Value of MR cholangiography in the evaluation of postoperative biliary complications following orthotopic liver transplantation

The aim of this study was to describe the spectrum of abnormal biliary findings as seen with magnetic resonance cholangiography (MRC) in symptomatic patients after orthotopic liver transplantation (OLT). In our study we included 12 consecutive patients post-OLT who presented with clinical and/or biochemical suspicion of biliary complications. In all patients MRC was performed on a 1.0-T whole-body magnet and breathhold half-Fourier acquired single-shot turbo spin echo and rapid acquisition with relaxation enhancement sequences were used. Diagnostic confirmation was obtained with percutaneous transhepatic cholangiography (PTC; n = 3 patients), endoscopic retrograde cholangiography (ERC; n = 3 patients), or clinical follow-up. A vast array of biliary abnormalities were detected in 11 of 12 patients: high-grade, obstructive, anastomotic stricture was the most common unique abnormality. Findings consistent with bile duct necrosis, the second most common abnormality, were accompanied by arterial abnormalities in 2 of 5 patients on subsequent MR- and digital subtraction angiography. Compared with the findings obtained with direct cholangiography (n = 5 patients), MRC was highly accurate for the detection and characterization of postoperative biliary complications. Compared with the final diagnosis, which was based on PTC-ERC findings and/or all available clinical data, MRC imaging alone was able to provide a specific diagnosis in 9 of 12 patients. Magnetic resonance cholangiography is an accurate, time-saving, and non-invasive imaging modality in the evaluation of post-OLT patients in whom suspicion of biliary complications exists. Although the precise value of MRA in this patient group requires larger dedicated studies, single session “all-in-one” MR evaluation of both biliary and arterial system in our series proved to be a substantial benefit in obtaining an accurate and complete diagnosis. Received: 2 December 1999; Revised: 24 February 2000; Accepted: 24 February 2000     

Clinical use of Levovist, an ultrasound contrast agent, in the imaging of liver transplantation: assessment of the pre- and post-transplant patient

Colour Doppler US is well established for imaging of hepatic vessels in the assessment of pre- and post-liver transplant patients. Unfortunately, a full colour Doppler US examination of the portal or hepatic venous and hepatic arterial systems is frequently precluded by technical factors. Ultrasound contrast agents are useful in enhancing vascular Doppler signal and play an important role in liver transplantation assessment. A series of patients with vascular problems illustrates the role of US contrast in the pre-transplant candidate, where portal vein patency and direction of flow is assessed, presence of portal vein thrombus is confirmed and cavernous transformation demonstrated. Occlusion of hepatic veins in Budd-Chiari syndrome is confidently confirmed. Following liver transplantation, US contrast allows a comprehensive assessment of hepatic artery thrombosis, hepatic artery stenosis and pseudoaneurysm formation. The need for further imaging is reduced or confidently deferred in many instances. Ultrasound contrast agents play an important role in the liver transplant candidate. Received: 15 April 1999; Revised: 21 June 1999; Accepted: 22 June 1999     

Pulmonary MALT lymphoma: imaging findings in 24 cases

The aim of this study was to describe the imaging features of pulmonary mucosa-associated lymphoid tissue (MALT) lymphoma. The chest radiographs (n = 18) and CT scans (n = 17) of 24 patients (18 men and 6 women) aged 27–78 years (mean = 56 years), with a known diagnosis of pulmonary MALT lymphoma, were retrospectively reviewed by two radiologists and the imaging findings are described. Six of the 24 patients had a history of an autoimmune disorder and 1 patient had acquired immune deficiency syndrome. Multiple pulmonary lesions were identified in 19 of 24 patients (79 %) and solitary lesions in 4 of 24 patients (17 %). Diffuse pulmonary infiltration was present in 1 patient. Lesions included masses or mass-like areas of consolidation (n = 21) and pulmonary nodules (n = 18). Associated findings were air bronchograms, airway dilatation, a positive angiogram sign and a halo of ground-glass shadowing at lesion margins. Peribronchovascular thickening was also observed, as were hilar or mediastinal lymph node enlargement and pleural effusions or thickening. Although rare, the diagnosis of pulmonary MALT lymphoma should be considered in patients with the imaging features described, particularly when in association with an indolent clinical course or a history of autoimmune disease. Received: 4 October 1999; Revised: 24 February 2000; Accepted: 18 April 2000     

The radiological management of biliary complications following liver transplantation

Purpose: Biliary complications contribute significantly to morbidity and mortality in the liver transplant recipient. Surgery has been the mainstay of therapy, but interventional radiological techniques have made significant progress. Methods: Diagnostic percutaneous transhepatic cholangiography (PTC) was performed in 12 patients; percutaneous transhepatic drainage (PTD) was performed in 10 patients. Additional interventional procedures included laser lithotripsy, biopsy, dilatation, and stent implantation. Results: In 6 patients PTC revealed anastomotic, and in 6 patients nonanastomotic biliary strictures. Four patients had intrahepatic stones. Biliary strictures were treated by implantation of Palmaz stents in 5 of 6 patients with anastomotic strictures, and in 3 of 6 patients with nonanastomotic strictures. The intrahepatic stones were fragmented with dye laser lithotripsy under cholangioscopic control in 3 of 4 patients. One spontaneous stent migration after 24 months and one stent occlusion were observed; the remaining stents are still patent. Patients with anastomotic strictures had a more favorable outcome: 5 of 6 of these patients are still alive and symptom-free after an average of 27.4 months, but only 3 of 6 patients with nonanastomotic strictures are alive after an average of 9.8 months. Conclusion: The different outcomes in patients with anastomotic versus nonanastomotic strictures may be explained by the different causes of these types of stricture.     

Image-guided automated needle biopsy of 106 thoracic lesions: a retrospective review of diagnostic accuracy and complication rates

We reviewed the diagnostic accuracy and complication rates of transthoracic needle biopsy (TNB) with an automated 18-gauge core biopsy needle and gun, using either fluoroscopic or CT guidance. One hundred six lesions were biopsied in 103 patients between 1992 and 1998. Hard-copy images, imaging reports, pathology reports and clinical notes were reviewed. In 3 patients it was not possible to establish the lesion as either malignant or benign from the available follow-up, so these were removed from the analysis of diagnostic accuracy. Adequate samples for histological diagnosis were obtained in 104 of 106 (98 %) biopsies. There were 75 of 85 (88 %) true-positive core biopsies for malignant lesions and a specific cell type was identified in 70 of 85 (82 %) cases. A specific histological diagnosis was obtained in 12 of 18 (66 %) biopsies. There was a 19 % rate of pneumothorax with only 2.4 % requiring drainage. Minor haemoptysis occurred in 3.8 % of procedures. The TNB technique with an automated core biopsy needle provides a high level of diagnostic accuracy, effectively distinguishes cell type in malignancy and provides a definite diagnosis in benign disease more frequently than fine needle aspiration (FNA). There is no increased complication rate compared with FNA. Received: 26 January 1999; Revision received: 1 July 1999; Accepted: 2 July 1999     

Acute pulmonary embolism: visualization of high attenuation clot in the pulmonary artery on noncontrast helical chest CT

Helical CT of the thorax is frequently utilized for the evaluation of chest pain or shortness of breath affecting the emergency patient. To improve diagnostic accuracy, thoracic CT examinations are frequently tailored to address specific conditions. Although tailored protocols may enhance diagnostic accuracy, implementing the wrong protocol could result in a misdiagnosis. The proper protocol choice may particularly difficult in the emergency patient due to the nonspecific nature of many chest pain syndromes. Recently, helical CT has been used for the evaluation of suspected pulmonary embolism (PE). Demonstration of an intravascular filling defect surrounded by contrast-enhanced blood is diagnostic of PE. However, because the clinical presentation of PE is frequently nonspecific, awareness of the many potential imaging manifestations of PE is important. Therefore, we present the rare circumstance of high-attenuation clot visible within the pulmonary arteries on noncontrast helical CT; PE was confirmed after the administration of iodinated contrast medium.     

Angiomyolipoma of the liver: ferumoxides-enhanced MR imaging

We report a case of angiomyolipoma of the liver with emphasis on the appearance at MRI after administration of ferumoxides. Post-contrast T1- and T2-weighted images showed a frank decrease of signal intensity in a rim on the margin of the tumor. This unusual finding was related to the presence of an increased number of CD68-positive histiocytic cells in the periphery of the lesion and in the adjacent liver parenchyma. Received: 13 September 1999; Revised: 2 February 2000; Accepted: 3 February 2000     

Imaging in ureteral complications of renal transplantation: value of static fluid MR urography

Ureteral obstruction is an infrequent complication after renal transplantation that may cause rapid loss of transplant function. We tested static fluid MR urography for determining the cause of graft hydronephrosis. Magnetic resonance urography was performed in nine transplants with dilated collecting systems on ultrasound. A heavily T2-weighted 3D turbo spin-echo sequence on a 1.5-T scanner was used and maximum intensity projections were obtained. The patients also underwent excretory urography (n = 1), renal scintigraphy (n = 1), antegrade pyelography (n = 3), voiding cystourethrography (n = 4), and non-enhanced CT (n = 2). Six patients had pathologic conditions including ureteral stricture, compression by lymphoceles, implantation stenosis, vesicoureteral reflux, and late-occurring transitional cell carcinoma at the implantation site. Static MRU was able to diagnose or exclude a dilation of the graft collecting system. It visualized the course of the ureters and localized the obstruction site in four of five obstructed transplants. In one case the ureter was obscured by lymphoceles, which were demonstrated by hydrographic MRU as well. The definite cause for obstruction was provided in only 2 of 5 cases. Dilation due to vesicoureteral reflux could not be differentiated. The current multimodality approach to renal transplant imaging already provides comprehensive assessment of graft hydronephrosis. Static MRU may be useful in some cases since complications associated with intravenous iodinated contrast or antegrade pyelography can be avoided. Its main drawback, the lack of functional information, may be overcome by combining it with contrast-enhanced MRU. Received: 18 February 1999; Revised: 23 July 1999; Accepted: 18 November 1999     

Swan-Ganz catheter-induced pulmonary artery pseudoaneurysm: review of radiological diagnosis and management

Iatrogenic pulmonary pseudoaneurysm is a well-known but uncommon complication related to Swan-Ganz catheters. The radiologist can play an important role in the diagnosis, prevention, and treatment of this potentially lethal complication. The authors present a case demonstrating the typical clinical and radiological presentation, and describe the role of interventional radiologic management.     

Initial technical and clinical evaluation of a new universal image receptor system

The aim of this study was to evaluate the physical performance of an experimental flat-panel digital X-ray detector plate (FDXD), and to assess its clinical potential in radiographic and fluoroscopic mode. The efficiency of the detector was assessed by calculating the low-frequency detective quantum efficiency (DQE(0)), and a measure of image quality was obtained using a threshold contrast detail detectability (TCDD) test object. A range of clinical examinations were also carried out, and the results reviewed by members of the radiology staff. The DQE(0) of the system was calculated to be almost 75 %, compared with a value of approximately 20 % for modern computed radiography equipment, offering the potential for increased image quality or significant dose reduction. Measurements using the TCDD test object demonstrated a corresponding advantage for the FDXD in image quality and dose efficiency. Clinical studies are producing radiographic results which are at least the equal of the best currently available digital technology, and a limited number of examinations using fluoroscopic mode at 25 frames per second have been equally encouraging. Equipment using FDXD technology could potentially fulfill all the radiographic and fluoroscopic requirements of the digital department, with improved image quality and dose efficiency. Received: 20 April 2000; Accepted: 25 April 2000     

Littoral cell angioma of the spleen: CT and MR imaging appearance

We report a case of littoral cell angioma (LCA) of the spleen, a recently described splenic pathology, which imaging characteristics and pathologic morphology have been discussed only by a few authors. The imaging findings in unenhanced and contrast-enhanced MRI and CT as well as histologic specimen are presented. Diagnosis was made after elective splenectomy. Differential diagnosis of splenic tumors as well as the imaging findings in this particular case are discussed. Received: 7 July 1999; Revised: 18 January 2000; Accepted: 19 January 2000     

Characterization of unifocal liver lesions with pulse inversion harmonic imaging after Levovist injection: preliminary results

The aim of this study was to evaluate capabilities of pulse inversion harmonic imaging (PIHI) in characterization of unifocal liver lesions. We evaluated with PIHI (HDI5000, ATL, Bothell, Wash.) and spiral CT 46 consecutive patients with a single liver lesion identified by fundamental US [7 hepatocellular carcinomas (HCC), 2 cholangiocarcinomas, 7 focal nodular hyperplasias (FNH), 17 hemangiomas and 13 metastases]. The PIHI was performed before and 30 s, 2 and 4 min after bolus administration of Levovist (2.5 g, 300 mg/ml). Scans were digitally stored and reviewed using a dedicated software. Hepatocellular carcinoma was hyperechoic on 30-s scan, and hypoechoic (n = 5) or isoechoic (n = 2) on 2-min scan. Cholangiocarcinoma had inhomogeneous persistent enhancement. Focal nodular hyperplasia was hyperechoic (n = 5) or isoechoic (n = 2) on 30-s scan, hyperechoic (n = 4), isoechoic (n = 2) or slightly hypoechoic (n = 1) on 2-min scan. Large hemangioma revealed peripheral enhancement on 30-s scan which extended centripetally on 2-min scan. Small hemangioma appeared isoechoic on 2-min scan in all but two cases in which they were hypoechoic on 2-min scans and hyperechoic on 4-min scan. Metastasis was hypoechoic on all scans, 70% with rim enhancement. Similar changes in enhancement pattern have been observed at spiral CT. The 30-s and the 2-min scans revealed a conclusive importance in characterization of HCC, cholangiocarcinoma, and large hemangioma. The 2-min scan often furnished enough information for characterization of small hemangioma and metastasis. The 4-min scan allowed characterization of two hemangiomas which appeared hypoechoic on 2-min scans. In the other cases it did not provide further information. Diagnosis of FNH is usually reached with Colour Doppler US; PIHI should be used when colour Doppler is biased by artefacts or when colour Doppler findings are not characteristic. Our results seem to show that PIHI could be a valuable alternative diagnostic approach to spiral CT for unifocal liver lesion characterization. This hypothesis needs to be confirmed with an increased number of lesions.     

Multifocal nodular fatty infiltration of the liver mimicking metastatic disease on CT: imaging findings and diagnosis using MR imaging

The aim of this study was to describe the MR appearance of multifocal nodular fatty infiltration of the liver (MNFIL) using T1-weighted in-phase (IP) and opposed-phase (OP) gradient-echo as well as T2-weighted turbo-spin-echo sequences with fat suppression (FSTSE) and without (HASTE). Magnetic resonance imaging examinations at 1.5 T using T1-weighted IP and OP-GRE with fast low angle shot (FLASH) technique, and T2-weighted FSTSE, T2-weighted HASTE of 137 patients undergoing evaluation for focal liver lesions were reviewed. Five patients were identified in whom CT indicated metastatic disease; however, no liver malignancy was finally proven. Diagnosis was confirmed by biopsy (n = 3), additional wedge resection (n = 1) or follow-up MRI 6–12 months later (n = 5). Regarding the identified five patients, the number of focal liver lesions was 2 (n = 2) and more than 20 (n = 3). The MR imaging characteristics were as follows: OP-image: markedly hypointense (n = 5); IP image: isointense (n = 2) or slightly hyperintense (n = 3); T2-weighted FSTSE-image: isointense (n = 5); T2-weighted HASTE image isointense (n = 1); slightly hyperintense (n = 4). On OP images all lesions were sharply demarcated and of almost spherical configuration (n = 5). Further evaluation by histology or follow-up MR imaging did not give evidence of malignancy in any case. Histology revealed fatty infiltration of the liver parenchyma in three patients. Magnetic resonance follow-up showed complete resolution in two patients and no change in three patients. Multifocal nodular fatty infiltration can simulate metastatic disease on both CT and MR imaging. The combination of in-phase (IP) and opposed-phase (OP) gradient-echo imaging can reliably differentiate MNFIL from metastatic disease. Received: 15 September 1999 Revised: 3 February 2000; Accepted: 7 February 2000     

Signal changes in liver and spleen after Endorem administration in patients with and without liver cirrhosis

The goal of this study was to compare the effect of Endorem on the signal intensity of the spleen in patients with normal liver tissue and in patients with liver cirrhosis. Thirty patients with normal liver tissue and 47 with liver cirrhosis were examined before and after i. v. Endorem administration. The patients were examined with a 1.5-T magnet system (Magnetom Vision) using a semiflexible cp-array coil. Three different pulse sequences were used: a T1-weighted gradient-echo sequence, a T2-weighted fast spin-echo sequence with spectral fat suppression, and a T2^*-weighted gradient-echo sequence. The signal-to-noise ratios (SNRs) of two areas of the liver and spleen were determined. The mean SNRs of the liver and spleen in patients with and without liver cirrhosis were compared. For assessment of statistical significance, the t-test at a level of P < 0.05 was applied. After i. v. administration of Endorem, no differences were seen with the T1-weighted gradient-echo sequence for the liver and spleen and, with the T2-weighted fast spin-echo sequence, no differences were found for the spleen. Significant differences between both groups were seen for the liver with the T2-weighted fast spin-echo sequence. The SNR in the noncirrhotic liver group was 57.4 % lower than the SNR in the cirrhotic liver group. With the T2^*-weighted gradient-echo sequence, the SNRs of the liver and spleen in the noncirrhotic liver group, compared with the cirrhotic liver group, were 126.8 % and 45.6 % less, respectively. The effect of Endorem on the liver in patients with Child C-stage liver cirrhosis was 32.1 % less than in patients with Child B-stage liver cirrhosis. Likewise, the Endorem effect on the spleen was 27.1 % less in patients with Child C-stage compared with Child B-stage liver cirrhosis. Hepatic and splenic uptake of Endorem is significantly decreased in patients with liver cirrhosis. Received: 3 February 1999; Revision received: 21 October 1999; Accepted: 27 October 1999     

Hemobilia, intrahepatic hematoma and acute thrombosis with cavernomatous transformation of the portal vein after percutaneous thermoablation of a liver metastasis

A 53-year-old-man underwent US-guided percutaneous thermal ablation with a cooled-tip needle of three liver metastases from gastric cancer. Six days later, the patient was re-admitted for melena, scleral jaundice, and anemia. Abdominal US disclosed echogenic material in the gallbladder lumen (hemobilia) and a focal lesion with mixed echotexture in segment III (hepatic hematoma). On day 5 portal cavernomatosis was diagnosed at US and confirmed by color Doppler and a helical CT exam. The case described emphasizes that radio-frequency interstitial hyperthermia may cause not only traumatic injury of the liver parenchyma but also thermally mediated damage of vascular structures. Received: 14 January 1999; Revised: 21 May 1999; Accepted: 9 June 1999     

Therapeutic effect of transcatheter arterial chemoembolization on hepatocellular carcinoma: evaluation with contrast-enhanced harmonic power Doppler ultrasound

The aim of this study was to investigate the usefulness of contrast-enhanced harmonic power Doppler ultrasound (US) for the detection of residual viable hepatocellular carcinoma (HCC) after treatment with transcatheter arterial chemoembolization (TACE). Forty-seven patients with 68 HCC lesions 1.8–9.5 cm in diameter (mean ± SD 4.3 ± 1.7 cm) underwent contrast-enhanced power Doppler US, in the harmonic mode, before and after treatment with TACE. Unenhanced spiral CT and contrast-enhanced dynamic MR imaging were also performed to help establish the outcome of therapy. Before treatment, intratumoral blood flow signals were detected at contrast-enhanced harmonic power Doppler US in 65 (95 %) of 68 lesions. After TACE, flow signals were no longer detectable in 22 of these 65 lesions, which showed complete response at spiral CT and dynamic MR imaging. In 38 (88 %) of the 43 lesions with partial response, intratumoral flow signals were still identified at contrast-enhanced harmonic power Doppler US. Twenty-eight of these 38 lesions underwent additional treatment with percutaneous ethanol injection (PEI) using contrast-enhanced harmonic power Doppler US guidance. Complete response was seen after PEI in 23 of 28 lesions. Contrast-enhanced harmonic power Doppler US proved useful for assessing the therapeutic effect of TACE on HCC and for guiding additional treatment with PEI in cases of partial response. Received: 25 January 2000; Revised: 21 April 2000; Accepted: 25 April 2000     

Extraosseous manifestation of Gaucher's disease type I: MR and histological appearance

Gaucher's disease type I is the most prevalent lysosomal storage disorder caused by an autosomal-recessive inherited deficiency of glucocerebrosidase activity with secondary accumulation of glucocerebrosides within the lysosomes of macrophages. The storage disorder produces a multisystem disease characterized by progressive visceral enlargement and gradual replacement of bone marrow with lipid-laden macrophages. Skeletal disease is a major source of disability in Gaucher's disease. Extraosseous extension of Gaucher cells is an extremely rare manifestation of skeletal Gaucher's disease. This is a report on the MRI and histopathological findings of an extraosseous Gaucher-cell extension into the midface in a patient with Gaucher's disease. Received: 27 January 2000; Revised: 15 March 2000; Accepted: 16 March 2000     

An experimental approach of the Doppler perfusion index of the liver in detecting occult hepatic metastases: histological findings related to the hemodynamic measurements in Wistar rats

Image-directed colour Doppler sonography has been successfully introduced for the detection of hepatic haemodynamic changes in the presence of liver metastases. The aim of our study was to correlate these haemodynamic changes with the liver histology at the time of measurement. We experimentally induced liver metastases in 30 male Wistar rats by inoculating Walker 256 tumour subcutaneously. The animals were assigned into three groups of ten and were studied sonographically at 4, 7 and 15 days after tumour implantation. Another group of ten normal animals were used as controls. Portal vein and hepatic artery measurements included resistance index (PVRI, HARI) and flow volume (PVFV, HAFV). Doppler perfusion index (DPI) of the liver was calculated as the ratio of HAFV/PVFV + HAFV. Liver histology followed each Doppler measurement. Metastases were first encountered on day 4, as small groups of cells in the connective tissue of the porta hepatis and the portal triads without apparent vascular association. Distinct elevation of HAFV and DPI was recorded in comparison with the controls (p = 0.0004 and p = 0.0005, respectively). PVFV reduction was subtle. Up to day 15 there were no significant changes in the measurements. Our data suggest that HAFV and DPI can efficiently detect early liver metastases and this is in accordance with existing clinical reports. Haemodynamic changes seem to originate from the early non-vascular phase of the metastases. Received: 8 March 1999; Revision received: 30 March 1999; Accepted: 4 June 1999     

原位肝移植术后肝动脉并发症介入治疗患者的预后及其影响因素分析

目的 探讨介入治疗肝移植术后肝动脉并发症患者的预后及影响预后的相关因素.方法 回顾性分析21例肝移植术后发牛肝动脉并发症,并接受肝动脉溶栓、腔内血管成形术(PTA)和(或)肝动脉支架置入术等治疗患者的临床资料,根据移植肝脏预后分为预后不良组和预后较好组,比较2组胆道并发症、肝动脉再狭窄、早期并发症或是晚期并发症等15项变量,采用二值Logistic回归分析方法 筛选出影响介入治疗疗效的因素.结果 21例患者平均随访时间为436 d,中位随访时问464 d(3～1037 d).其中预后不良组11例(5例再次肝移植,6例死亡),平均存活时间191 d,中位存活时问为73 d(3～616 d);预后较好组10例,移植肝平均存活时间706 d,中位存活时间692 d(245～1037 d).单因素分析两组患者差异有统计学意义的因素有:胆道并发症、血总胆红素水平以及间接胆红素水平;经Logistic回归分析显示,主要危险因素是胆道并发症(P=0.027,OR=22.818). .结论 伴有胆道并发症,是影响肝移植术后肝动脉并发症患者介入治疗效果的主要因素.     

Change in regional pulmonary perfusion as a result of posture and lung volume assessed using technetium-99m macroaggregated albumin single-photon emission tomography

The purpose of this study was to evaluate the effects of gravity and lung volume on regional pulmonary perfusion using technetium-99m macroaggregated albumin (^99mTc MAA) single-photon emission tomography (SPET). Twenty-five subjects were classified into three groups according to their position during the injection of the tracer [11 subjects sitting, six supine and eight both supine and prone (S+P) positions]. All of these subjects were injected with the tracer during normal tidal breathing. In the S+P group, half of the tracer was injected while the subject was in each position. The remaining 11 subjects were classified into two groups according to their lung volume during the injection. Supine patients were instructed to hold their breath at residual volume (RV) (five subjects) or total lung capacity (TLC) (six subjects) while receiving the tracer injection. A region of interest with a ventrodorsal axis was defined in the centre of each lung. Profile curves were produced by plotting and normalizing the perfusion values as a percentage of the maximum value. The perfusion distributions for the sitting and S+P positions and at RV were relatively uniform. However, the distributions for the supine position and at TLC showed a gravitational influence [sitting vs TLC: 87.8%±10.4% vs 67.3%±8.7% for % maximum perfusion at +5 pixels from the midpoint of the upper lobe (P<0.00002)]. The gravity-related perfusion inhomogeneity was more prominent in the lower lobe than in the upper lobe. It is concluded that the physiological vertical gravitational gradient should be taken into consideration during the interpretation of lung SPET images. Preferably, patients should be injected with the tracer twice, once in the supine position and once in the prone position, while breathing normally. Alternatively, they may be injected with the tracer once while in the supine position and holding their breath at RV. Either of these protocols should ensure a uniform distribution of tracer. Received 4 November 1999 and in revised form 18 January 2000