ORIGINAL RESEARCH
Dynamic MR Angiography in Acute Aortic Dissection Sonja Kinner, MD,1* Holger Eggebrecht, MD,2 Stefan Maderwald, PhD,1 J€ org Barkhausen, MD,3 Susanne C. Ladd, MD,1 Harald H. Quick, PhD,1 Peter Hunold, MD,3 and Florian M. Vogt, MD3 Background: To evaluate the benefit (additional flow information), image quality, and diagnostic accuracy of a dynamic magnetic resonance angiography (MRA) combining high spatial and temporal resolution for the preinterventional assessment of acute aortic dissection. Methods: Nineteen patients (12 men, 7 women; aged 32–78 years) with acute aortic dissection underwent contrastenhanced four-dimensional (4D) MRA and 3D conventional high-resolution MRA (3D MRA) within one examination on a 1.5 Tesla MR system. Both MRA datasets for each patient were evaluated and compared for image quality and visualization of vascular details on a 5-point scale (5 5 excellent image quality, 1 5 nondiagnostic image quality). In addition, presence and relevance of additional hemodynamic information (flow direction and organ perfusion delay) gained by dynamic MRA were assessed. Results: Conventional 3D MRA provided significantly higher values for image quality of the aorta and aortic side branches compared with dynamic MRA (aorta: 4.3 versus 3.3; P 5 0.006 side branches: 4.2 versus 3.3; P 5 0.02). However, in 10 of the 19 patients (53%) the additionally available information on flow dynamics due to dynamic MRA (e.g., delayed perfusion of parenchymal organs) led to a change in therapy planning and realization. Conclusion: Dynamic MRA is a technique that combines functional flow and morphological information. Thus, the combination of 3D and dynamic MRA provides all requested information for treatment planning in patients suffering from acute aortic dissection. J. MAGN. RESON. IMAGING 2015;42:505–514.
A
cute Aortic Dissection (AD) is reported to be the most common aortic emergency with an increasing incidence of approximately 3–4 per 100,000 people per year.1 Typical for an acute dissection and its main symptom is a fierce and sudden thoracic pain (presented by 80–96% of patients) associated with symptoms of ischemia, e.g., of the heart, the cerebrum, or abdominal organs such as the gut or kidneys. A chronic aortic dissection mostly presents with back pain and symptoms like hoarseness or dysphagia due to organ displacement. Despite recent advances in surgical and interventional techniques, AD is still associated with a high morbidity and mortality rate.2 Mortality rates for acute dissections involving the ascending aorta (type A according to the Stanford classification) reaches 40–50% within 48 h, if left untreated.3
Therefore, almost all patients with acute type A dissection require emergency surgical repair. In contrast, dissection limited to the descending aorta (type B) has better survival rates and is usually treated with medical therapy, reserving endovascular therapy (i.e., stent-graft placement) for evolving complications.4 Malperfusion syndromes due to either dynamic true lumen collapse or static aortic branch involvement resulting in visceral or limb ischemia are among the most important complications of AD, affecting almost every third patient. For these patients, thoracic endovascular aortic repair (TEVAR) is considered to be the treatment of choice.5–8 In terms of TEVAR planning, MRI appears to be particularly useful as it can provide a comprehensive preinterventional diagnostic evaluation of the vascular aortic
View this article online at wileyonlinelibrary.com. DOI: 10.1002/jmri.24788 Received Sep 12, 2014, Accepted for publication Oct 14, 2014. *Address reprint requests to: S.K., Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Hufelandstrasse 55, 45122 Essen, Germany. E-mail: [email protected] From the 1Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Germany; 2Cardioangiological Center ubeck, L€ ubeck / Bethanien (CCB), Frankfurt, Germany; and 3Clinic for Radiology and Nuclear Medicine, University Hospital Schleswig-Holstein, Campus L€ Germany
C 2014 Wiley Periodicals, Inc. V 505
Journal of Magnetic Resonance Imaging
TABLE 1. Image Acquisition Parameters for Dynamic MRA and Conventional 3D MRA
Dynamic MRA
Conventional 3D MRA
Field of view [mm2]
500*450
500*417
TR [ms]
2.8
3.0
1.2
0.97
flip angle [ ]
25
25
slices
64
72
Voxel [mm ]
1.9 x 1.6 x 2.1
1.4 x 1.3 x 1.8
Matrix
231*320
289*384
GRAPPA reduction factor
2
2
Acquisition time [s]
70
25
temporal resolution true/interpolated [s]
3.3/1.65
n/a
TE [ms]
3
morphology.9,10 However, standard MR angiography (MRA) techniques provided no information on flow and tissue perfusion. This is particularly of interest for decisionmaking concerning a surgical access or stenting. Therefore, time-resolved, dynamic MRA techniques have been introduced which can overcome these limitations.11,12 Dynamic MRA techniques provide a higher temporal resolution giving the chance to evaluate how flow/ contrast progresses, which has been shown for other body parts already.13,14 Until now, sufficient temporal resolution and coverage of a longer aortic segment could only be achieved with reduced spatial resolution, thus requiring acquisition of an additional high-resolution MRA. The purpose of our study, therefore, was to evaluate image quality, diagnostic accuracy, and benefits like flow information of a recently introduced dynamic MRA technique (TWIST, Time-resolved angiography With Interleaved Stochastic Trajectories) combining high spatial and temporal resolution for the preinterventional assessment of aortic dissection. In addition, we wanted to find out if dynamic MRA can be used as a stand-alone modality in the evaluation of AD.
Materials and Methods Patients The study was approved by the local institutional review board. The study collective consisted of nineteen hemodynamically stable patients (12 men, 7 women; aged 32–78 years; mean age 61 years) with newly diagnosed acute aortic dissection (type A 5 5, type B 5 14) in computed tomography angiography (CTA). CTA of the entire aorta and its branches was performed on a Sensation 16 scanner (Siemens, Erlangen, Germany) using CARE bolus technique and served as standard of reference. After written informed consent, patients underwent contrastenhanced three-dimensional (3D) and dynamic MRA of the entire thoracic and abdominal aorta in one combined examination 506
MR Imaging Technique All imaging was performed on a 1.5 Tesla (T) MR scanner (Avanto, Siemens AG, Healthcare Sector, Erlangen, Germany) equipped with two flexible phased array radiofrequency (RF) coils and the integrated spine RF coil for signal reception covering the entire thoracic and abdominal aorta. Following automatic injection of 5 mL gadobutrol (Gadovist, Bayer Schering Pharma, Berlin, Germany) at 3 mL/s, 15 consecutive coronal T1w 3D datasets (repetition time/echo time [TR/TE] 2.8/1.2 ms; flip angle (FA) 25 ; slices 64; matrix 231 3 320; spatial resolution 1.9 3 1.6 3 2.1 mm3, true temporal resolution 3.3 s) were acquired using the TWIST sequence and parallel imaging technique (GRAPPA; R 5 2). Following the acquisition of an entire nonenhanced dataset within one breathhold (23 s), 14 consecutive undersampled 3D datasets were obtained. Patients were asked to hold their breath for as long as possible and to breathe shallowly onward. In the TWIST sequence all k-space points are sorted according to their radial distance in k-space; k-space is divided into A and B.11 While the center region of k-space A is completely sampled, region B is undersampled by a factor of n. The k-space trajectory within region B follows a spiral pattern in the ky-kz plane with every trajectory in B being slightly different. The individual trajectories of B are interleaved during the execution of the TWIST sequence. After a pause of 10 min, a conventional 3D high spatial resolution MRA (0.1 mmol/kg gadobutrol) was acquired (breathhold 25 s) using a T1-weighted fast 3D spoiled gradient-echo sequence FLASH (TR/TE 3/0.97 ms, FA 25 , slices 72, matrix 289 3 384; voxel size 1.4 3 1.3 3 1.8 mm3). Details on sequence parameters for both MRA sequences can be found in Table 1.
Image Evaluation All dynamic MRA and 3D MRA examinations were transferred to a postprocessing workstation (Leonardo, Siemens AG, Healthcare Sector, Erlangen, Germany) and archived on a PACS. Two radiologists with 8 and 12 years’ experience in interventional radiology and MR angiography assessed the MRA datasets Volume 42, No. 2
Kinner et al.: 4D MRA in Acute Aortic Dissection
FIGURE 1: Comparison of image quality of dynamic MRA (A) and conventional 3D MRA (B) revealed statistically significant better image quality for 3D MRA due to slightly higher spatial resolution, which generally leads to lower SNR and thus lower image quality. Image quality for dynamic MRA was sufficient to diagnose pathologies. Corresponding CT angiography of the patient in volume rendering technique (VRT; C).
separately. Source images, subtracted images, as well as maximum intensity projection (MIP) images of all MRA datasets were used for analysis. Both MRA datasets were evaluated and compared for image quality and visualization of vascular details. Image quality for the aorta and the visceral side branches was assessed using a 5-point Likert scale (5 5 excellent image quality, 4 5 good image quality, 3 5 fair image quality, 2 5 poor image quality, 1 5 nondiagnostic image quality). The type of aortic dissection and its extension as well as number and localization of entries and re-entries were assessed together and compared for both datasets. Involvement of supraaortic and visceral as well as iliac arteries were assessed for both datasets and the presence of thrombi, their localization, and affiliation to the true or false lumen were recorded. Additional diagnostic information of the dynamic MRA concerning contrast enhancement and tissue perfusion were also evaluated: For the perfusion of the true and false lumen, the perfusion speed was visually classified into slow (>14 frames), medium (8–14 frames), and high velocity (1–7 frames from aortic root/ ascending aorta contrast until contrast in the false lumen). Furthermore, the flow direction for the false lumen (antegrade versus retrograde) as well as perfusion delays of parenchymal tissue was evaluated. For perfusion delays, we used a purely visual qualitative evaluation. A difference of contrasting the false lumen compared with the true lumen of more than one timeframe was considered delayed. Both readers assessed the presence and relevance of hemodynamic information obtained by dynamic MRA. In addition, an interventional cardiologist (14 years’ experience) familiar with MRA datasets (8 years’ experience) independently evaluated if the additional hemodynamic information resulted in a change of therapy options and procedures, e.g., additional or different stenting or the missing need of an intervention at all.
Statistical Analysis Mean values for image quality of the aorta and the side branches were calculated and compared for dynamic MRA and conventional 3D August 2015
MRA using a Wilcoxon signed-rank test. A P-value of 0.8 excellent interobserver agreement). For statistical analysis, PASW statistics, release version 18 (SPSS Inc., Chicago, IL), was used.
Results According to the Stanford classification, all patients were diagnosed correctly by both conventional 3D MRA and dynamic MRA. Dynamic and 3D MRA could successfully be performed in all patients; no technical or image reconstruction problems occurred. No patient had to be taken out of the scanner due to aggravation of the disease or instability. Image reconstruction time amounted to 5 min for the dynamic MRA datasets. Image Quality Image quality for the assessment of the aorta by dynamic MRA amounted to 3.3/ 3.3 compared with 4.3/ 4.2 in 3D MRA (P 5 0.006; k 5 1.0/0.88). For the visceral side branches, image quality in dynamic MRA reached a value of 3.3/ 3.4 in comparison to 4.2/ 4.2 for 3D MRA (P 5 0.02; k 5 0.88/1.0). Image quality for the aorta as well as for aortic side branches, therefore, showed statistically significant improved image quality for the 3D MRA when compared with dynamic MRA. Figure 1 provides a typical example comparing image quality of dynamic MRA with 3D MRA and CT. 3D MRA showed comparable image quality as CT. Interobserver agreement proved to be excellent. Morphology of Aorta, Aortic Dissection, and Involvement of Side Branches Type and origin of AD was diagnosed correctly in both MRA datasets for all patients as compared to CTA. 507
Journal of Magnetic Resonance Imaging
FIGURE 2: Images of two patients with different flow velocities: In A, the patient shows a slow velocity of the false lumen as the contrasting of the false lumen needs more than 14 time frames between aortic root contrast and filling of the false lumen (interpolated images with a time delay of 1.6 s between images shown). The patient in B shows high velocity in the false lumen with a rapid contrasting of the false lumen (
Dynamic MR Angiography in Acute Aortic Dissection Sonja Kinner, MD,1* Holger Eggebrecht, MD,2 Stefan Maderwald, PhD,1 J€ org Barkhausen, MD,3 Susanne C. Ladd, MD,1 Harald H. Quick, PhD,1 Peter Hunold, MD,3 and Florian M. Vogt, MD3 Background: To evaluate the benefit (additional flow information), image quality, and diagnostic accuracy of a dynamic magnetic resonance angiography (MRA) combining high spatial and temporal resolution for the preinterventional assessment of acute aortic dissection. Methods: Nineteen patients (12 men, 7 women; aged 32–78 years) with acute aortic dissection underwent contrastenhanced four-dimensional (4D) MRA and 3D conventional high-resolution MRA (3D MRA) within one examination on a 1.5 Tesla MR system. Both MRA datasets for each patient were evaluated and compared for image quality and visualization of vascular details on a 5-point scale (5 5 excellent image quality, 1 5 nondiagnostic image quality). In addition, presence and relevance of additional hemodynamic information (flow direction and organ perfusion delay) gained by dynamic MRA were assessed. Results: Conventional 3D MRA provided significantly higher values for image quality of the aorta and aortic side branches compared with dynamic MRA (aorta: 4.3 versus 3.3; P 5 0.006 side branches: 4.2 versus 3.3; P 5 0.02). However, in 10 of the 19 patients (53%) the additionally available information on flow dynamics due to dynamic MRA (e.g., delayed perfusion of parenchymal organs) led to a change in therapy planning and realization. Conclusion: Dynamic MRA is a technique that combines functional flow and morphological information. Thus, the combination of 3D and dynamic MRA provides all requested information for treatment planning in patients suffering from acute aortic dissection. J. MAGN. RESON. IMAGING 2015;42:505–514.
A
cute Aortic Dissection (AD) is reported to be the most common aortic emergency with an increasing incidence of approximately 3–4 per 100,000 people per year.1 Typical for an acute dissection and its main symptom is a fierce and sudden thoracic pain (presented by 80–96% of patients) associated with symptoms of ischemia, e.g., of the heart, the cerebrum, or abdominal organs such as the gut or kidneys. A chronic aortic dissection mostly presents with back pain and symptoms like hoarseness or dysphagia due to organ displacement. Despite recent advances in surgical and interventional techniques, AD is still associated with a high morbidity and mortality rate.2 Mortality rates for acute dissections involving the ascending aorta (type A according to the Stanford classification) reaches 40–50% within 48 h, if left untreated.3
Therefore, almost all patients with acute type A dissection require emergency surgical repair. In contrast, dissection limited to the descending aorta (type B) has better survival rates and is usually treated with medical therapy, reserving endovascular therapy (i.e., stent-graft placement) for evolving complications.4 Malperfusion syndromes due to either dynamic true lumen collapse or static aortic branch involvement resulting in visceral or limb ischemia are among the most important complications of AD, affecting almost every third patient. For these patients, thoracic endovascular aortic repair (TEVAR) is considered to be the treatment of choice.5–8 In terms of TEVAR planning, MRI appears to be particularly useful as it can provide a comprehensive preinterventional diagnostic evaluation of the vascular aortic
View this article online at wileyonlinelibrary.com. DOI: 10.1002/jmri.24788 Received Sep 12, 2014, Accepted for publication Oct 14, 2014. *Address reprint requests to: S.K., Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Hufelandstrasse 55, 45122 Essen, Germany. E-mail: [email protected] From the 1Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Germany; 2Cardioangiological Center ubeck, L€ ubeck / Bethanien (CCB), Frankfurt, Germany; and 3Clinic for Radiology and Nuclear Medicine, University Hospital Schleswig-Holstein, Campus L€ Germany
C 2014 Wiley Periodicals, Inc. V 505
Journal of Magnetic Resonance Imaging
TABLE 1. Image Acquisition Parameters for Dynamic MRA and Conventional 3D MRA
Dynamic MRA
Conventional 3D MRA
Field of view [mm2]
500*450
500*417
TR [ms]
2.8
3.0
1.2
0.97
flip angle [ ]
25
25
slices
64
72
Voxel [mm ]
1.9 x 1.6 x 2.1
1.4 x 1.3 x 1.8
Matrix
231*320
289*384
GRAPPA reduction factor
2
2
Acquisition time [s]
70
25
temporal resolution true/interpolated [s]
3.3/1.65
n/a
TE [ms]
3
morphology.9,10 However, standard MR angiography (MRA) techniques provided no information on flow and tissue perfusion. This is particularly of interest for decisionmaking concerning a surgical access or stenting. Therefore, time-resolved, dynamic MRA techniques have been introduced which can overcome these limitations.11,12 Dynamic MRA techniques provide a higher temporal resolution giving the chance to evaluate how flow/ contrast progresses, which has been shown for other body parts already.13,14 Until now, sufficient temporal resolution and coverage of a longer aortic segment could only be achieved with reduced spatial resolution, thus requiring acquisition of an additional high-resolution MRA. The purpose of our study, therefore, was to evaluate image quality, diagnostic accuracy, and benefits like flow information of a recently introduced dynamic MRA technique (TWIST, Time-resolved angiography With Interleaved Stochastic Trajectories) combining high spatial and temporal resolution for the preinterventional assessment of aortic dissection. In addition, we wanted to find out if dynamic MRA can be used as a stand-alone modality in the evaluation of AD.
Materials and Methods Patients The study was approved by the local institutional review board. The study collective consisted of nineteen hemodynamically stable patients (12 men, 7 women; aged 32–78 years; mean age 61 years) with newly diagnosed acute aortic dissection (type A 5 5, type B 5 14) in computed tomography angiography (CTA). CTA of the entire aorta and its branches was performed on a Sensation 16 scanner (Siemens, Erlangen, Germany) using CARE bolus technique and served as standard of reference. After written informed consent, patients underwent contrastenhanced three-dimensional (3D) and dynamic MRA of the entire thoracic and abdominal aorta in one combined examination 506
MR Imaging Technique All imaging was performed on a 1.5 Tesla (T) MR scanner (Avanto, Siemens AG, Healthcare Sector, Erlangen, Germany) equipped with two flexible phased array radiofrequency (RF) coils and the integrated spine RF coil for signal reception covering the entire thoracic and abdominal aorta. Following automatic injection of 5 mL gadobutrol (Gadovist, Bayer Schering Pharma, Berlin, Germany) at 3 mL/s, 15 consecutive coronal T1w 3D datasets (repetition time/echo time [TR/TE] 2.8/1.2 ms; flip angle (FA) 25 ; slices 64; matrix 231 3 320; spatial resolution 1.9 3 1.6 3 2.1 mm3, true temporal resolution 3.3 s) were acquired using the TWIST sequence and parallel imaging technique (GRAPPA; R 5 2). Following the acquisition of an entire nonenhanced dataset within one breathhold (23 s), 14 consecutive undersampled 3D datasets were obtained. Patients were asked to hold their breath for as long as possible and to breathe shallowly onward. In the TWIST sequence all k-space points are sorted according to their radial distance in k-space; k-space is divided into A and B.11 While the center region of k-space A is completely sampled, region B is undersampled by a factor of n. The k-space trajectory within region B follows a spiral pattern in the ky-kz plane with every trajectory in B being slightly different. The individual trajectories of B are interleaved during the execution of the TWIST sequence. After a pause of 10 min, a conventional 3D high spatial resolution MRA (0.1 mmol/kg gadobutrol) was acquired (breathhold 25 s) using a T1-weighted fast 3D spoiled gradient-echo sequence FLASH (TR/TE 3/0.97 ms, FA 25 , slices 72, matrix 289 3 384; voxel size 1.4 3 1.3 3 1.8 mm3). Details on sequence parameters for both MRA sequences can be found in Table 1.
Image Evaluation All dynamic MRA and 3D MRA examinations were transferred to a postprocessing workstation (Leonardo, Siemens AG, Healthcare Sector, Erlangen, Germany) and archived on a PACS. Two radiologists with 8 and 12 years’ experience in interventional radiology and MR angiography assessed the MRA datasets Volume 42, No. 2
Kinner et al.: 4D MRA in Acute Aortic Dissection
FIGURE 1: Comparison of image quality of dynamic MRA (A) and conventional 3D MRA (B) revealed statistically significant better image quality for 3D MRA due to slightly higher spatial resolution, which generally leads to lower SNR and thus lower image quality. Image quality for dynamic MRA was sufficient to diagnose pathologies. Corresponding CT angiography of the patient in volume rendering technique (VRT; C).
separately. Source images, subtracted images, as well as maximum intensity projection (MIP) images of all MRA datasets were used for analysis. Both MRA datasets were evaluated and compared for image quality and visualization of vascular details. Image quality for the aorta and the visceral side branches was assessed using a 5-point Likert scale (5 5 excellent image quality, 4 5 good image quality, 3 5 fair image quality, 2 5 poor image quality, 1 5 nondiagnostic image quality). The type of aortic dissection and its extension as well as number and localization of entries and re-entries were assessed together and compared for both datasets. Involvement of supraaortic and visceral as well as iliac arteries were assessed for both datasets and the presence of thrombi, their localization, and affiliation to the true or false lumen were recorded. Additional diagnostic information of the dynamic MRA concerning contrast enhancement and tissue perfusion were also evaluated: For the perfusion of the true and false lumen, the perfusion speed was visually classified into slow (>14 frames), medium (8–14 frames), and high velocity (1–7 frames from aortic root/ ascending aorta contrast until contrast in the false lumen). Furthermore, the flow direction for the false lumen (antegrade versus retrograde) as well as perfusion delays of parenchymal tissue was evaluated. For perfusion delays, we used a purely visual qualitative evaluation. A difference of contrasting the false lumen compared with the true lumen of more than one timeframe was considered delayed. Both readers assessed the presence and relevance of hemodynamic information obtained by dynamic MRA. In addition, an interventional cardiologist (14 years’ experience) familiar with MRA datasets (8 years’ experience) independently evaluated if the additional hemodynamic information resulted in a change of therapy options and procedures, e.g., additional or different stenting or the missing need of an intervention at all.
Statistical Analysis Mean values for image quality of the aorta and the side branches were calculated and compared for dynamic MRA and conventional 3D August 2015
MRA using a Wilcoxon signed-rank test. A P-value of 0.8 excellent interobserver agreement). For statistical analysis, PASW statistics, release version 18 (SPSS Inc., Chicago, IL), was used.
Results According to the Stanford classification, all patients were diagnosed correctly by both conventional 3D MRA and dynamic MRA. Dynamic and 3D MRA could successfully be performed in all patients; no technical or image reconstruction problems occurred. No patient had to be taken out of the scanner due to aggravation of the disease or instability. Image reconstruction time amounted to 5 min for the dynamic MRA datasets. Image Quality Image quality for the assessment of the aorta by dynamic MRA amounted to 3.3/ 3.3 compared with 4.3/ 4.2 in 3D MRA (P 5 0.006; k 5 1.0/0.88). For the visceral side branches, image quality in dynamic MRA reached a value of 3.3/ 3.4 in comparison to 4.2/ 4.2 for 3D MRA (P 5 0.02; k 5 0.88/1.0). Image quality for the aorta as well as for aortic side branches, therefore, showed statistically significant improved image quality for the 3D MRA when compared with dynamic MRA. Figure 1 provides a typical example comparing image quality of dynamic MRA with 3D MRA and CT. 3D MRA showed comparable image quality as CT. Interobserver agreement proved to be excellent. Morphology of Aorta, Aortic Dissection, and Involvement of Side Branches Type and origin of AD was diagnosed correctly in both MRA datasets for all patients as compared to CTA. 507
Journal of Magnetic Resonance Imaging
FIGURE 2: Images of two patients with different flow velocities: In A, the patient shows a slow velocity of the false lumen as the contrasting of the false lumen needs more than 14 time frames between aortic root contrast and filling of the false lumen (interpolated images with a time delay of 1.6 s between images shown). The patient in B shows high velocity in the false lumen with a rapid contrasting of the false lumen (
![[MR of acute aortic dissection].](/assets/img/hold.png)
