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Portable Abdominal CT:

Analysis of Quality and Clinical Impact in More Than 100 Consecutive Cases

OBJECTIVE.

Our aim was to determine portable abdominal CT image quality and clini-cal content in a consecutive series of scans obtained in ICUs at a single center.

MATERIALS AND METHODS.

All helical portable abdominal CT scans obtained be-tween June 1999 and December 2000 were reviewed by two observers. Image quality was as-sessed for feature detection and overall quality compared with the patients’ contemporaneousstationary CT scans. Hospital records were used to determine patient demographics, scanningindications, and clinical impact and to verify portable CT findings when possible.

RESULTS

.

One hundred twenty-two helical portable CT scans (47 contrast-enhanced) and 41contemporaneous stationary CT scans in 107 patients were included. IV contrast material im-proved portable CT scan quality, but quality scores for portable CT scans were consistently lowerthan those for stationary CT scans, both with and without contrast material. Thirty-three conditionssuspected before scanning were supported by findings on portable CT scans, which detected evi-dence of infection in 18 and hemorrhage in 16 cases and motivated seven laparotomies and six per-cutaneous drainage procedures. Thirty-three portable CT scans (27%) contributed to a change inpatient treatment. Results of surgery or autopsy confirmed portable CT findings in 12 of 17 cases.

CONCLUSION.

Although image quality is inferior to conventional stationary CT, portableabdominal CT provides important diagnostic information without requiring patient transport out-side the ICU. Radiologists should avoid overconfident interpretation of portable CT scans.

echnical developments have addedhelical CT to the abdominal imag-ing techniques that can be per-

formed at the bedside. CT scanners usedworldwide, capable of portable operation, havebeen deployed in many large hospitals [1]. How-ever, little information has been published aboutthe performance of such scanners or how theyare being used. The purpose of this study was toevaluate a consecutive series of portable helicalabdominal CT scans at a single institution to as-sess image quality and clinical impact.

Materials and Methods

This study was approved by the Subcommitteeon Human Studies, the hospital institutional re-view board. Because it was a retrospective study,patient informed consent was not required.

Portable CT Hardware

In all cases, portable CT scans were obtainedon an Anatom 2000 (Analogic), marketed as theTomoscan M (Philips Medical Systems) (Fig. 1).Components of the Anatom 2000 system include a

lightweight gantry with built-in computer andpower supplies, patient table, and operating con-sole with display [1]. The gantry, driven by a DCmotor, can translate horizontally for a total dis-tance of approximately 356 mm, so the patient re-mains stationary during scanning [2]. For scanningof the abdomen, patients must be moved fromtheir beds, usually onto the patient table placed atthe bedside. A slide board resting between the footof the patient’s bed and a small table serves as thescanning table when bedside space is limited.

For 18 months before the study, 68 portable CT ex-aminations were performed using an axial CT protocolthat did not permit use of IV contrast enhancement.Subsequently, a software upgrade enabled portable ab-dominal CT examinations to be performed with a heli-cal protocol. In this mode, two volumes 285–355 mmlong were scanned in 5-mm contiguous sections, eachacquired during 70 sec at 120–130 kV, 30–40 mA, anda pitch of 1.0–1.5:1. Study indication and patient con-dition determined whether IV contrast material wasadministered during helical portable CT.

Case Material

Helical portable CT scans obtained after thesoftware upgrade in June 1999 through December

Michael M. Maher

1

Peter F. Hahn

1

Debra A. Gervais

1

Brid Seoighe

1,2

James B. Ravenscroft

1

Peter R. Mueller

1

Received December 19, 2003; accepted after revision March 16, 2004.

1

Department of Radiology, Division of Abdominal Imaging and Intervention, Massachusetts General Hospital, 55 Fruit St., White 270, Boston, MA 02114. Address correspondence to P. F. Hahn (phahn@partners.org).

2

Present address: Faculty of Health Sciences, University of Dublin, Trinity College, Dublin 2, Ireland.

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2000 were identified using the radiology informa-tion system (RIS) (IDXrad, IDX Systems). ForRIS purposes, the portable CT scanner was as-signed a specific unique examination location, en-suring a complete and consecutive listing of allportable CT scans. Those examinations with ab-dominal examination codes were included, exceptthat 19 portable abdominal CT scans during thestudy period were excluded because they were ob-tained with the older axial protocol. All CT scans(portable and stationary nonportable) obtained atour institution during the study period were ar-chived digitally and without lossy compression onthe PACS (Agfa). During evaluation of the porta-ble CT scans of each patient, the investigators de-termined from PACS whether a stationary(nonportable) CT scan had been obtained within14 days of any portable CT scans. If so, the tempo-rally closest stationary scan was included for com-parison, regardless of whether it matched theportable CT scan in contrast agent usage. Morethan one stationary CT scan was included for com-parison if the patient had more than one portable

CT scan. All portable and stationary abdominalCT scans evaluated included those of the pelvisand the abdomen.

Assessment of Image Quality

All helical portable abdominal CT scans ob-tained during the study period were retrieved fromPACS and displayed on a diagnostic-quality work-station. In addition, any conventional abdominalCT scan obtained within 2 weeks of the portablescan was also retrieved for the purpose of compar-ison with the portable CT scan.

Each abdominal CT scan retrieved was as-sessed in consensus by two radiologists with 6 and17 years’ experience in CT interpretation. Al-though both portable and conventional nonpor-table studies were included, no attempt was madeto blind the observers as to which type they werereviewing. Indeed, the acquisition of separate vol-umes to cover the entire abdomen and pelvisreadily identified the portable CT scans.

First, the presence of IV and oral contrast mate-rial was determined by inspection of the images.

The observers then assessed the quality of thescans. The quality assessment proceeded by deter-mining the visibility of nine selected anatomicstructures at different levels in the abdominal CTscan. Visualization of each of the selected anatomicstructures was assessed and graded as 1 point (seen)or 0 points (not seen). The anatomic structures inthe liver chosen as indicators of image quality in-cluded the main portal vein, both right and left por-tal veins, and secondary portal vein branches. Threepoints were awarded if all three sets of structureswere visible; otherwise, the point score was re-duced. An additional point was awarded for visual-ization of the lobular pattern of the pancreas (Fig.2). Renal visualization was assessed for identifica-tion of the renal collecting system at the hilum (1point) and the intrarenal collecting system (1 point).The quality of small-bowel visualization at the levelof the iliac crest was determined from the definitionof bowel against mesenteric fat (1 point), the bowellumen against the bowel wall (1 point), and the val-vular pattern (1 point).

The CT scan was then assessed for major tech-nical problems such as streak artifact, motion arti-fact, skip areas of coverage, or other major defectslimiting the diagnostic value of the study. The scanreceived an additional point if no major technicalproblems existed. An optimal scan would receive3 points for visualization of the portal vein, 1 pointfor visualization of the lobular pattern of the pan-creas, 2 points for visualization of the collectingsystem of the kidney, 3 points for small-bowelvisualization at the iliac crest, and 1 point for ab-sence of technical problems. Thus, the maximalscore that could be awarded was 10 points.

The overall scan quality was then assessed fol-lowing a modification of the qualitative scoringsystem of Stockberger et al. [3]. Scans weregraded from 1 to 5 (5, exceptional quality; 4, stan-dard quality; 3, less than standard quality; 2, diag-nosis in doubt because of poor scan quality; 1,inadequate quality for diagnosis). The same scor-ing system was used to assess both the portablescans and the contemporaneous stationary scans.

Assessment of Clinical Impact

All portable CT scans were reviewed on thePACS by the two radiologists, first without knowl-edge of the original interpretation and then incomparison with the radiology report recorded inthe RIS. Any discrepancies in interpretation be-tween the two reviews were recorded. Contempo-raneous stationary CT scans, whether obtainedbefore or after the same patient’s portable CTscans, were also evaluated by the observers.

Using the database of patients generated from theRIS, we retrospectively reviewed the computerizedhospital medical record for patient demographics,including age, sex, principal reason for hospital ad-mission, date of admission, date of performance ofportable CT, and indication for portable CT. Usingthis information, we calculated the interval betweenhospital admission and date of portable abdominalCT. In addition, patients were categorized into seven

Fig. 1.—Photograph of portable CT scanner (Tomoscan M, Philips Medical Systems).

Fig. 2.—Portable CT scan of 44-year-old man with fever shows normal lobulations of pancreas (P). Only 49% ofportable CT scans delineated this feature.

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groups on the basis of the principal medical problemthat led to hospital admission: neurologic disease,cardiovascular disease, multiorgan failure, congeni-tal disease, postoperative complications, trauma, andrespiratory failure.

Portable abdominal CT scans were evaluatedfor positive radiologic findings, particularly thosesuggesting infection, hemorrhage, or importantunsuspected diagnoses. In addition, portable CTfindings were correlated with the indication forCT, and a decision was made as to whether theportable CT findings confirmed the clinical indica-tion that precipitated the test.

The hospital record was then evaluated for out-come in the aftermath of portable CT. The reportsof all abdominal imaging examinations performedduring the subsequent hospital course were evalu-ated. The reports of any examinations performedwithin 2 weeks of the portable CT were comparedwith findings on portable CT. In addition, thequestion of whether the portable CT led to achange in patient treatment was addressed. Finally,the findings on portable CT were correlated withinformation from other events in the 7-day periodafter portable CT was performed, including ab-dominal surgery and autopsy.

Statistical Analysis

Characteristics of portable and nonportable CTscans were compared using the chi-square testwith continuity correction. Means of summedscores for feature visualization were compared us-ing the two-tailed unpaired Student’s

t

test. Pairedscores for portable CT scans and stationary CTscans in the same patient were compared using theWilcoxon’s signed rank test. The null hypothesiswas rejected if the difference would have arisen bychance in less than 5% of trials.

Results

Patient Characteristics

From June 22, 1999, to December 31, 2000,122 helical portable abdominal CT studieswere performed on 107 patients (64 males, 43females; age range, 1–86 years; mean age, 55years). Except for two patients less than 3 yearsold, all were 19 years old or older. Eleven pa-tients underwent two helical portable CT ex-aminations; two patients had three. In addition,four patients who underwent helical portableCT examinations each had one axial portableCT (of 19 axial portable CT examinations dur-ing this period) not included in the analysis.Portable CT examinations were performed at amean of 16 days (1–134 days) after hospitaladmission. All portable CT examinations wereperformed in medical, cardiac, pediatric, neu-rologic, or surgical ICUs. The patients’ prin-cipal medical problems were as follows:neurologic disease (

n

= 36), cardiovascular

disease (

n

= 20), multiorgan failure (

n

= 34),congenital disease (

n

= 2), postoperative com-plications (

n

= 5), trauma (

n

= 4), and respira-tory failure (

n

= 6).

Assessment of Portable CT Quality

IV contrast agent was not administered for31 portable CT examinations in which the in-dication for the study was exclusion of hemor-rhage. This decision followed departmentalprotocol for abdominal–pelvic CT performedfor exclusion of this condition. Contrast mate-rial was withheld in the remaining 44 patientsbecause of renal insufficiency or allergy to io-dinated contrast material.

One hundred twenty-two helical portableCT scans averaged 6.19 ± 1.64 (mean ± SD)of a possible 10 points for feature visualiza-tion and 1.49 ± 0.55 of a possible 5 points onthe modified qualitative scoring system ofStockberger et al. [3]. Contrast administra-tion improved portable CT scans for featurevisualization, which averaged 6.83 ± 1.49 for47 contrast-enhanced portable CT scans and5.79 ± 1.60 for 75 scans without contrast en-hancement (

p

< 0.001) and on the qualitativescale 1.64 ± 0.57 and 1.40 ± 0.52 (

p

< 0.02),respectively. When scores for portable CTscans were compared with 41 nearly contem-poraneous stationary CT scans in this cohort,the stationary CT scans were rated betterboth for feature visualization (7.22 ± 1.44,

p

<0.001) and on the qualitative scale (2.02 ±0.72,

p

< 0.0001). Inferior ratings of portableCT versus stationary CT persisted when onlythe 75 unenhanced portable CT scans were

compared with 26 unenhanced stationary CTscans (5.79 ± 1.60 vs 6.92 ± 1.55,

p

< 0.01and 1.40 ± 0.52 vs 1.92 ± 0.74,

p

< 0.001)and for comparison of 47 contrast-enhancedportable CT scans with 15 contrast-enhancedstationary CT scans (6.83 ± 1.49 vs 7.73 ±1.10,

p

< 0.05 and 1.64 ± 0.57 vs 2.20 ± 0.68,

p

< 0.01). Chi-square analysis showed nodifference in contrast agent use between por-table CT and stationary CT (

p

= 0.82).Table 1 shows, for each of nine features

scored, the number of portable and stationaryCT scans on which that feature could be iden-tified. An additional point was awarded toscans with no major problems. Stationary CToutscored portable CT in all 10 features. Infour of the 10 (left and right portal vein, branchportal vein, intrarenal collection system, andabsence of serious technical problems), thedifferences were statistically significant.Thirty-nine portable CT scans (32%) had a to-tal of 41 major problems including motion (

n

=23) or streak (

n

= 4) artifact, gaps in coverage(

n

= 13), and equilibrium phase contrast en-hancement (

n

= 1). In comparison, only three(7%) of the same patients’ 41 stationary CTscans had major problems (streak artifact [

n

=1], motion [

n

= 1], and inadequate field ofview [

n

= 1],

p

< 0.01). In 24 cases, portable and stationary CT

examinations of the same patient were per-formed within 2 weeks of each other, andboth were performed either with (10 cases)or without (14 cases) IV contrast material.Two patients were represented twice amongthese 24 cases. In these 24 cases, both

aSignificant difference (p < 0.01) using chi-square test.bSignificant difference (p < 0.05) using chi-square test.

TABLE 1 Visibility of Individual Features on Portable and Stationary CT Scans

FeaturePortable Scans (n = 122) Stationary Scans (n = 41)

No. % No. %

Portal veinMain 100 82 38 93Left and right 83a 68 39 95Secondary branches 63a 52 31 76

Pancreatic lobules 60 49 24 59Renal hila 113 93 41 100Collecting system 85b 70 35 85Bowel

Wall 110 90 39 95Lumen 35 29 13 32Folds 3 2 3 7

Major defects absent 83a 68 37 90

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summed scores for feature visualization andqualitative scores for image quality could becompared between pairs of portable and sta-tionary CT scans. The mean 10-point summedassessment scores for these patients were 6.2± 1.68 for portable CT and 7.2 ± 1.53 for sta-tionary CT. The mean 5-point qualitativeassessment scores were 1.40 ± 0.51 and 2.10± 0.78, respectively. For both 10- and 5-pointscales, the scores rated quality of stationaryCT better than that of portable CT in the samepatients. For both scales, the differences be-tween the two cohorts were significant by theWilcoxon’s signed rank test (

p

< 0.02 and0.01, respectively).

Portable CT Indications and Findings

In 116 cases, the indication for portableCT fell into a single indication category,whereas in six cases, the indications fell intotwo categories, for a total of 128 indicationsamong 122 scans. The two most commonlystated indications for portable CT were sus-pected infection (

n

= 79) and hemorrhage (

n

=31). Eighteen portable CT studies includedindications other than these. For 33 (26%) of128 indications, portable CT confirmed thesuspected diagnosis, whereas in 95 cases(74%), the imaging findings did not supportthe suspected clinical diagnosis promptingthe CT scan.

Air-space or pleural disease was present invirtually all patients, reducing the value ofclinical signs of infection. When abdominalinfection was the indication for portable CT(79 scans), potential sources were found on15 scans (19%). Categories of infection in-cluded infectious colitis (

n

= 4) (Fig. 3), focalintraabdominal collection (

n

= 10), acalcu-lous cholecystitis (

n

= 2), and bowel perfora-tion (

n

= 1) (Fig. 4). In 64 other portable CTexaminations (81%) performed for a sus-pected intraabdominal infection, no sourceof intraabdominal infection was identified. Inone CT scan obtained to exclude infection,the findings were reported as negative for in-fection, but the result of laparotomy showednecrotic bowel. Retrospective review showedevidence of bowel wall thickening.

Among 43 portable CT examinations per-formed for indications that did not includeinfection, three scans (7%) had findings sug-gestive of infection. Specifically, these casesall showed colonic thickening suspicious forinfectious colitis.

On 31 portable CT scans obtained becauseof clinical concern for hemorrhage, hemor-rhage was found in 11 cases (35%) (Figs. 5

and 6). In one patient, a portable CT scan ob-tained to exclude hemorrhage showed a rec-tus sheath hematoma that was not identifiedprospectively. Portable CT also revealedhemorrhage in five (5%) of 91 scans ob-tained for indications that did not includesuspected hemorrhage. Similarly, among thisgroup, a psoas hematoma was not reportedon an initial CT but was detected on retro-spective review. Thus, hemorrhage was afinding in 13% (16/122) of the portable CTscans in the series. As mentioned previously,in two cases, retrospective evaluation of por-table CT scans showed hemorrhage that wasnot detected at the initial interpretation.

Among 18 scans obtained for an indica-tion other than infection or hemorrhage, find-ings of portable CT scans supported theclinically suspected diagnosis in six cases(33%). These included one case each of ad-vanced metastatic disease and fetal hydropsand four cases of acute pancreatitis (Fig. 7).

Other than three scans with findings unex-pectedly raising concern for infection andfive unexpectedly showing hemorrhage,eight portable CT scans had eight unex-pected findings. Portable abdominal CT re-vealed two pneumothoraces. There was onelarge obstructing ventral hernia and an addi-tional case of small-bowel obstruction. Onepatient had shock liver, one additional pa-tient had acute tubular necrosis, and portableCT detected unsuspected pneumatosis coli intwo patients.

Impact of Portable CT on Patient Treatment

Findings on portable CT contributed to achange in patient treatment in 33 cases (27%).Thirteen patients with bleeding had correction

of their coagulopathy within 48 hr of the por-table CT. In six patients, findings of infectiouscolitis resulted in a change of antibiotic regi-men (Fig. 8). The finding of advancing meta-static disease influenced the decision towithdraw support of one patient. Portable CTled to laparotomy for seven patients.

In six patients, portable CT precipitatedpercutaneous catheter drainage. In two pa-tients, intraabdominal collections depictedon portable CT were drained under portablesonographic guidance. Two other patientstraveled to the radiology department for ab-scess drainage, either under stationary CT (1patient) (Fig. 4) or under combined fluoros-copy and sonography. Two patients with dis-tended gallbladders visualized on theirportable CT scans underwent portable sono-graphically guided cholecystostomy for sus-pected acalculous cholecystitis.

Surgical and Autopsy Correlation

Within 7 days after their portable CT, 17patients underwent either autopsy or abdom-inal surgery. Ten patients underwent autopsywithin 7 days of portable CT (mean, 3 days;range, 1–7 days). Seven patients had surgeryat a mean duration of 1.2 days (0–5 days) af-ter portable CT.

In seven (70%) of the 10 patients pre-sented for postmortem examination, therewas concordance between the portable CTand the autopsy findings. Diagnoses not visual-ized on CT included hepatic necrosis (

n

= 1)and pancreatitis (

n

= 1). In a third case, intra-peritoneal hemorrhage was suspected onportable CT performed 5 days before the pa-tient’s death, but there was no support forthis finding at autopsy.

Fig. 3.—Contrast-enhanced portable CT scan of 50-year-old man shows pancolitis, with heterogeneously enhancingcolonic wall surrounding narrowed lumen (arrows). Patient was successfully treated for pseudomembranous colitis.

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The findings on portable abdominal CTwere confirmed in five (71%) of seven patientswho had a laparotomy. In two cases, colitiswas not detected on preoperative portable CT,one each from ischemia and infection–necro-sis. One patient underwent a laparotomy thatproved to have negative findings despite a por-table CT scan showing pneumatosis coli andportal venous gas (Fig. 9).

On review of all portable CT scans, threesignificant findings were not seen at the timeof the initial evaluation: infectious colitiswith necrosis found at laparotomy as de-scribed previously (

n

= 1), psoas hematoma(

n

= 1), and rectus sheath hematoma (

n

= 1).In a fourth patient in whom necrotic bowelwas diagnosed at laparotomy, preoperativeportable CT findings were remarkably mini-mal, even when reviewed retrospectively.

Discussion

Since the introduction of portable CT toour hospital in 1998, use of this technologyhas steadily increased. Cranial CT remainsthe principal application for portable scan-

ning, which accounts for the disproportion-ate number of neurologic patients whoseabdomens also were imaged portably. Unlikecranial CT, which can be accomplished withthe patient still lying in his or her ICU bed,

portable acquisition of an abdominal scan re-quires moving the patient. Nevertheless, allthe portable abdominal CT scans were ac-quired at the bedside, without the need of pa-tient transport away from the ICU.

Fig. 5.—Portable CT scan obtained in 83-year-old woman with coagulopathy shows large pelvic hematoma (H)of distinctly higher density than adjacent uterus (U).

Fig. 4.—65-year-old man with intraabdominal abscesses after breakdown of en-teric anastomosis.A, Portable CT scan shows large extraluminal pelvic collection (E) containing leak-ing gastrointestinal contrast material. This collection was drained surgically. B, Repeat portable CT scan shows interloop abscess (I) from peritoneal contamination. C, Stationary (nonportable) CT scan obtained during percutaneous drainage shows de-ployment of drainage catheter. Note that stationary CT scan exhibits better image quality.

BA

C

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On the basis of the stated indication or indi-cations for performing abdominal CT, portableCT confirmed one or more clinical suspicionsin more than one quarter of the cases. Themost common indication for portable CT wassuspected intraabdominal infection, which wassupported by the portable CT findings in 15(19%) of 79 cases and detected in an addi-tional three cases when infection was not thestated indication for the examination. This re-sult is similar to that for stationary CT in thestudy of Barkhausen et al. [4], who detected anabdominal source in 16% of septic patients.

Understandably, our proportion of positiveportable CT findings for infection is not aslarge as the 58% reported in patients aftertrauma [5], which represented (including post-operative patients) only 7% (9/122) of the por-table CT scans in our series. It was reassuring

to find that in five (71%) of seven patients inwhom portable CT findings precipitated lap-arotomy, the suspected abnormality was con-firmed. The two false-negative cases in whichlaparotomy was performed despite normalportable CT findings revealed ischemia and in-fection–necrosis. However, these two mayhave represented cases in which the ischemiawas predominantly on the mucosal side andtherefore may have been better detected at en-doscopy. One patient underwent a laparotomythat proved to have negative results despite aportable CT scan showing pneumatosis coliand portal vein gas (Fig. 9). This case reem-phasizes that pneumatosis coli and portal veingas can occasionally be benign findings and donot always imply bowel ischemia.

Many patients who underwent portable CTsubsequently left the ICU for special proce-

dures or surgery or for other imaging proce-dures performed on nonportable equipment. Insome cases, the portable CT forced an inter-vention that could not be performed at the bed-side, and in others, the patients’ conditionsimproved to the point at which they could besafely moved. Nevertheless, undoubtedly insome patients, abdominal CT was performedportably more for convenience than for truemedical necessity. McCunn et al. [6] surveyedICU physicians who had access to a portableCT scanner. Extracorporeal life support andcardiovascular, respiratory, or neurologic insta-bility were the principal indications for per-forming CT portably. However, when portableCT was unavailable, these physicians willinglytransported their patients to a stationary CTscanner to obtain diagnostic information.

ICU staff reluctance to transport unstablepatients to the radiology department is under-standable. Risks associated with transport ofICU patients to hospital locations outside theICU have been documented in several studies[7–9], although serious injuries and deleteriouseffects on outcome are rare. In a comparison ofneurologic ICU patients transported for sta-tionary head CT scans with similar patientswho underwent portable scans, Gunnarsson etal. [10] found that the 20–25% incidence ofcomplications and deterioration associatedwith transport fell to 0–4% when the examina-tion was performed portably. Those research-ers also noted a beneficial effect on staffworkload when transport could be avoided.

Besides portability, several other technicalfactors distinguish the portable CT instrumentused in our study from conventional stationaryCT scanners. The lower tube current (maxi-

Fig. 6.—Portable CT scan of 64-year-old woman shows extensive perihepatic hematoma (arrows), incompletelyevacuated during trauma surgery 1 day before.

Fig. 7.—Comparison of stationary and portable CT scans in 47-year-old man with acute pancreatitis.A, Contrast-enhanced stationary CT scan shows peripancreatic fluid (arrow) but homogeneous pancreatic enhancement.B, Although features of acute pancreatitis, including peripancreatic fluid (arrow), are visible on portable CT scan, they are better seen on stationary CT scan (A).

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mum, 50 mA) imposes a prolonged scanningtime (2 sec) and results in a reduced milliam-pere-second compared with conventional CT.Even with high-efficiency detectors [11], boththe longer scanning time and lower milliam-pere-second have their greatest effect in scan-ning the abdomen, in which the combinedeffects of motion and patient density are mostsevere. Motion sensitivity was readily apparenton many of the portable CT scans in our studyand caused major artifacts in 23 (19%) of the122 scans. In many cases, monitoring equip-ment or support devices such as intraaortic bal-loons showed characteristic artifacts (Fig. 10).Reduced milliampere-second correlates with

increased image noise, the major determinantobscuring small differences in contrast [12,13]. Among the effects of reduced contrast res-olution might be diminished capacity to detecthemorrhage as abnormal hyperdensity, partic-ularly in patients whose anemia already limitsthis effect. Hemorrhage was the second mostcommon indication for portable CT in our se-ries. Two of the three cases with missed find-ings visible in retrospect had hematomas, andin the third case, hemorrhagic ascites provideda clue to the diagnosis of colonic infarctionsubsequently found at laparotomy.

Battery charge and tube heating place limitsnot applicable to stationary units on the num-

ber of sections that can be performed on porta-ble CT. Particularly when the chest andabdomen are to be scanned together, these fac-tors restrict portable multiphase scanning. As aresult, in a study performed for hemorrhage,the operator’s choice was between unenhancedportable CT optimized for detection of bloodand the benefits in scanning quality associatedwith IV contrast administration [14].

During this study, most stationary scanswere obtained on either single-detector helicalCT scanners or 4-MDCT scanners. Despitethis, analysis of image quality showed that10% of stationary scans had a major defect(Table 1). On initial analysis, this finding

Fig. 8.—Comparison of portable and stationary CT scans in 80-year-old woman with pseudomembranous colitis.A, Portable CT scan shows early sigmoid colon thickening (arrows). High-density ascites partially obscures colon wall. B, Stationary CT scan obtained 4 days after portable CT scan (A) shows layering hematocrit (open arrow) and marked progression of mural abnormality (solid arrows).Patient underwent colectomy for toxic megacolon.

BA

Fig. 9.—73-year-old man with fever and sepsis.A and B, Contrast-enhanced portable CT scans show portal vein gas (arrows, A) and pneumatosis coli (arrows, B). Edematous but nonnecrotic bowel was found at laparotomy.

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seems disappointing but can be explained, inmost cases, by excessive respiratory motion ar-tifacts in this severely ill cohort of patients andby streak artifact from monitoring equipmentand support devices. The continued techno-logic refinements currently being seen withMDCT should improve the image quality ofscans acquired in this patient cohort. Althoughspatial resolution of MDCT is equal to that ofsingle-detector CT scanners, substantial im-provement in temporal resolution with MDCT,because of markedly reduced acquisitiontimes, reduces artifacts caused by voluntary orinvoluntary patient motion [15].

The value of CT for noninvasive diagno-sis of acute abdominal conditions has beenwell established [16, 17]. However, even inthe same cohort of severely ill patients, sta-tionary nonportable abdominal CT offers alevel of quality that the portable CT couldnot provide. Physicians caring for ICU pa-tients must avoid thinking of portable ab-dominal CT as a conventional abdominalCT performed portably. Portable CT shouldbe regarded as a different examination withspecial limitations that do not apply to sta-tionary CT. Whenever possible, patientswho need the valuable diagnostic informa-tion that CT can provide should be moved toa stationary CT installation for their study inpreference to a portable CT scanner.

The unique imaging opportunity afforded byportable abdominal CT is itself the major limi-tation of this study. Although many of the posi-tive findings on portable CT were subsequentlyconfirmed by the intervention they required,most negative studies have no correlation. Wedo not know the accuracy of portable CT, espe-cially the negative predictive value. Portable CTexaminations must be interpreted with specialcaution. Uncertainties inherent in each scanshould be part of the report communicated tothe physicians caring for the patient.

Nevertheless, portable abdominal CT offers awindow on the critically ill population thatwould not otherwise be available. Alternativeportable imaging techniques do not offer reliabledetection or exclusion of many conditions thatCT can image very well, including bowel ob-struction, interloop abscess, and many forms ofhemorrhage [16]. Radiologists who interpret CTscans in major medical centers will probably seecontinued use of portable CT. They must learn torecognize abnormalities on CT scans of less thanthe usual quality and to overcome the effects ofartifacts. Most important, radiologists shouldavoid overstating the information provided byany given portable CT scan. A balanced assess-ment of each study will prevent overconfidentreliance on portable CT for decisions in patienttreatment and encourage the use of portable CTfor abdominal imaging only when transport for

stationary CT would expose the most fragile pa-tients to high risk of decompensation.

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Fig. 10.—Portable CT scan obtained without IV contrast material in 50-year-old man with cardiac failure showsintraaortic balloon pump in lumen of aorta (straight arrow). Note typical fan-shaped artifact from intraaortic bal-loon pump (curved arrows). Gas bubble in ascites, anterior to liver, was introduced during recent paracentesis.