Point-of-Care Ultrasonography: A How-To for the Primary Care Provider
Abstract: Portable ultrasound devices are increasingly used at the point of care in emergency departments and intensive care units by physicians seeking to safely guide procedures and rapidly answer focused clinical questions. The applications of this powerful tool also extend into the outpatient setting where it has the potential to clarify physical exam findings and rapidly expedite diagnostic workups, particularly in resource poor settings. With sophisticated pocket-sized ultrasounds now readily available on the market this technology is more accessible to primary care providers than ever before. The following review highlights a few of the many applications of focused ultrasound for the primary care provider.
Keywords: Point-of-care ultrasound, ultrasonography, internal medicine, clinical care, primary care
In the last 2 decades, medicine has undergone tremendous change due to technologic advances. Many of these advances have occurred in imaging, but few have touched clinical practice as much as point-of-care ultrasonography (US).
Scanning a patient at the point of care is vastly different from consultative imaging services provided by cardiovascular and radiology laboratories, where a technologist performs an US examination that is interpreted by a physician often hours to days later, after which a report is transmitted to the ordering clinician. While this approach typically results in quality imaging, it has numerous limitations, including tremendous delays in patient care and simple inefficiency in giving patients what they crave: an immediate answer. Not only do patients want this, but so, too, do their care providers.
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The beauty of point-of-care US is the rapid answer it provides to a clinician who is unsure whether a pulsating mass is present in the hypotensive patient’s abdomen, whether a pericardial effusion is the cause of cardiac arrest, and to many other questions about critical conditions that are virtually immune from high diagnostic accuracy via a physical examination. US diagnosis of these and many other processes at the bedside is simple and highly accurate when clinicians have appropriate training. The basis of a point-of-care US examination typically is a binary question, such as whether there is free fluid in a patient’s abdomen, whether there is cardiac activity in a patient with arrest, and whether a pneumothorax is present in a trauma case.
Point-of-care US in its many forms has arrived at the doorstep of primary care, the largest and most important room in the house of medicine. While much of the literature supporting point-of-care US use currently comes from the emergency department and inpatient settings, there is increasing interest in utilizing this technology among primary care providers. This article presents an overview of US applications with utility in primary care that can benefit any primary care clinician. Point-of-care US can improve patient safety, diagnostic and procedural accuracy, patient and provider satisfaction, and the overall efficiency of patient care.
Abdominal Aortic Aneurysm
The use of US to screen for abdominal aortic aneurysm (AAA) represents one of its most well-established applications in primary care. Sensitivity and specificity for the detection of AAA with US are reportedly 98.9% and 99.9%, respectively.1 US screening programs have been shown to decrease mortality while being cost effective,2,3 and the United States Preventive Services Task Force recommends that all men aged 65 to 75 who have a history of smoking receive a 1-time US screening for AAA.4 Despite this, screening for AAA is significantly underutilized in the United States5 and represents an opportunity for clinicians to incorporate US into primary care practices and increase compliance with screening guidelines.
The literature supports the use of US to detect AAA at the bedside. A systematic review of symptomatic patients presenting to an emergency department (ED) found that emergency medicine physicians could detect AAA with a pooled sensitivity and specificity of 99% and 98%, respectively.6 Results of a small study of rural outpatients performed by a resident in family medicine showed high concordance of a “quick screen” protocol with results from a formal US examination.7 In this outpatient study, the US examination took place within a 15-minute office appointment and generally took less than 4 minutes to perform, successfully imaging most patients without their having to return for a fasting study.7 A pilot study8 of primary care resident physicians at a U.S. academic institution showed that after approximately 2.5 hours of training, residents detected all 4 AAAs among the 79 patients screened; 10 residents were able to attain abdominal aortic US-independent competence level after an average of only 3.4 proctored examinations.
Start with a low-frequency probe in the transverse plane in the epigastric region (Figure 1). Ensure that the depth is set to visualize the hyperechoic (white) vertebral bodies with shadowing posteriorly. The pulsatile, anechoic (black) aorta is seen just anterior to the vertebral bodies and should be distinguished from the inferior vena cava (IVC). The probe is slid from the epigastric region toward the umbilicus, ensuring the aorta is visualized just cephalad to the renal arteries through its bifurcation to the iliac arteries. Graded compression techniques to move aside air-filled bowel may be necessary. The aorta is measured at its maximum diameter, with >3 cm being the cutoff for a positive AAA screening result. Three measurements are obtained for documentation—proximal, middle, and distal.
Deep Venous Thrombosis
Limited compression ultrasound (LCUS) for deep venous thrombosis (DVT) is based on the principle that if a deep vein is visualized to be completely collapsible with gentle probe pressure, a DVT is excluded at that site. By evaluating the deep venous system at key branch points in the popliteal and inguinal
regions, the trained clinician can exclude proximal DVT at the bedside in a few minutes without requiring the use of advanced Doppler techniques.
Over the last 2 decades, there has been increasing evidence in the literature supporting the use of LCUS to evaluate for DVT. A large, multicenter, randomized, controlled trial9 of symptomatic outpatients with suspected DVT of the lower extremities compared a full-leg US strategy with a strategy pairing LCUS with a D-dimer test. After 3 months of follow-up, no significant difference was observed in the detection rate of venous thromboembolism, suggesting that either strategy is reasonable in symptomatic outpatients.
Several studies from the emergency medicine literature support the use of bedside LCUS. A 2008 systematic review10 reported an overall sensitivity of 95% and specificity of 96% for detection of proximal DVT. A 2010 study showed that residents, fellows, and staff physicians in an ED with limited LCUS training were able to attain 100% sensitivity and 99% specificity compared with an US of the proximal venous system performed in the radiology suite.11 The American College of Emergency Physicians lists bedside US for DVT as 1 of 11 core US applications for emergency physicians.12
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Place a high-frequency linear probe just below the inguinal ligament in the transverse plane over the region of the femoral pulse (Figure 2). Compress the vein so the anterior and poster walls completely touch, and then release. Deformation or collapse of the paired artery without complete collapse of the vein is considered a DVT positive test. Ensure that the common femoral vein is completely collapsible with gentle compression from the saphenofemoral junction proximally through its bifurcation distally. Repeat the same technique in the popliteal region from just above the crease through the trifurcation of the vein distally.
Musculoskeletal and Soft Tissue
Soft-tissue and musculoskeletal US is one of the most rapidly expanding areas of point-of-care US and potentially is one of the more useful areas of focus for primary care providers. In our experience, rapid evaluation of “lumps and bumps” in the outpatient clinic helps immediately clarify physical examination findings and confidently distinguish lymph nodes, lipomas, and cysts.
Regarding patients with joint pain, a recent evidenced-based consensus paper13 found that musculoskeletal US is highly indicated for detecting joint synovitis, effusions, and fluid collections, and it may become a useful tool for general practitioners in addition to specialists. There is emerging evidence that US is useful in identifying and monitoring patients with inflammatory arthritis, often altering diagnosis and management.14 An additional benefit of scanning in real time at the bedside is that it allows for dynamic assessments of muscles and tendons that is not currently possible with computed tomography (CT) or magnetic resonance imaging.
At some institutions, US of the median nerve is used preferentially as a noninvasive first-line alternative to potentially painful electrophysiology studies in the diagnosis of carpal tunnel syndrome. For patients who are not responsive to splinting and conservative therapy, US can be used to safely guide corticosteroid injections around the median nerve.
In superficial soft-tissue infections, US not only can help distinguish abscess from cellulitis, but also can define its depth/extent, its relation to nearby vascular structures and nerves, as well as distinguish associated foreign bodies. One ED study found US to be more sensitive for abscess identification than CT.15 As one of the more readily acquired skills in point-of-care US, a clinician can quickly learn how to spare patients without focal fluid collections unnecessary incision and drainage procedures and avoid potentially serious diagnostic errors (eg, mistaking a hematoma or pseudoaneurysm for an abscess).16
To distinguish abscess from cellulitis, use a high-frequency probe with ample gel, and slide the probe over the entire area of interest (Figure 3). A focal fluid collection will appear as a dark (hypoechoic) region, often with irregular borders, with no flow on Doppler imaging. In cases of cellulitis, there will be no discrete fluid collection, and the skin and subcutaneous tissue will be edematous, often with a “cobblestone” appearance. It sometimes is helpful to scan a normal area prior to scanning the region in question. For very small or difficult to scan regions such as the hands and feet, using a water bath instead of gel often can bring objects into better focus and result in higher detail.
Although since the 1970s US has been shown to be superior to chest radiography in the detection and characterization of pleural effusion,17 due to significant artifact caused by the interaction of ultrasound waves and air in the lung itself, it was long believed that US of the lung parenchyma was not clinically helpful. This conception was disproven in the 1990s by French intensivists who linked important patterns of lung ultrasonography (LUS) artifact with corresponding findings on CT.18 The ability to correctly interpret these artifacts allows the clinician to diagnose a number of conditions (eg, pleural effusion, pneumothorax, pulmonary edema, pneumonia) at the bedside markedly better than physical examination alone, and in many cases, equal to or better than chest radiography.19
For the primary care physician, a particularly useful and relatively easy to learn LUS application is the ability to distinguish between patients with chronic obstructive pulmonary disease (COPD) exacerbation and patients with heart failure (HF) exacerbation, and to monitor pulmonary congestion in HF patients. In patients with COPD, the lung remains aerated, and a horizontal reverberation artifact pattern (A-lines) is seen throughout both lungs bilaterally. In patients with HF, increased extravascular lung water causes a vertical reverberation artifact pattern (B-lines) (Figure 4). Cardiogenic B-lines are seen in both lungs in a gravity-dependent manner, with a greater number of B-lines at the bases compared with the apices.
A recent large study of ED patients20 showed that a LUS-implemented approach had a significantly higher accuracy (sensitivity 97%, specificity 97.4%) in differentiating acute decompensated HF from noncardiac causes of acute dyspnea than the initial clinical workup, chest radiography, and natriuretic peptide tests. Because the number of B-lines present correlates with the degree of pulmonary congestion,21 it is possible that LUS in the primary care setting could help identify patients earlier in the disease process and guide interventions targeted at decreasing decompensation and hospital admissions.
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To distinguish acute decompensated HF from COPD exacerbation, start with a low-frequency probe in the sagittal plane at the anterior second intercostal space (Figure 4). In the near field, identify the pleural line (a shimmering, hyperechoic/white, horizontal line just beneath the ribs that moves with respiration). Evaluate for bright vertical lines extending down to the bottom of the screen that move like searchlights or lasers (B-lines). Check the upper and lower anterior lung fields, as well as the upper and lower lateral lung fields, on either side. In the absence of B-lines, multiple horizontal lines equidistant from the pleural line are often seen (A-lines), which argue against decompensated HF and suggest an alternative cause of dyspnea, such as COPD or asthma.
Focused Cardiac US
Several well-established protocols exist for cardiac point-of-care US, which is also known as focused cardiac ultrasonography (FCU). One of the best-studied in the internal medicine setting is the cardiovascular limited ultrasound examination (CLUE), which combines cardiac and pulmonary views to assess for left ventricular dysfunction, left atrial enlargement, pulmonary edema, pleural effusions, right ventricular dilation, and IVC dilation. Each of the views or “signs” have demonstrated diagnostic accuracy and prognostic importance,22 in addition to the demonstration that this protocol can be mastered by more than 80% of internal medicine residents over the 3-year curriculum.23
When FCU and abdominal US were used by Norwegian residents, on medical admission more than one-third of patients had their primary diagnosis changed or verified, or had another diagnosis added.24 The majority of the primary diagnosis changes (77%) were due to cardiac findings, and the authors posited that this resulted in expedited appropriate workup and treatment. Hand-carried FCU also has been shown to increase the accuracy of hospitalists’ assessment of left ventricular function, cardiomegaly, and pericardial effusion.25
Even medical students can be trained in FCU: In a study of patients with confirmed findings on formal echocardiography,26 medical students without clinical experience were found to have better diagnostic accuracy in detecting valvular disease, left ventricular dysfunction, chamber enlargement, and hypertrophy than were experienced cardiologists performing physical examinations.
Another component of FCU is central venous volume as detected by IVC size and respiratory collapse.27 Although there are caveats outside the scope of this review, a dilated IVC over 2 cm without respiratory collapse can help distinguish acute HF from COPD in a dyspneic patient.
While the majority of FCU studies have been conducted in acute care settings, it certainly has value in the outpatient setting, as well. The signs and symptoms of congestive heart failure are notoriously insensitive, and in a study of more than 500 outpatients in Portugal with confirmed cardiac dysfunction,28 the classic symptoms of orthopnea, paroxysmal nocturnal dyspnea, and dyspnea on flat walking were less than 36% sensitive, while a ventricular gallop, a heart rate over 110 beats/min, crackles on auscultation, and jugular venous pressure greater than 6 cm with hepatic enlargement and edema had a sensitivity of less than 10%. However, in another study,29 FCU performed by a general internist in a underserved minority health care clinic detected significant valvular or ventricular dysfunction in 12% of the 196 screened patients.
First, ensure you have the “cardiac” examination type selected. This will optimize frame rates for cardiac motion. For the parasternal long axis view, place the probe in the fourth intercostal space on the left lateral border of the sternum, with the probe marker pointed toward the patient’s right shoulder (Figure 5A). The resultant images are shown in Figures 5B and 5C. For the subcostal view, place the probe in the patient’s epigastric area, with the probe marker toward the patient’s left and the beam angled cephalad, with firm pressure to get under the patient’s ribs (Figure 5D). A 4-chamber view is obtained (Figure 5E), which is ideal for identifying pericardial effusions (Figure 5F). To visualize the IVC, rotate the probe 90° so that the marker is pointed cephalad (Figure 5G), then tilt the probe so that the ultrasound beam is pointed posteriorly (Figure 5H).
Detection of free intraperitoneal fluid was one of the first studied uses of point-of-care US via the focused assessment with sonography for trauma (FAST), which consists of 4 dependent abdominal views and is capable of detecting as little as 225 mL of free fluid, with a sensitivity of 97% for 1 L of fluid.30 While the FAST examination is considered the standard of care in trauma by the American College of Emergency Physicians and the American College of Surgeons,31 it also has use in less-acute settings. The same views can assist in ascites assessment in end-stage liver disease, assessment for fluid versus bowel gas in a patient for weight gain with increasing abdominal girth and bloating, and free fluid in the pelvis. Additionally, bladder volume measurement based on bladder dimensions has been shown to correlate well with volume from catheterization.32 Postvoid residual tests can be performed in the office using this technique without a formal bladder scanner to assist in the diagnosis of urinary retention.
While US has a low sensitivity for detecting ureteral calculi themselves, hydronephrosis is present with obstructing stones. In a multicenter comparative effectiveness trial,33 patients randomly assigned to undergo point-of-care US had lower radiation exposures than those assigned to CT, without significant differences in complications or the rate of serious adverse events. Patients presenting with flank pain can be assessed easily in the office. Additionally, the authors felt that US is an appropriate way to rule out hydronephrosis as a source of acute kidney injury; however, future studies are needed in this area.
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Place a low-frequency probe at posterior axillary line at the level of the 11th to 12th ribs, with the probe marker facing cephalad (Figure 6A), and identify the hepatorenal space (Figure 6B). Fluid can be identified as an anechoic (black) stripe (Figure 6C). Place the probe at the left posterior axillary line at the level of the 10th to 11th ribs, with the probe marker facing cephalad (Figure 6D), and identify the spleen and perisplenic spaces (Figure 6E). Moving inferiorly in either the right or the left quadrants will demonstrate the kidney. Place the probe just superior to the symphysis pubis (Figure 6F) in the transverse and sagittal planes and visualize the cul-de-sac and/or take bladder measurements (Figure 6G). See the focused cardiac US section regarding the subxiphoid view.
The presence of thyroid enlargement, nodules, and other abnormalities on physical examination have implications on patient management. For instance, with signs of hyperthyroidism, detection of goiter increases the likelihood of thyrotoxicosis being present.34 However, when compared with postmortem thyroid weight and US-determined thyroid volume, palpation has been shown to be highly inaccurate for estimating thyroid size.35 Additionally, thyroid US will demonstrate hypoechogenicity consistent with subacute thyroiditis,36 which could minimize additional testing in a patient presenting with neck or throat pain and/or fever. The American Thyroid Association recommends formal thyroid US for all patients with known or suspected nodules, while at the same time acknowledging that high-resolution point-of-care thyroid US is able to detect nodules in 19% to 67% of individuals.38
In surveys of medical students, interns, residents, and attending internists, detecting a thyroid nodule had one of the lowest self-confidence scores of 15 key physical examination skills, along with one of the highest perceived utility scores.39 No studied thyroid point-of-care US protocols exist; however, the authors propose that point-of-care US is a logical first follow-up tool for patients with a suspected thyroid nodule, sending them on for formal thyroid US only if a nodule is detected at the bedside or if otherwise clinically indicated.
A high-frequency linear probe should be used to examine the thyroid in most patients (Figures 7A-7C). Obese patients may require a lower-frequency probe. Images will be obtained in both the transverse and sagittal planes, scanning across both lobes and the isthmus (Figures 7D and 7E). The normal thyroid is uniformly echoic (gray) without nodules, stranding, or calcifications. The normal thyroid can have a range of appearances, and differences from a normal appearance may not be indicative of significant pathology (Figure 7F).
The Politics of US
The politics of point-of-care US are time challenging, to say the least, and in the past they have been simply distasteful. However, primary care physicians can benefit from the years of fighting and open hostility that had existed between specialists in radiology and cardiology and the earlier adopters such as emergency physicians, surgeons, and intensivists. Fortunately, for the most part, the dark days are far behind us, and there are simply too many clinicians using US who understand its value for their patients, and far too much research supporting the utilization and effectiveness of US.
Still, when starting to use US, it is best to assess potential pitfalls, must do’s, and must nots. In out-of-hospital settings, no one in the group might oppose the use of US. However, in a multispecialty setting or on hospital wards, it is best to investigate who may be an ally and who may be in opposition before undertaking US.
The future of primary care US is wide open. Considering even just the applications discussed here, the daily practice of every primary care physician could be radically altered. In fact, if we look at colleagues overseas and across specialties, there appears to be no end to the applications for which clinicians may use US. The key is what application is useful in a particular practice setting.
Point-of-care US benefits more than the primary care physician whose patient has a swollen tender lower leg, or has a prominent abdominal atrial pulse, or requires a tendon injection. In fact, even the basic physical examination is being altered by point-of-care US. No longer will we wonder whether there was an extra heart sound, murmur, or a sign of left ventricular hypertrophy on the office electrocardiogram. Why not just look and see in real time with US?
- Lindholt JS, Vammen S, Juul S, Henneberg EW, Fasting H. The validity of ultrasonographic scanning as screening method for abdominal aortic aneurysm. Eur J Vasc Endovasc Surg. 1999;17(6):472-475.
- Cosford PA, Leng GC. Screening for abdominal aortic aneurysm. Cochrane Database Syst Rev. 2007;(2):CD002945.
- Glover MJ, Kim LG, Sweeting MJ, Thompson SG, Buxton MJ. Cost-effectiveness of the National Health Service Abdominal Aortic Aneurysm Screening Programme in England. Br J Surg. 2014;101(8):976-982.
- LeFevre ML; U.S. Preventive Services Task Force. Screening for abdominal aortic aneurysm: U.S. Preventive Services Task Force recommendation statement. Ann Intern Med. 2014;161(4):281-290.
- Olchanski N, Winn A, Cohen JT, Neumann PJ. Abdominal aortic aneurysm screening: how many life years lost from underuse of the Medicare screening benefit? J Gen Intern Med. 2014;29(8):1155-1161.
- Rubano E, Mehta N, Caputo W, Paladino L, Sinert R. Systematic review: emergency department bedside ultrasonography for diagnosing suspected abdominal aortic aneurysm. Acad Emerg Med. 2013;20(2):128-138.
- Blois B. Office-based ultrasound screening for abdominal aortic aneurysm. Can Fam Physician. 2012;58(3):e172-e178.
- Bailey RP, Ault M, Greengold NL, Rosendahl T, Cossman D. Ultrasonography performed by primary care residents for abdominal aortic aneurysm screening. J Gen Intern Med. 2001;16(12):845-849.
- Bernardi E, Camporese G, Büller HR, et al; Erasmus Study Group. Serial 2-point ultrasonography plus D-dimer vs whole-leg color-coded Doppler ultrasonography for diagnosing suspected symptomatic deep vein thrombosis: a randomized controlled trial. JAMA. 2008;
- Burnside PR, Brown MD, Kline JA. Systematic review of emergency physician-performed ultrasonography for lower-extremity deep vein thrombosis. Acad Emerg Med. 2008;15(6):493-498.
- Crisp JG, Lovato LM, Jang TB. Compression ultrasonography of the lower extremity with portable vascular ultrasonography can accurately detect deep venous thrombosis in the emergency department. Ann Emerg Med. 2010;56(6):601-610.
- American College of Emergency Physicians. Emergency ultrasound guidelines. Ann Emerg Med. 2009;53(4):550-570.
- Klauser AS, Tagliafico A, Allen GM, et al. Clinical indications for musculoskeletal ultrasound: a Delphi-based consensus paper of the European Society of Musculoskeletal Radiology. Eur Radiol. 2012;22(5):1140-1148.
- Karim Z, Wakefield RJ, Conaghan PG, et al. The impact of ultrasonography on diagnosis and management of patients with musculoskeletal conditions. Arthritis Rheum. 2001;44(12):2932-2933.
- Gaspari R, Dayno M, Briones J, Blehar D. Comparison of computerized tomography and ultrasound for diagnosing soft tissue abscesses. Crit Ultrasound J. 2012;4(1):5.
- Blaivas M, Adhikari S. Unexpected findings on point-of-care superficial ultrasound imaging before incision and drainage. J Ultrasound Med. 2011;30(10):1425-1430.
- Gryminski J, Krakówka P, Lypacewicz G. The diagnosis of pleural effusion by ultrasonic and radiologic techniques. Chest. 1976;70(1):33-37.
- Lichtenstein D, Mézière G, Biderman P, Gepner A, Barré O. The comet-tail artifact: an ultrasound sign of alveolar-interstitial syndrome. Am J Respir Crit Care Med. 1997;156(5):1640-1646.
- Volpicelli G, Elbarbary M, Blaivas M, et al; International Liaison Committee on Lung Ultrasound (ILC-LUS) for International Consensus Conference on Lung Ultrasound (ICC-LUS). International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med. 2012;38(4):577-591.
- Pivetta E, Goffi A, Lupia E, et al; SIMEU Group for Lung Ultrasound in the Emergency Department in Piedmont. Lung ultrasound-implemented diagnosis of acute decompensated heart failure in the ED: a SIMEU multicenter study. Chest. 2015;148(1):202-210.
- Jambrik Z, Monti S, Coppola V, et al. Usefulness of ultrasound lung comets as a nonradiologic sign of extravascular lung water. Am J Cardiol. 2004;93(10):1265-1270.
- Kimura BJ, Yogo N, O’Connell CW, Phan JN, Showalter BK, Wolfson T. Cardiopulmonary limited ultrasound examination for “quick-look” bedside application. Am J Cardiol. 2011;108(4):586-590.
- Kimura BJ, Amundson SA, Phan JN, Agan DL, Shaw DJ. Observations during development of an internal medicine residency training program in cardiovascular limited ultrasound examination. J Hosp Med. 2012;7(7):537-542.
- Andersen GN, Graven T, Skjetne K, et al. Diagnostic influence of routine point-of-care pocket-size ultrasound examinations performed by medical residents. J Ultrasound Med. 2015;34(4):627-636.
- Martin LD, Howell EE, Ziegelstein RC, et al. Hand-carried ultrasound performed by hospitalists: does it improve the cardiac physical examination? Am J Med. 2009;122(1):35-41.
- Kobal SL, Trento L, Baharami S, et al. Comparison of effectiveness of hand-carried ultrasound to bedside cardiovascular physical examination. Am J Cardiol. 2005;96(7):1002-1006.
- Beigel R, Cercek B, Luo H, Siegel RJ. Noninvasive evaluation of right atrial pressure. J Am Soc Echocardiogr. 2013;26(9):1033-1042.
- Fonseca C, Morais H, Mota T, et al; EPICA Investigators. The diagnosis of heart failure in primary care: value of symptoms and signs. Eur J Heart Fail. 2004;6(6):795-800, 821-822.
- Kirkpatrick JN, Davis A, Decara JM, et al. Hand-carried cardiac ultrasound as a tool to screen for important cardiovascular disease in an underserved minority health care clinic.J Am Soc Echocardiogr. 2004;17(5):399-403.
- Branney SW, Wolfe RE, Moore EE, et al. Quantitative sensitivity of ultrasound in detecting free intraperitoneal fluid. J Trauma. 1995;39(2):375-380.
- O’Brien KM, Stolz LA, Amini R, Gross A, Stolz U, Adhikari S. Focused assessment with sonography for trauma examination: reexamining the importance of the left upper quadrant view. J Ultrasound Med. 2015;34(8):1429-1434.
- Chan H. Noninvasive bladder volume measurement. J Neurosci Nurs. 1993;25(5):309-312.
- Smith-Bindman R, Aubin C, Bailitz J, et al. Ultrasonography versus computed tomography for suspected nephrolithiasis. N Engl J Med. 2014;371(12):1100-1110.
- Gurney C, Hall R, Harper M, Owen SG, Roth M, Smart GA. Newcastle thyrotoxicosis index. Lancet. 1970;2(7686):1275-1278.
- Langer P. Normal thyroid size versus goiter--postmortem thyroid weight and ultrasonographic volumetry versus physical examination. Endocrinol Exp. 1989;23(2):67-76.
- Park SY, Kim E-K, Kim MJ, et al. Ultrasonographic characteristics of subacute granulomatous thyroiditis. Korean J Radiol. 2006;7(4):229-234.
- Cooper DS, Doherty GM, Haugen BR, et al. Revised American Thyroid Association management guidelines for patients with thyroid nodules and differentiated thyroid cancer. Thyroid. 2009;19(11):1167-1214.
- Wu EH, Fagan MJ, Reinert SE, Diaz JA. Self-confidence in and perceived utility of the physical examination: a comparison of medical students, residents, and faculty internists.J Gen Intern Med. 2007;22(12):1725-1730.