CHAPTER 9

DIAGNOSTIC IMAGING IN ACUTE CARE SURGERY

BEHROZE A. VACHHA, LEO L. TSAI, KAREN S. LEE, AND MARC A. CAMACHO

The management of acute care surgical patients, as that of all patients, has largely benefited by advances in radiology. Current imaging techniques and modalities have vastly improved the ability to noninvasively diagnose and characterize disease and injury, which has largely supplanted the need for invasive exploratory surgery. Almost equally important, imaging can reliably exclude severe illness or injury in the acute presentation. This has resulted, for example, in a major shift in the management of trauma patients, in particular, with a greater emphasis on conservative management and reduction in the attendant morbidity and mortality introduced by surgical intervention and perisurgical (e.g. ICU) care.

There is a vast array of imaging modalities in the diagnostic armamentarium currently. While radiography, angiography, and nuclear medicine have undergone advances in their own right, the advances in magnetic resonance imaging (MRI), ultrasound (US), and, in particular, computed tomography (CT) have resulted in a major expansion in applications and extraordinary increases in utilization. This has led to justified concerns regarding health care costs, the need for outcomes research, and potential deleterious effects introduced by the imaging modalities themselves (e.g. radiation dose and CT). However, these imaging modalities each exhibit unique advantages that can be exploited for specific indications to help increase accuracy of, and decrease delays in, diagnosis. Additionally, these modalities also may have associated relative and absolute contraindications, which should be considered when considering utilization for a particular patient and clinical scenario.

In this chapter, we present these three imaging modalities in detail, introduce basic background concepts, and detail advantages and disadvantages as well as optimal target patient populations, clinical scenarios, and indications. Each subsection concludes with a gallery of images representing commonly encountered acute care surgical pathology, with detailed captions.

ULTRASOUND

Ultrasound is one of the most utilized and widespread diagnostic imaging modalities in medicine. Ultrasound images are generated by transmitting an ultrasonic pulse and detecting the amplitudes and delay times from returning acoustical echoes reflecting off of tissue interfaces. Typical operating frequencies range from 2 to 12 MHz. The major advantages of this modality include the ability to perform real-time noninvasive imaging without the use of ionizing radiation, its high degree of portability, its relatively low cost, and the capability to acquire images in an infinite number of planes. Additionally, by evaluating Doppler frequency shifts from moving blood, ultrasound can provide blood flow information by producing arterial or venous waveforms and quantitatively measuring blood flow velocities.

Ultrasound is particularly well suited to evaluate the abdomen and pelvis, owing to excellent acoustical windows provided by the liver, spleen, and bladder. The liver window provides a view of the gallbladder, pancreas, right kidney, heart, and right pleural space. The spleen is used to image the left kidney and left pleural space. A full bladder can allow a thorough assessment of the uterus and adnexa. Transvaginal, transrectal, transesophageal, and endoscopic probes are also available for more detailed imaging of the pelvis, prostate, rectum, heart, upper gastrointestinal tract, and pancreas. Ultrasound is also the primary imaging method used for assessment of the fetus, as no ionizing radiation or contrast materials are required, and the transmitted acoustic energy in typical obstetric examinations is accepted as safe.¹ Furthermore, thoracentesis,² paracentesis, superficial abscess drainage, percutaneous cholecystostomy, and other minimally invasive bedside procedures are commonly performed under real-time ultrasound guidance.

Common uses of ultrasound in the acute surgical setting include evaluation for acute cholecystitis (Fig. 9.1),^–46 renal obstruction or calculi (Fig. 9.2), vascular occlusion,⁷ ovarian or testicular torsion (Fig. 9.3),^9,10 and appendicitis (Fig. 9.4).¹¹ Because of its portability and real-time imaging capability, ultrasound is also often employed in the trauma setting to assess patients who may be too unstable for a CT examination. Ultrasound can readily evaluate for solid organ injury within the liver, spleen, and kidneys. Additionally, the Focused Assessment with Sonography for Trauma (FAST) exam is a common bedside ultrasound study used to detect free fluid within the peritoneal, pericardial, and pleural spaces as a first-step measure to identify an internal hemorrhage (Fig. 9.5).^–1315

Drawbacks of ultrasound include its limited ability to assess the brain, thorax, and bowel due to the presence of tissue–air and tissue–bone interfaces that are nearly completely acoustically reflective and therefore prevent the penetration of ultrasound waves. Ultrasound imaging of obese patients is also challenging, often resulting in low-quality images due to poor penetration of the ultrasound beam. Additionally, ultrasound can be subject to imaging artifacts from oblique scattering and multiple reflections.¹⁶ Most importantly, quality ultrasound imaging is highly dependent on operator skill and experience.

Table 9.1 summarizes the advantages and disadvantages of ultrasound.

Figures 9.1 through 9.5 demonstrate common acute care surgical pathology optimally diagnosed via ultrasound.

MAGNETIC RESONANCE IMAGING

Magnetic resonance imaging (MRI) is performed through manipulation of the magnetic moment of water hydrogen protons within a static magnetic field. The behavior of these moments, or spins, is directly related to the environment surrounding the water molecule through local effects such as magnetic fields from neighboring spins and energy exchange with nearby nuclei. Images are created after applying pulsed radiofrequency (RF) waves and magnetic field gradients at calculated times, then extracting spatial and temporal data from the subsequent signal patterns. Each programmed set of RF and gradient settings is referred to as a pulse sequence. The signal from protons within fat, protein, or free water can be isolated and highlighted through different pulse sequences, allowing for excellent soft tissue contrast that is superior to other cross-sectional imaging methods.¹⁷ Isometric 3D MRI is now commonly used for multiplanar and curved reconstructions of vascular and ductal structures. Multiple types of contrast agents can be used to highlight the gastrointestinal tract, vascular structures, or bile ducts.^18,19 Interventional MRI continues to be an active area of research, particularly in oncology.²⁰

A particular advantage of MRI is that this modality does not employ ionizing radiation, and, therefore, is favored whenever there is a particular desire to limit or avoid radiation exposure, such as in the evaluation of pregnant patients (Figs. 9.6–9.8).^22,23 MRI is also increasingly considered as a primary imaging modality in the evaluation of chronic diseases where the cumulative radiation dose from frequent follow-up CT examinations would be quite substantial. Inflammatory bowel disease is one such entity, where rapid pulse sequences are now able to compensate for peristaltic motion, and T2-weighted, fluid-sensitive sequences are particularly useful for detecting fistulas or abscesses (Figs. 9.9 and 9.10).^–2527

While magnetic resonance cholangiopancreatography (MRCP) is commonly used for assessment of the biliary tree, liver, and pancreas for oncologic staging, surgical planning, or radiotherapy targeting,^–2831 in acute settings, MRCP is well suited for diagnosing emergent gallbladder and biliary tract conditions, including acute cholecystitis, choledocholithiasis, and acute cholangitis (Figs. 9.11 and 9.12).³² Furthermore, magnetic resonance angiography provides an excellent alternative to CT for the imaging of vascular abnormalities (Fig. 9.13), particularly when iodinated contrast agents cannot be used due to contrast allergies or poor renal function.

Drawbacks of MRI include the need for a large static magnetic field to achieve an adequate signal-to-noise ratio, requiring large superconducting coils that are relatively expensive to maintain. An MRI suite must also be specially shielded to avoid signal interference from electromagnetic noise. Furthermore, typical magnets have a narrow cylindrical bore that can induce claustrophobia and may not be able to accommodate obese patients or patients who require large support devices.

Another primary disadvantage of MRI is the inability to image patients with MRI-incompatible devices. The high magnetic field strengths of MRI are not compatible with ferromagnetic materials, such as many intracranial aneurysm clips. Additionally, RF pulses used to acquire MR images may interfere with electronic implants, including many current and older generations of cardiac pacemakers. MRI centers, therefore, require prescreening of all patients with a detailed checklist to ensure that the patient is safe for imaging and that all support equipment are MRI compatible.³⁴ Even the suspected presence of embedded metal within the orbits warrants further evaluation, typically with a radiograph, as fluctuations in magnetic fields can cause current-induced heating and particle movement. Furthermore, metallic implants that are deemed safe for MRI still cause local magnetic field artifacts that distort and obscure adjacent structures, thereby degrading image quality.

Contrast reactions with gadolinium-based agents can occur, as they do with CT agents; however, particular care must be paid to patients with severely impaired renal function, who have an increased risk of developing nephrogenic systemic fibrosis, or NSF. Typical symptoms include swelling and tightening of the skin within the extremities, but can progress to involve the internal organs, including skeletal muscle, myocardium, lungs, kidneys, and dura mater, with potentially fatal outcomes and no effective treatments.^–3436 The proposed pathogenesis is attributed to an abnormal activation of circulating fibrocytes as a response to residual gadolinium in tissues that has remained long after initial administration. Since the first description of NSF in 1997, there have been approximately 335 biopsy-confirmed cases worldwide, but virtually no new cases since 2008 due to widespread restrictions on gadolinium-based contrast use in patients with severe renal failure.³⁷ The use of intravenous gadolinium-based contrast agents is also not considered safe in pregnancy.³⁸ With the recent development of robust noncontrast MRI sequencing techniques, these issues related to contrast administration, fortunately, can be circumvented in many cases.

Motion-related artifacts are a common occurrence in MRI. Many sequences require the patient to lie still for a substantially longer amount of time than that needed for a CT or ultrasound, with some sequences lasting several minutes. Additionally, the total duration of an MRI examination can be lengthy compared to other modalities, typically lasting 40 minutes. As a result, sedation is often required in the pediatric population in order to obtain images free of motion artifact. Breath-hold imaging acquisitions can reduce the total imaging time and are frequently used to eliminate respiratory-related motion artifacts. Acutely ill patients who are unable to suspend their respiration, however, pose a challenge for MRI. The use of non–breath-hold, rapid imaging strategies can allow interpretable MR images to be acquired in these sicker patients.

Finally, MRI has a limited capability to image structures that have low water proton density. As a result, the detection of calcifications on MRI can be difficult. Furthermore, imaging of the lungs with MRI is particularly challenging as these structures contain mostly air and, therefore, generate little signal. The multiple air–tissue interfaces within the lung parenchyma also increase the rate of signal decay, further limiting the imaging of the pulmonary parenchyma.

Table 9.2 summarizes the advantages and disadvantages of MRI.

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