Projection radiographs are viewed as the radiologist looks at the patient, placing the patient’s right side at his own left side. Coronal cross-sectional images use the same orientation, generally presented, from anterior to posterior. Axial images are viewed from the perspective of standing at the supine patient’s feet, again placing the patient’s right side at the radiologist’s own left side. Sagittal images are generally presented from the patient’s left side to the right side, although this convention is not always applicable to all studies, particularly MRI. It is helpful to reconcile the position of the heart in determining whether sagittal images are on the right or left side of the patient’s body. The Visible Human Project and the proliferation of web-based medical education materials now allow easy access to comparison images for anatomic identification in all three of these planes. Sophisticated image processing of volumetric CT data sets increasingly enables radiologists to provide data for surgery in the perspective of the surgeon. Surgeons have also learned to use conventional axial CT images to determine operability and plan specific surgeries.
The manner in which a standard radiological examination is performed may be modified to accommodate the inability of a hospitalized patient to have a more standard examination. Thus, a frontal chest radiograph may be made with the patient sitting up on a stretcher, sitting in a wheelchair, or lying in bed. In these instances, the radiograph will be made with projection from anterior to posterior rather than the standard erect imaging approach from posterior to anterior. The standing position facilitates obtaining radiographs in maximum inspiration. Gravitational effects are not as favorable in the supine position and other positions introduce a variety of variable complications. At minimum on an AP radiograph, the heart will appear larger due to magnification that varies with the inverse square law, whereby increasing the distance of the heart by a factor of 2 will increase the magnification by a factor of 4. This effect increases with increasing size of the patient, being particularly prominent when imaging an obese patient.
For a radiology examination to have maximal benefit in patient care, it should be performed at the right time. Sedation decreases cooperation and depth of respiration. Likewise, it may be better to presumptively treat an acutely tachypneic patient and obtain the diagnostic study after the patient is hemodynamically stable, able to lie flat, and breathe less rapidly for image acquisition.
The acute hospitalization may not be the right time for obtaining many radiologic examinations. The patient participates in most imaging, whether by staying still, following breathing instructions, or moving through a series of positions. CT, MR, and fluoroscopic examinations that are not essential to address the immediate treatment of the acute process should not be performed when the images will most certainly be limited by uncontrolled respiration and superimposed acute processes. Clinicians rarely help patients by obtaining an inpatient CT or MR to either spare the patient the outpatient trip or to work up a nonacute incidental finding, such as an indeterminate subcentimeter pulmonary nodule.
The most fundamental undertaking of all radiology examinations is the largely noninvasive visualization of tissues that make up the body, with differentiation of normal structures from pathology. Projection radiographs detect five categories of density: air, fat, water (including soft tissue and muscle), calcium, and metal. The relative radiodensities of various substances and tissues will determine the ability of plain films to differentiate between them. For example, blood, muscle, and liver will have an almost identical medium gray appearance as will most solid or fluid filled organs and tissue masses, greater than air, but less than bone or metal. The muscular heart filled with blood will appear homogeneous relative to the air-filled lungs on both sides of it.
The radiologist is able to process two-dimensional data in three dimensions; focusing through various layers, perhaps starting with the posterior portions of the ribs and then the anterior, thinking about superimposed masses and the anatomic structures responsible for them. The lung is much thicker medially where it borders the mediastinum and inferiorly. There are more vessels superimposed on each other in the medial half of the lung field and in the lower half of the lung than the upper half. The radiologist notes an abnormal shadow by its proximity to a particular rib or interspace. “Fool’s triangle” refers to the right cardiophrenic angle where many vascular trunks overlap due to the anteriorly placed middle lobe superimposed on the vessels of the posterior lower lobe. Novices in radiology interpretation may over read infiltrates in this area or note the most obvious abnormality, whereas radiologists systematically study each film looking at various structures in a deliberate order, concentrating on the anatomy of each, while excluding superimposed shadows of other structures. The integration of two orthogonal views provides the maximal localization of individual structures and abnormalities. A single radiograph will not be able to precisely locate a foreign body or support line. Bony fractures may not be apparent with a single film and a second film at right angles should be ordered to identify the lack of alignment and possible fragments.
Serial radiographs may be more helpful than more sophisticated imaging through the introduction of the fourth dimension, following the course of disease and physiology over time. Radiography can be performed at the bedside when the patient is unable to travel to the radiology department and can be made available very rapidly.
US imaging takes advantage of sonographic properties such as augmented transmission through fluid and textures that may be influenced by fat, blood vessels, and other structures. However, there are important limitations because ultrasound is stopped by air, limiting its utility in lungs and in the setting of gas collection throughout the body. Ultrasound cannot penetrate bone and many medical devices (such as joint replacement).
The Doppler shift refers to the change in frequency that occurs when a sound wave is reflected by moving blood. This change in frequency is proportional to the velocity of the blood flow in the vessel being sampled. Since World War II Doppler technology has evolved from crude, continuous-wave Doppler flow detectors blindly applied to the skin surface to color flow mapping systems. Duplex instruments combine pulsed Doppler techniques with real-time ultrasound imaging, made possible through the introduction of electronically steered, phased array transducer systems and the application of signal processing and display techniques for analyzing ultrasound echoes.
Duplex Doppler imaging with two-dimensional US provides anatomic information with pulsed-wave Doppler analysis to calculate a color overlay containing information about the direction and velocity of blood flow. Pulsed Doppler permits a smaller sample size, thereby permitting analysis of the arterial lumen without the associated vein. The spectral tracing is a quantitative depiction of red blood cell movement within a sample volume. A semiquantitative color encoding of the Doppler information is superimposed on the gray-scale, real-time image providing color Doppler imaging. The color depends on the mean velocity of flow and the direction of flow. Blue does not necessarily mean venous flow as seen in the reverse component of triphasic flow characteristic of normal arterial flow. Red does not necessarily mean arterial flow as seen in reflux of venous flow in incompetent veins. The hue reflects the relative blood velocity, so that fast flow just proximal to a critical stenosis may appear white whereas slow flow beyond the stenosis would have a deeper hue. Color duplex imaging increases the sensitivity of duplex imaging. Color Doppler imaging makes it easier to evaluate deep veins in obese or edematous individuals and to identify flow around a thrombus.
Doppler technology has largely replaced venography in the diagnosis of venous thrombosis involving the legs. Although noninvasive, safe, and less expensive, US may not always be able to image vessels for the following reasons using the 5 MHz probe: (1) the vessels too deep to be imaged due to overlying fat or edema, (2) iliac veins and the inferior vena cava obscured by overlying bowel gas and depth of vessels, (3) inability to visualize a segment of the distal superficial femoral vein in the adductor canal (isolated clot in this area unlikely), (4) the small caliber and multiple branches of the calf veins, (5) inability to definitively distinguish between acute and chronic thrombus, and (6) limited expertise of operator.
Duplex or color flow Doppler US may be used to confirm arterial perfusion of organ transplants and exclude venous thrombosis in portal, splenic, and renal veins. Doppler US indicates the direction of blood flow, which may be helpful in diagnosing subclavian steel or portal hypertension with altered hemodynamics or in the diagnosis of pseudoaneurysms or mesenteric ischemia. Doppler US may be used to characterize tumors, varices, or ectopic pregnancies that have characteristic flow patterns.
Pulse Doppler quantitates the degree of arterial stenosis. Real-time US may be able to characterize arterial plaque as calcified or soft. The reliability depends on whether the vessel can be imaged adequately, whether the vessel is straight or tortuous, whether there are tandem lesions, and on the skill of the sonographer. For vessels that can be imaged easily, the reliability of US is excellent. If the vessel is not well suited to imaging, the anatomic and hemodynamic information is unreliable, especially in inexperienced hands. The mere presence of a hemodynamically significant vascular lesion does not reliably prove that it is the cause of a particular symptom or that it is otherwise functionally significant.
Ultrasound guidance is being used with increasing frequency for central venous catheter placement, thoracentesis, and paracentesis. Owing to increased concerns about patient exposure to ionizing radiation, ultrasound is being used increasingly for novel applications including joint examination. It is actively being explored as an adjunct for bedside physical examination in ICU settings.
COMPUTED TOMOGRAPHY (CT) SCANS
In addition to the five categories of density, air, fat, water (includes soft tissue and muscle), calcium, and metal detected by plain films, CT scans are able to detect the differences between water and a variety of specific soft tissues including liver and kidneys and in the case of the brain, between white and gray matter. CT provides detailed anatomic images in which a variety of soft tissues can be recognized; the resulting basic transaxial images are the in vivo equivalent of transaxial anatomic pictures of a cadaver. State-of-the-art multidetector CT scanners are capable of acquiring ever-increasing numbers of individual slices of data at one time. Four-detector scanners image the chest in approximately 20 seconds, a practical time for patient breath holding. Readily available clinical models with the capability of producing 16 to 64 slices at one time can scan the chest in 10 seconds or less. Alternatively, very small structures may be studied using ever-smaller slice thickness. Along with cardiac gating, providing up to 320 slices at one time permits unprecedented in vivo evaluation of ultrastructure in lungs as well as the heart.
Specialized CT examinations are performed according to disease-specific algorithms. The reconstructed image thickness of the CT determines its sensitivity and specificity for identifying certain underlying conditions, based primarily on the size of the structure being assessed. For example, a PE-protocol CT requires thinner images than a venous-CT of the lower extremities.
Hounsfield units derive their name from the developer of the CT scanner, Nobel Laureate Sir Godfrey N. Hounsfield. The scale arbitrarily assigns water the attenuation value of zero, and air 1000, with the attenuation of other materials defined in relation to these set points. These numerical values of normalized x-ray attenuation define the gray scale of all CT images. The display windows highlight various structures based on the relationships between the underlying fundamental gray scale and the composition of various tissues in the body.
Intravenous iodinated contrast material commonly provides optimum delineation of vascular structures, particularly when they lie in close proximity to the pathologic entity. Thus, lung cancer staging is most often performed with IV contrast. Oral contrast is used to aid delineation of gastrointestinal (GI) tract structures. A routine abdomen/pelvis CT performed for nonspecific abdominal pain or cancer restaging typically employs both intravenous and oral contrast for optimal tissue characterization. Alternative routes of contrast material administration are also used for nonvascular examinations such as cystography and myelography. Nonionic contrast agents have replaced older ionic contrast agents as they produce fewer side effects. The use of well-functioning 20-gauge or larger peripheral IV is required for administration of iodinated contrast agents for optimal imaging, particularly for vascular CT applications that require high contrast flow rates. Whether in the GI fluoroscopy suite, CT fluoroscopy suite, or angiography suite, fluoroscopy provides physiologic information along with anatomic information but they are invasive tests, best preceded by appropriate subspecialty consultation.
Not all CT scans require IV or oral contrast material. For example, to diagnose the presence of a renal stone, a dedicated renal stone CT would not use intravenous or oral contrast as neither is needed to detect a high-density renal stone. High-resolution CT to assess interstitial lung disease is generally performed without contrast. Follow-up CT scans may be performed without contrast materials depending upon the tissue contrast between the structures of continuing interest.
PRACTICE POINT CT
CT scans of contiguous body parts such as chest, abdomen, and pelvis are frequently performed together. The sequence of scanning and the volume of contrast material utilized will be selected to maximize scanning efficiency and answer the clinical questions posed.
Separate doses of IV contrast material are generally not required. The same bolus of contrast material may be followed through the body with attention to circulation time and distribution of contrast material within organs to optimize imaging. Bolus tracking methods in modern scanners provide individualized selection of delay. The chest may easily be imaged during the delay required for liver enhancement on an abdomen CT scan.
In many instances, the chest portion of the CT scan will be equally diagnostic with or without IV contrast enhancement. The chest portion of the CT scan should be performed prior to contrast administration to assess interstitial lung disease or detect calcification in very small lung nodules. Chest CT scanning may also be performed during the administration of IV contrast material for some contrast enhanced head CT scans.
To ensure that the CT scan is tailored to the clinical questions and patient-specific needs, speak directly to the radiologist before ordering the CT scan. The radiologist may suggest potentially valuable alternatives and prevent waiting for test results from the wrong test that cannot further clinical decision-making.
PET-CT is a lengthy examination that is often better suited to outpatient follow-up. Patients are asked to avoid strenuous activities the day before the examination and may be given special preparatory dietary instructions to consume a fatty meal the evening before the examination. The patient should be NPO for at least 4 to 6 hours. While PET-CT has become a central tool for the staging of malignancies, significant overlap in results between neoplastic and inflammatory processes limits the value of the study during hospitalization for acute illness.
MAGNETIC RESONANCE IMAGING (MRI)
MR uses very specific depolarizing pulse sequences to detect tiny differences in signal from soft tissues that may be otherwise indistinguishable. Gadolinium has paramagnetic properties that make it the most common contrast agent used for MR examinations. While inert, gadolinium does pose potential risk for nephrogenic systemic sclerosis (NSF), particularly in the setting of renal failure. MRI examinations are customized to the problem being evaluated. Coils used to perform the examination not only provide improved imaging but also control technical parameters such as field of view. The bore of the available MRI scanner, itself, may limit the size of patients who can have MRI. Larger bore and open scanners have decreased this limitation, but a patient may have to go to a special location to have such an examination.
Hospitalized patients are often unable to cooperate adequately to allow the full benefit of the MRI technology in their care. MRI examinations do not use ionizing radiation but may be quite lengthy, lasting 1 hour or more in duration. The request for wider coverage such as adjacent body parts is not easily accommodated in the same scanning session. It is therefore imperative to have a specific goal for the MRI from the outset. MRI and CT are equivalent for imaging lymphadenopathy. MRI imaging is by nature less than contiguous and should otherwise be viewed as complementary to CT imaging. In many instances, the patient will be better served by MRI as part of outpatient follow-up following recovery from the acute illness requiring hospitalization. The need for critical information determines the best study; MRI/MRA may more rapidly diagnose potentially surgical aortic disease such as aortic dissection compared with conventional angiography. Selected for the wrong reason, MRI will increase the stress on the acutely ill patient and delay institution of needed therapy.
Always inquire about patient claustrophobia before ordering the MRI examination. Patients often benefit from oral premedication; those with severe claustrophobia or difficulty remaining still in the confines of the scanner may require sedation with an anesthesiologist present during the scan. All patients should be screened for possible contraindications prior to scanning as routine practice. Contraindications to MRI include aneurysm clip, recent surgery (generally within 10 days) and incompatible cardiac pacing devices. Some patients who have metallic implants such as joint prostheses will experience unacceptable heating of the region that will prevent completion of the examination. This is sometimes quite specific to the location of scanning relative to the location of the implant. These effects vary with the field strength of the MRI unit. Relative contraindications also include cochlear implants and neurostimulators. In addition, patients who have potential to have a metallic foreign body in an eye, typically due to occupational or other machine shop exposure, may need to have radiographs or even CT scanning of the orbits prior to MRI. There is no known adverse effect of MRI on the fetus but the decision to scan during pregnancy should be made on an individual basis.
MRI exceeds CT for the multifactorial differentiation of fat and other tissue planes, properties that are particularly useful when studying the musculoskeletal system and in localizing boundaries of pathology, and fluids, including the differentiation of the various states of hemoglobin. In the brain, MRI images provide significantly more information than CT images, resulting in greater sensitivity for small and subtle lesions such as early brain metastases. The use of MRI for clinical problem solving is more apt to reflect the problem under consideration than a standardized approach. MR angiography (MRA) may provide high-quality images of many parts of the body often adequate to replace conventional angiography.
The use of MRI for direct acquisition of multiplanar, sagittal, coronal, and axial images has diminished with increasing availability of PET-CT as well as multidetector CT scanners that permit data to be acquired with voxels of equal dimension in all three planes, thus providing high-quality sagittal and coronal reformatted CT images. MRI imaging of calcium as a signal void is a particular pitfall in MRI imaging that highlights the complementary nature of CT for the study of bones and potentially calcified pathology.