Key words abdomen - ultrasound - conventional radiography - CT
Introduction
Free intra-abdominal air, referred as pneumoperitoneum (PP), represents a critical radiological appearance in severely ill patients. In patients presenting with an acute onset of abdominal pain without any major abdominal surgery immediately before the onset, PP as an indirect sign of gastrointestinal perforation has to be excluded. The most sensitive radiological method for the diagnosis of small amounts of PP is usually computed tomography [1 ]. Still posterior-anterior chest and erect abdominal radiographs are commonly used alternatives for the daily clinical workflow resulting in fast imaging results and therefore fast diagnosis in these critically ill patients [2 ]
[3 ]. Even small amounts of as little as 1 ml of free intra-abdominal air space can be recognized with these modalities [2 ]
[4 ]. However, due to the patient’s condition, upright radiographs cannot always be obtained. In these cases, left-lateral decubitus abdominal radiography seems to be the best alternative with comparable findings [5 ]
[6 ].
Nevertheless, ultrasound is the first chosen imaging modality in patients with acute abdominal pain in most emergency departments because it represents a reliable, inexpensive and therefore economical examination which can easily be performed in the examination room without the need to transport the patient [7 ]
[8 ]
[9 ]
[10 ]
[11 ]
[12 ]
[13 ]. Additionally, ultrasound examination can frequently provide an immediate diagnosis for several queries such as intra-abdominal fluid collection as an indicator of hemorrhage or peritonitis. Furthermore, the whole abdomen can be probed during one examination without radiation exposure [1 ]
[8 ]
[9 ]
[14 ]
[15 ]
[16 ]. Ultrasound is also the first chosen imaging modality in traumatology for the detection of trauma-associated lesions, known as focused assessment with sonography for trauma (FAST) [17 ].
Until nowadays, sonography has not been commonly used for the detection of free intra-abdominal air although the possibility of the detection of PP has been suggested in the literature [1 ]
[5 ]
[18 ]
[19 ]
[20 ].
In the last few years, further development generated miniaturized ultrasound systems, leading to the introduction of hand-carried ultrasound (HCU) imagers. These HCU devices were initially developed for the military or air medical environment but have made their way into the daily clinical workflow [21 ]
[22 ]
[23 ]. The major advantages of these devices are fast bedside availability and excellent transportability enabling prompt diagnosis and immediate therapeutic measures combined with low costs [24 ].
Additionally, the image quality of high-end ultrasound equipment has dramatically increased over the last few years resulting in better and faster high-resolution imaging of the whole abdomen.
Therefore, the aim of the study was the evaluation of the diagnostic yield of a new generation hand-carried ultrasound (HCU) imager and a high-end ultrasound (HUS) system in comparison to erect or left-lateral decubitus abdominal radiographs (AR) as well as to multidetector computed tomography (MDCT) as the standard of reference in patients with acute abdominal pain for the presence or absence of PP.
Materials and Methods
Design and Setting
We prospectively included all consecutive patients who received an erect or left-lateral decubitus abdomen radiograph and a multidetector computed tomography because of acute abdominal pain in a period over one month in our department and for whom two experienced examiners performed the ultrasound examinations. To guarantee the presence of both examiners, only patients examined within the core service hours of the department of radiology, i. e. 8 am to 5 pm on working days, were included in the study. All patients meeting these criteria were consecutively enrolled in this study irrespective of their underlying disease. The study was performed in a tertiary care university medical center. Study participant operators were neither aware of the indication for AR or MDCT nor of the underlying disease. During this period we were able to include a total of 31 patients. Their basic demographic data are reported in [Table 1 ].
Table 1
Basic demographic characteristics of enrolled patients
number (No.)
percentage (%)
patients
31
100
gender
male
female
18
13
58
42
median age and age range (years)
57 (23 – 83)
median weight and weight range (kilograms)
83 (56 – 108)
underlying disease
inflammatory bowel disease
cholecystitis
diverticulitis
appendicitis
percutaneous endoscopic gastrostomy
mesenteric ischemia
renal colic
gastric perforation
myocardial infarction
8
6
4
4
3
2
2
1
1
26
19
13
13
9
7
7
3
3
Abdomen radiograph and multidetector computed tomography
The indication for erect abdomen radiograph or left-lateral decubitus abdomen radiograph was acute abdominal pain in all 31 patients combined with the clinical suspicion of ileus in 13 cases and the clinical suspicion of hollow organ perforation in another 18 cases. All AR were acquired on a flat panel detector (Axiom Aristos Multix FDX, Siemens Healthcare AG, Erlangen, Germany). In these 31 patients an MDCT scan was acquired as well (128-slice CT, SOMATOM Definition Flash, Siemens Healthcare AG, Erlangen, Germany) for further diagnosis. The presence or absence of PP in AR and MDCT was diagnosed by an experienced board-certified radiologist with a specialization in abdominal imaging, who was blinded to the ultrasonographic and clinical findings.
Abdominal ultrasound with hand-carried ultrasound device
We employed a new generation state-of-the-art HCU device (VScan, GE Healthcare, Waukesha. WI, USA) with a plane 1.7 – 3.8 MHz transducer for two-dimensional imaging, a 3.5-inch screen and a total weight of 390 g. The independent and experienced operator was a radiologist who had performed more than 1000 documented ultrasound examinations within 12 months under the supervision of an experienced ultrasound examiner who has conducted more than 5000 ultrasound examinations each year for more than 10 years. The operator was unaware of the findings of the clinical examination, HUS or AR. The examination with the hand-carried ultrasound imager was performed instantaneously before HUS yet before the acquisition of AR and MDCT.
Abdominal ultrasound with high-end ultrasound device
Furthermore, abdominal ultrasound examination was performed with a high-end ultrasound system (LOGIQ E9, GE Healthcare, Waukesha. WI, USA) with a curved-array 1 – 5 MHz transducer. The independent and experienced operator was a radiologist who has conducted more than 5000 ultrasound examinations each year for more than 10 years. The operator was unaware of the findings of the clinical examination, HCU or AR. The examination with the high-end ultrasound system was performed within a maximum of five minutes after HCU yet before the acquisition of AR and MDCT.
Ultrasound technique
To standardize and simplify the ultrasound examination, all patients were examined within a maximum of ten minutes before the acquisition of AR and MDCT. The mean scanning time was 1.5 minutes (range from 1 to 3 minutes) with HCU and HUS. The ultrasound examination was not intended to examine the complete abdomen, but was focused on detecting or excluding PP. The examination for free intra-abdominal air was carried out with patients in a supine and left-lateral position. The transducer was set parasagittally onto the right paramedian epigastric area in a supine position. Afterwards the transducer was set in the right mid-axillary line, rib-parallel in between the 8th and 10th intercostal space. These scans allowed the detection of PP by the recognition of strong reverberations as well as the shifting phenomenon and the enhancement of the peritoneal stripe [19 ]
[20 ]
[25 ]. Diagnosis of PP was therefore made if a highly echoic area with a highly echoic tail and a mirror image in the ventral space of the liver could be detected [1 ]. Free intra-abdominal air must be differentiated from air in the lung, air in the gastrointestinal tract and subcutaneous emphysema [1 ].
Quality of ultrasound images
A simple scoring system concerning the quality and adaptability of the ultrasound images for the diagnosis of PP was employed for the ultrasound study. For the HCU and HUS device the best achievable image quality was scored from 0 (lowest quality) to 5 (best quality) by each operator. Scores of 0 and 1 were not suitable for a diagnostic decision of PP, whereas scores of 2, 3, 4 and 5 were considered as suitable. The different levels of this simple scoring system are outlined in [Table 2 ]. A decisive criterion is the glance at Morison’s Pouch and the view of the costodiaphragmatic recess.
Table 2
Scoring system concerning the quality and adaptability of the ultrasound images for the diagnosis of PP achieved with HCU and HUS. Decisive criterion is the glance at Morison’s Pouch and the view of the costodiaphragmatic recess.
5
visible in an optimal way
4
visible to the greatest possible extent
3
visible to some extent
2
visible to small extent, still utilizable for diagnosis
1
incompletely visible, not utilizable for diagnosis
0
not visible, not utilizable for diagnosis
Statistical analysis
Data was acquired using Excel tables (Excel 2007, Microsoft, Redmond. WA, USA). Statistical analysis was performed using PASW (PASW V.18, IBM SPSS Inc., Armonk, NewYork, USA). Agreement between the findings of HCU and HUS with the diagnosis made by AR and MDCT as the standard of reference was assessed from 2 × 2 tables.
Results
Imaging quality
Both HCU and HUS were technically feasible in all 31 patients. As mentioned before, the mean scanning time was 1.5 minutes (range from 1 to 3 minutes) with HCU and HUS.
In 29 of 31 cases (94 %) the image quality of the HUS device was rated 5, in one case the score was 4 and in one another case the score was only 3.
The image quality of the HCU imager was rated 5 in 22 of 31 patients (71 %) and 4 in another 5 cases (16 %), whereas a score of 3 was given in the 4 remaining patients (13 %).
In summary all ultrasound images acquired using the HCU and HUS devices were suitable for diagnosis but the image quality was slightly lower for HCU compared to HUS.
Diagnostic value
The findings of AR and MDCT were congruent in all patients and PP was depicted in 5 of 31 patients, resulting in a prevalence of 16 %. Examination with HCU revealed PP correctly in 4 of 5 patients, but was false positive in 5 more patients. The absence of PP was correctly verified by HCU in 21 of 26 cases. Therefore, a sensitivity and specificity of 80 % and 81 % was calculated, respectively. In the one remaining patient HCU was false negative, the amount of free intra-abdominal air identified by AR and MDCT was minimal. The positive predictive value (PPV) and negative (NPV) predictive value for the diagnosis of PP with HCU were 44 % and 95 %, respectively.
Examination with HUS revealed PP correctly in 4 of 5 patients as well, and was false positive in 3 more patients. The absence of PP was correctly verified by HUS in 23 of 26 cases, but was also false negative in one more patient with a small amount of free intra-abdominal air identified by AR and MDCT. Therefore, a sensitivity and a specificity of 80 % and 89 % were calculated, respectively. The PPV and NPV for the diagnosis of PP with HUS were 57 % and 96 %, respectively.
The results of the diagnosis of PP by means of HCU and HUS are summarized in [Table 3 ].
Table 3
Sensitivity, specificity, positive (PPV) and negative predictive value (NPV) for the diagnosis of PP using HCU and HUS with MDCT and AR as the standard of reference
HCU
diagnosis of PP
sensitivity
80 %
specificity
81 %
PPV
44 %
NPV
95 %
HUS
diagnosis of PP
sensitivity
80 %
specificity
89 %
PPV
57 %
NPV
96 %
The images presented in [Fig. 1 ] show congruent findings of HCU, HUS, AR and MDCT in the diagnosis of PP, whereas [Fig. 2 ] shows the finding of a Chilaiditi Syndrome in AR and MDCT which was mistaken for PP in HCU and HUS.
Fig. 1 For a 61-year-old male patient with acute abdominal pain, an erect abdomen radiograph was ordered by the ward physician under the suspicion of PP. Free intraabdominal air space was correctly diagnosed by HCU and HUS – a perforated gastric ulcer was diagnosed with MDCT and confirmed intraoperatively. a HCU B-mode image, showing free intraabdominal air space (marked with white arrow) b HUS B-mode image, showing free intraabdominal air space (marked with black arrow) c AR image, showing free intraabdominal air space (marked with black arrow) d MDCT scan, showing free intraabdominal air space (marked with white arrow)Abb. 1 Bei einem 61-jährigen Mann wurde unter der Verdachtsdiagnose eines Pneumoperitoneum bei abdominellen Schmerzen eine konventionelle Abdomenübersichtsaufnahme angefertigt. Es wurde freie intraabdominelle Luft mittels HCU und HUS korrekt festgestellt. In der MDCT wurde schließlich ein perforiertes Magenulcus diagnostiziert, was sich intraoperativ bestätigte. a HCU B-Bild-Sonografie, freie intraabdominelle Luft markiert mittels weißem Pfeil b HUS B-Bild-Sonografie, freie intraabdominelle Luft markiert mittels schwarzem Pfeil c AR Bild, freie intraabdominelle Luft markiert mittels schwarzem Pfeil d MDCT-Scan, freie intraabdominelle Luft markiert mittels weißem Pfeil
Fig. 2 For a 64-year-old male patient with acute abdominal pain, a left-lateral decubitus abdomen radiograph was ordered under the suspicion of ileus by the ward physician. A Chilaiditi Syndrome was correctly diagnosed by AR and MDCT, while HCU and HUS were false positive for the diagnosis of PP. a HCU B-mode image, the Chilaiditi Syndrome was misinterpreted as free intraabdominal air space (marked with white arrow) b HUS B-mode image, the Chilaiditi Syndrome was misinterpreted as free intraabdominal air space (marked with black arrow) c AR image, the Chilaiditi Syndrome can easily be diagnosed (marked with black arrow) d MDCT scan, the Chilaiditi Syndrome can easily be diagnosed (marked with black arrow).Abb. 2 Bei einem 64-jährigen Mann mit abdominellen Schmerzen wurde eine konventionelle Abdomenübersichtsaufnahme in Linksseitenlage zur Beurteilung eines Ileus angefertigt. Mittels AR und MDCT wurde ein Chilaiditi-Syndrom erkannt, während mittels HCU und HUS fälschlicherweise freie intraabdmominelle Luft diagnostiziert wurde. a HCU-B-Bild-Sonografie, als freie intraabdominelle Luft fehlinterpretiertes Chilaiditi-Syndrom, markiert mittels weißem Pfeil b HUS-B-Bild-Sonografie, als freie intraabdominelle Luft fehlinterpretiertes Chilaiditi-Syndrom, markiert mittels schwarzem Pfeil c AR Bild, das Chilaiditi Syndrom kann leicht erkannt werden, markiert mittels schwarzem Pfeil d MDCT-Scan, das Chilaiditi-Syndrom kann leicht erkannt werden, markiert mittels schwarzem Pfeil.
Discussion
Pneumoperitoneum is a common finding in hospitalized patients suffering from acute abdominal pain or in emergency departments. It may result from laparoscopy, percutaneous endoscopic gastrostomy, gastrointestinal perforation or as a consequence of abdominal trauma. MDCT as well as posterior-anterior chest and erect abdominal radiographs are widely used in the daily clinical workflow for fast diagnosis and detection of the underlying causes. Ultrasonography is increasingly used to demonstrate PP [2 ]
[3 ]
[20 ]
[26 ]
[27 ].
In the past some authors pointed out that ultrasonography is as effective as radiography for the diagnosis of pneumoperitoneum [18 ]
[19 ]
[20 ]
[25 ]. Chang-Chien et al. reported the results of a comparison between ultrasonography and chest or abdomen radiography in patients with perforated peptic ulcers and found that ultrasonography could detect pneumoperitoneum in nine of ten patients while abdomen radiography was only able to reveal PP correctly in eight of ten cases [19 ]. Karahan et al. found a sensitivity of 94 % and a specificity of 100 % for the detection of PP in 72 patients with suspected gastrointestinal perforation [20 ].
Our own results indicate that the quality of the images acquired with the HCU imager and the HUS device was suitable for making the diagnosis of pneumoperitoneum, while the image quality was slightly lower for HCU compared to HUS. However, for the question of free intra-abdominal air space image quality is not the decisive criterion resulting in comparable results between HCU and HUS. Therefore, a sensitivity and a specificity of 80 % and 81 %, respectively, for the examination with HCU and a sensitivity and a specificity of 80 % and 89 %, respectively, for the examination with HUS with an erect or left-lateral decubitus abdomen radiography as the standard of reference were calculated. Consequently, our results are inferior compared to the literature [18 ]
[19 ]
[20 ]
[25 ]. HCU and HUS missed the correct diagnosis of PP in one case each and were false positive in 5 and 3 cases, respectively. Especially the high rate of false-positive cases is problematic as further investigations depend on the correct diagnosis of pneumoperitoneum and an unnecessary laparotomy could even be performed in false-positive cases.
Ultrasonography therefore seems to only be an ideal additional tool in immediate fast screening and as an initial examination because of its low costs and its ease of use for experienced operators. However, HCU and HUS can’t compete with AR or MDCT for the diagnosis of PP, especially because of the high rate of false-positive results in the diagnosis of free intra-abdominal air.
There are some limitations of the present study. Only 31 patients were enrolled and only 5 cases of pneumoperitoneum were found. Furthermore, two different operators were working on either the hand-carried or the high-end ultrasound system and ultrasound examination is naturally an operator-dependent modality. Nevertheless the results of HCU and HUS were comparable.
As pointed out before, ultrasonography is usually the first chosen imaging modality in patients with acute abdominal pain in emergency departments [1 ]
[7 ]
[8 ]
[9 ]
[10 ]
[14 ]
[16 ]
[17 ]. It can initially be employed because of its fast bedside availability and excellent transportability enabling prompt diagnosis and immediate therapeutic measures combined with low costs and the abandonment of radiation but ultrasonography has not been common for the detection of free intra-abdominal air which is coincident with our own findings [20 ]
[28 ]
[29 ]
[30 ].
Conclusion
In conclusion, examination with HCU and HUS for the presence or absence of PP can be used initially as a screening method in patients with acute abdominal pain and it can lead to a rapid extension of the diagnostic possibilities. However, it seems that an abdominal ultrasound examination performed with an HCU device or an HUS imager is inferior compared to the findings of AR and MDCT. Nevertheless HCU and HUS can accelerate the triage of patients and help to make decisions regarding the necessity of further exams without the need for radiation and while reducing economic and logistic resources.
