate chapter.

Radiography

Despite being the earliest discovered & medically adopted imaging technique, radiographs (the preferred term for “plain films” or “x-rays”) remain the workhorse of radiologic imaging in the child (with the main exception being the head). They are widely used in the assessments of pediatric bone trauma, abdominal pain, & suspected lower respiratory tract infections, among many other indications. Radiographs remain the cheapest & most readily available imaging studies, though pediatric specific centers are most experienced with techniques to optimize not only the extraction of medical data from the study but also enhance the patient & family experience. Additionally, pediatric centers have been at the forefront of reducing the dose of ionizing radiation associated with these exams, which does require a balance with maintaining a diagnostically sufficient study.

In general, radiography uses a beam of x-rays coursing through the body to produce an image. The image generated will project as various shades of gray (from black to white) based on the density & thickness of the tissue being examined as well as the energy of the x-ray beam. The degree to which the x-ray beam is attenuated as it passes from the tube to the detector of the machine (with the patient in between these 2 components) determines how much of the detector is exposed to x-rays that then activate specific elements within the detector to create an image. Therefore, tissues such as the lung (which are filled with air) will attenuate relatively little of the x-ray beam on its way to the detector. In contrast, tissues such as bone will attenuate a much larger percentage of the x-ray beam, allowing relatively few x-rays to reach the detector. By convention, tissues of greater density (such as bone) will be displayed as relatively bright or white (radiopaque) while tissues of lesser density (such as lung) will be displayed as relatively dark or black (radiolucent). Classically, 5 main categories of density are recognized visually on radiographs (in order from least to most dense): Gas/air, fat, water (soft tissue), calcium, & metal.

Unlike the “cross-sectional” imaging modalities discussed below (i.e., computed tomography, magnetic resonance, ultrasound, & some nuclear medicine exams), where 2D images represent very thin slices of 3D patient anatomy, the 2D radiograph represents a single thick slice (or volume) of the entire patient where the 3D anatomy is all superimposed onto a single 2D image. Therefore, a particular point on the image likely represents the result of the x-ray beam passing through several different types of tissues. Structures will be most discrete when the interface of 2 tissues of differing densities (such as aerated lung vs. pneumonia) lies parallel to the x-ray beam. Overlapping structures (where 1 structure of interest lies deep to the other such that their interface is perpendicular to the x-ray beam) will be less apparent (e.g., a pneumonia hiding behind the heart). The loss of a normal interface can also represent pathology when 2 structures of similar density (such as the heart & pneumonia) lie directly adjacent to one another (obscuring the normal heart border). Because of this superimposition of structures & the need for visible interfaces of differing densities to increase the conspicuity of pathology, radiographs will be insensitive for some disease processes (such as a bowel obstruction that is fluid-filled rather than gas-filled) but highly sensitive for others (as with displaced fractures of bone).

Fluoroscopy

Whereas radiographs use a finite x-ray exposure to create a single image, fluoroscopy essentially creates real-time radiographic visualization of changing anatomy by continuous or pulsed x-ray beams. For decades, this technology has been used to study the gastrointestinal & urinary tracts when specific combinations of anatomic & physiologic information have been required. A fundamental example is the voiding cystourethrogram where key anatomy is only visualized during specific physiologic acts (e.g., voiding allows assessment of the urethra & can unmask previously occult vesicoureteral reflux).

As with many other radiologic studies, fluoroscopy frequently employs a relatively inert contrast agent to aid in viewing targeted anatomy. The interaction of these contrast agents with the x-ray beam (typically by increasing the density of the region of interest, thereby attenuating the x-ray beam) increases the conspicuity of the target organ in some way. For most pediatric fluoroscopic exams, this involves the introduction of the contrast agent into a hollow viscus (such as oral contrast into the stomach). The contrast is then allowed to propagate physiologically through the organ of interest (such as with bowel peristalsis) while images are intermittently obtained. A limitation of this type of imaging is that it does not directly visualize the tissue of interest. That is to say, as the contrast typically only outlines the mucosal surface of a viscus, it only allows inferences about the nature of the viscus wall & what is causing its distortion.