1991, The British Journal of Radiology, 64, 177-178
Short communications
Short communications Pulmonary embolus as a complication of therapeutic peripheral arteriovenous malformation embolization By *P. McCarthy, MRCP, FRCR, A. Kennedy, MRCP, FRCR, P. Dawson, MRCP, FRCR and D. Allison, M D , FRCR Department of Diagnostic Radiology, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road, London W12 OHS, UK (Received May 1990) Keywords: Pulmonary embolus, Arteriovenous, Embolization
Therapeutic embolization is an established technique in interventional radiology. In our unit we have considerable experience in the use of this technique in the management of systemic arteriovenous malformations. During the embolization of systemic vascular lesions containing arteriovenous communications, there is a danger of embolic material being swept into the venous side of the circulation and causing pulmonary embolism. In a previous study of 137 patients treated in this way, overt pulmonary embolism was observed to occur in two patients (1.5%); we did not, however, determine the incidence of subclinical embolism in that series and are unaware of any other available data concerning this (Hemingway & Allison, 1988). A small pilot study was undertaken to see whether the incidence of subclinical embolization in these circumstances was sufficiently high to merit more detailed investigation.
Patients and methods Ten consecutive patients referred for the therapeutic embolization of a peripheral arteriovenous malformation were exam*Current address: Section of Uroradiology, Hospital of University of Pennsylvania, 3400 Spruce Street, Philadelphia PA 19104, USA.
ined by ventilation/perfusion scanning 24 hours after the procedure. Informed consent to the scan was obtained from all patients. It was not felt that an additional pre-embolization scan to establish the baseline appearance of the lungs could be justified in this preliminary study. The 10 patients were all embolized with two or more of the following agents: sterile absorbable gelatin sponge (Sterispon), polyvinyl alcohol particles (Ivalon), hypertonic dextrose solution and steel coils. The exact combination of agents employed depended on the nature and size of the lesion being embolized.
Results Eight of the scans were normal, two showed multiple areas of ventilation/perfusion mismatch (i.e. defects of perfusion in areas of the lung that were normally ventilated) (Fig. 1). None of the 10 patients exhibited any clinical signs or symptoms suggestive of pulmonary embolism following therapeutic embolization. Of the two patients with abnormal scans, neither had a history of pre-existing lung disease and neither smoked. One was a 23-year-old male, the other a 41-year-old female. It is of some interest that in both the individuals with abnormal scans the operator suspected at the time of the embolization that some embolic material had passed into the venous circulation.
V
(a) (b) Figure 1. Ventilation perfusion scan, (a) The ventilation scan (V) is normal; (b) the perfusion scan (Q) shows multiple defects (arrows) in the left lung. Vol. 64, No. 758
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Short communications
1991, The British Journal of Radiology, 64, 178-181 A third patient in whom this occurrence had been suspected had a normal post-embolization scan.
Discussion It seems reasonable to attribute the abnormal findings in these patients to embolization, though this can only remain a strong presumption in the absence of a pre-embolization scan. In order to establish the true incidence of subclinical pulmonary embolism following systemic embolization it would be necessary to undertake a controlled trial in which both preand post-embolization scans were performed in every patient. It is difficult to justify such a study in patients with no clinical signs or symptoms referrable to pulmonary embolism and in whom any adverse long-term sequelae seem extremely
unlikely, but the results of the pilot study reported here seem to suggest that subclinical pulmonary embolism probably does occur in a significant proportion of patients undergoing the therapeutic embolization of complex lesions containing arteriovenous shunts. The results, albeit in a small and uncontrolled series, underline the need for assiduous care and attention to technique during the embolization of arteriovenous communications and the need to terminate or modify the procedure if there is any suspicion on fluoroscopy that venous embolization may be occurring. Reference HEMINGWAY, A. & ALLISON, D. J., 1988. Complications
of
embolization: analysis of 410 procedures. Radiology, 166, 669-672.
Measurement of time-averaged flow in the middle cerebral artery by magnetic resonance imaging By Maria Tarnawski, MSc, Soundrie Padayachee, PhD, Martin J. Graves, MSc, M. Graeme Taylor, PhD, Victoria T. Ayton, DCR(R) and 'Michael A. Smith, PhD, FIPSM Division of Radiological Sciences, UMDS, Guy's Hospital, London Bridge, London SE1 9RT, UK and •Department of Medical Physics, University of Leeds, Leeds General Infirmary, The Wellcome Wing, Great George Street, Leeds LS1 3EX, UK {Received December 1989 and in revised form August 1990) Keywords: Flow, MRI, Middle cerebral artery
Magnetic resonance imaging (MRI) sequences have been developed specifically for the quantitative evaluation of blood flow in vessels. The most promising sequences are those utilizing gradient field echoes to produce two interleaved images with a velocity-induced phase difference between images (Firmin et al, 1987; Ridgeway & Smith, 1987). Velocity values within a pixel can be extracted by subtraction of the images, producing a phase map where the phase is directly proportional to velocity. It is generally accepted that there is a lower limit to the size of vessel in which flow can be measured owing to the resolution restriction imposed by the image pixel size. This constraint is due to the partial volume effect, in which a pixel covering a vessel edge may contain both stationary tissue and flowing blood, and as a consequence produces limitations in the accurate measurement of vessel cross-sectional area. Conventional methods of flow calculation generally combine a measure of the vessel cross-sectional area with the mean velocity. In small vessels, where the pixel size is a significant proportion of the vessel diameter, significant errors can occur in both the measurement of the mean velocity and, more importantly, in the cross-sectional area. In the latter a 10% error results in a 20% error in the flow. The problems posed by the measurement of time-averaged flow using MR in small vessels can be solved using a simple theoretical evaluation of the partial volume effect. Under Reprint address: Maria Tarnawski, Division of Radiological Sciences, UMDS, Guy's Hospital, London Bridge, London SE1 9RT.
178
certain constraints the measured pixel phase is directly proportional to the velocity-induced phase and the proportion of the pixel occupied by the vessel. The measured phase, when combined with the pixel area, gives the flow in that part of the vessel. Summing the values in all pixels in a region of interest (ROI) containing the vessel then results in the flow rate. Thus area measurements are not required as long as we ensure that the ROI encompasses the complete vessel; in fact the ROI can be larger than the vessels. We have previously shown (Tarnawski et al, 1989), using in vitro phantom models where the pixel size to vessel diameter ratio simulates that of small vessels (0.1-0.4), that accurate flow measurements are possible using ROIs larger than the vessel and where a partial volume effect exists in a large proportion of the pixels. In addition the vessels need not be perpendicular to the slice. This method has been used for the measurement of timeaveraged blood flow in the middle cerebral arteries in normal asymptomatic volunteers. Measurement of flow in this vessel has not been possible previously using non-invasive techniques because of the inaccessibility and small size of these arteries. Other techniques such as transcranial Doppler ultrasound are used for measurements of the velocity (Kirkham et al, 1986) but flow cannot be measured as the temporal skull window, through which the beam passes, and the vessel orientation do not allow vessel visualization and therefore diameter measurements. Mathematical models have been used to suggest flow values in cerebral vessels (Zagzoule & Marc Vergnes, 1986) but are subject to simplifications and do not take into account the variable anatomy of the vessels.
The British Journal of Radiology, February 1991
Short communications
Short communications Pulmonary embolus as a complication of therapeutic peripheral arteriovenous malformation embolization By *P. McCarthy, MRCP, FRCR, A. Kennedy, MRCP, FRCR, P. Dawson, MRCP, FRCR and D. Allison, M D , FRCR Department of Diagnostic Radiology, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road, London W12 OHS, UK (Received May 1990) Keywords: Pulmonary embolus, Arteriovenous, Embolization
Therapeutic embolization is an established technique in interventional radiology. In our unit we have considerable experience in the use of this technique in the management of systemic arteriovenous malformations. During the embolization of systemic vascular lesions containing arteriovenous communications, there is a danger of embolic material being swept into the venous side of the circulation and causing pulmonary embolism. In a previous study of 137 patients treated in this way, overt pulmonary embolism was observed to occur in two patients (1.5%); we did not, however, determine the incidence of subclinical embolism in that series and are unaware of any other available data concerning this (Hemingway & Allison, 1988). A small pilot study was undertaken to see whether the incidence of subclinical embolization in these circumstances was sufficiently high to merit more detailed investigation.
Patients and methods Ten consecutive patients referred for the therapeutic embolization of a peripheral arteriovenous malformation were exam*Current address: Section of Uroradiology, Hospital of University of Pennsylvania, 3400 Spruce Street, Philadelphia PA 19104, USA.
ined by ventilation/perfusion scanning 24 hours after the procedure. Informed consent to the scan was obtained from all patients. It was not felt that an additional pre-embolization scan to establish the baseline appearance of the lungs could be justified in this preliminary study. The 10 patients were all embolized with two or more of the following agents: sterile absorbable gelatin sponge (Sterispon), polyvinyl alcohol particles (Ivalon), hypertonic dextrose solution and steel coils. The exact combination of agents employed depended on the nature and size of the lesion being embolized.
Results Eight of the scans were normal, two showed multiple areas of ventilation/perfusion mismatch (i.e. defects of perfusion in areas of the lung that were normally ventilated) (Fig. 1). None of the 10 patients exhibited any clinical signs or symptoms suggestive of pulmonary embolism following therapeutic embolization. Of the two patients with abnormal scans, neither had a history of pre-existing lung disease and neither smoked. One was a 23-year-old male, the other a 41-year-old female. It is of some interest that in both the individuals with abnormal scans the operator suspected at the time of the embolization that some embolic material had passed into the venous circulation.
V
(a) (b) Figure 1. Ventilation perfusion scan, (a) The ventilation scan (V) is normal; (b) the perfusion scan (Q) shows multiple defects (arrows) in the left lung. Vol. 64, No. 758
177
Short communications
1991, The British Journal of Radiology, 64, 178-181 A third patient in whom this occurrence had been suspected had a normal post-embolization scan.
Discussion It seems reasonable to attribute the abnormal findings in these patients to embolization, though this can only remain a strong presumption in the absence of a pre-embolization scan. In order to establish the true incidence of subclinical pulmonary embolism following systemic embolization it would be necessary to undertake a controlled trial in which both preand post-embolization scans were performed in every patient. It is difficult to justify such a study in patients with no clinical signs or symptoms referrable to pulmonary embolism and in whom any adverse long-term sequelae seem extremely
unlikely, but the results of the pilot study reported here seem to suggest that subclinical pulmonary embolism probably does occur in a significant proportion of patients undergoing the therapeutic embolization of complex lesions containing arteriovenous shunts. The results, albeit in a small and uncontrolled series, underline the need for assiduous care and attention to technique during the embolization of arteriovenous communications and the need to terminate or modify the procedure if there is any suspicion on fluoroscopy that venous embolization may be occurring. Reference HEMINGWAY, A. & ALLISON, D. J., 1988. Complications
of
embolization: analysis of 410 procedures. Radiology, 166, 669-672.
Measurement of time-averaged flow in the middle cerebral artery by magnetic resonance imaging By Maria Tarnawski, MSc, Soundrie Padayachee, PhD, Martin J. Graves, MSc, M. Graeme Taylor, PhD, Victoria T. Ayton, DCR(R) and 'Michael A. Smith, PhD, FIPSM Division of Radiological Sciences, UMDS, Guy's Hospital, London Bridge, London SE1 9RT, UK and •Department of Medical Physics, University of Leeds, Leeds General Infirmary, The Wellcome Wing, Great George Street, Leeds LS1 3EX, UK {Received December 1989 and in revised form August 1990) Keywords: Flow, MRI, Middle cerebral artery
Magnetic resonance imaging (MRI) sequences have been developed specifically for the quantitative evaluation of blood flow in vessels. The most promising sequences are those utilizing gradient field echoes to produce two interleaved images with a velocity-induced phase difference between images (Firmin et al, 1987; Ridgeway & Smith, 1987). Velocity values within a pixel can be extracted by subtraction of the images, producing a phase map where the phase is directly proportional to velocity. It is generally accepted that there is a lower limit to the size of vessel in which flow can be measured owing to the resolution restriction imposed by the image pixel size. This constraint is due to the partial volume effect, in which a pixel covering a vessel edge may contain both stationary tissue and flowing blood, and as a consequence produces limitations in the accurate measurement of vessel cross-sectional area. Conventional methods of flow calculation generally combine a measure of the vessel cross-sectional area with the mean velocity. In small vessels, where the pixel size is a significant proportion of the vessel diameter, significant errors can occur in both the measurement of the mean velocity and, more importantly, in the cross-sectional area. In the latter a 10% error results in a 20% error in the flow. The problems posed by the measurement of time-averaged flow using MR in small vessels can be solved using a simple theoretical evaluation of the partial volume effect. Under Reprint address: Maria Tarnawski, Division of Radiological Sciences, UMDS, Guy's Hospital, London Bridge, London SE1 9RT.
178
certain constraints the measured pixel phase is directly proportional to the velocity-induced phase and the proportion of the pixel occupied by the vessel. The measured phase, when combined with the pixel area, gives the flow in that part of the vessel. Summing the values in all pixels in a region of interest (ROI) containing the vessel then results in the flow rate. Thus area measurements are not required as long as we ensure that the ROI encompasses the complete vessel; in fact the ROI can be larger than the vessels. We have previously shown (Tarnawski et al, 1989), using in vitro phantom models where the pixel size to vessel diameter ratio simulates that of small vessels (0.1-0.4), that accurate flow measurements are possible using ROIs larger than the vessel and where a partial volume effect exists in a large proportion of the pixels. In addition the vessels need not be perpendicular to the slice. This method has been used for the measurement of timeaveraged blood flow in the middle cerebral arteries in normal asymptomatic volunteers. Measurement of flow in this vessel has not been possible previously using non-invasive techniques because of the inaccessibility and small size of these arteries. Other techniques such as transcranial Doppler ultrasound are used for measurements of the velocity (Kirkham et al, 1986) but flow cannot be measured as the temporal skull window, through which the beam passes, and the vessel orientation do not allow vessel visualization and therefore diameter measurements. Mathematical models have been used to suggest flow values in cerebral vessels (Zagzoule & Marc Vergnes, 1986) but are subject to simplifications and do not take into account the variable anatomy of the vessels.
The British Journal of Radiology, February 1991
