Abstract
Purpose
Diagnostic imaging of Abdominal aortic aneurysm (AAA) almost exclusively employs CT angiography (CTA) involving X-ray exposure and contrast medium that may harm some patients. Quiescent-Interval Slice Selective MR (QISS-MR) depicts vascular anatomy without radiation or contrast medium. The diagnostic quality of QISS-MRA and CTA were compared in regard to length and diameter measurements in AAA patients. Suitability of QISS-MRA for AAA treatment planning was evaluated.
Materials and Methods
The details of 30 patients with AAA who received both a QISS-MR and CTA for a known infrarenal AAA were obtained retrospectively that was approved by the local research ethics board. Two observers analyzed each dataset in terms of image quality and determined lumen diameter and length of 15 vessel segments.
Results
Highly accurate agreement between the diagnostic scores from the two observers was achieved. There was no significant difference between CTA and QISS-MRA for all 15 measured vessels. Although information on calcification was lacking and intraluminal thrombus was visualized in only 25 patients out of 30 patients, a founded decision to carry out OR or EVAR was possible with both imaging modalities.
Conclusion
QISS-MRA presents a radiation and contrast free method for preoperative diagnostic AAA imaging. While QISS-MRA does not deliver exact information regarding calcification and thrombus formation, it does accurately allow measurement of vessel diameter and length. Therefore, it is potentially useful for EVAR planning in selected patients with impaired renal function.
Keywords
Introduction
Most decisions regarding diagnosis and therapy of abdominal aortic aneurysm (AAA) are based on computed tomography angiography (CTA), which is the most frequently used imaging technique performed before intervention. 1 However, the use of contrast medium presents a potential risk for the development of acute or chronic renal insufficiency, especially in patients with preexisting impaired renal function.1,2 In AAA patients with compromised renal function and in patients allergic to the iodinated contrast media used in CTA, magnetic resonance angiography (MRA; combined with a noncontrast CT) is an alternative imaging modality to principally decide whether patients can be treated by standard endovascular aortic repair (EVAR). Nonetheless, MRA is also considered to be an unsafe imaging technique for some patients due to the use of gadolinium-based contrast agents (group 1 GBCAs). Nephrogenic systemic fibrosis (NSF) has been associated with the use of GBCAs, and evidence indicates that GBCAs are not entirely cleared from tissues, including the brain. 3 Consequently, the development of non-contrast-enhanced MRA (NC-MRA) techniques has been pursued in recent years.
The quiescent-interval slice selective MRA (QISS-MRA) is a non-contrast-enhanced MRA technique not yet evaluated for diagnosing and planning endovascular aortic repair for AAA. The primary inquiry in AAA revision is whether EVAR is feasible, and if so, adequate imaging is required for appropriate sizing and placement of the expandable stent graft to carry out the procedure. This includes vascular measurement, calcification estimation, and thrombus volume. This pilot study was designed to compare QISS-MRA with CTA (gold standard) in planning AAA treatment.
Materials and Methods
Between April 2016 and December 2018, 35 patients (29 men and 6 women, age 65–80 years) were enrolled in this retrospective study that was approved by the local research ethics board. Inclusion criteria were an asymptomatic infrarenal abdominal aortic aneurysm in patients who had consented to data collection and for whom both CTA and QISS-MRA had been performed. All patients had a regular heart rhythm and normal kidney function.
Imaging Protocol
Quiescent-Interval Slice Selective MRA
QISS-MRA is a technique allowing vascular imaging without radiation exposure and contrast agents that was presented by Edelman et al 4 in 2010. It is an electrocardiographic-triggered image acquisition method whereby image acquisition is synchronized with the patient’s heart signal without contrast agent administration. After R wave slice-selective saturation, the radiofrequency (RF) pulse is triggered in the readout layer, and the signal intensity of the stationary background tissue is reduced.4–8 This magnetization preparation follows a quiescent interval (QI), during which no RF pulse excitation occurs. The QI ideally lies at the moment of maximum systolic blood flow in the readout layer. The image acquisition takes place during diastole with slow blood flow.4–8
All QISS-MRA examinations were performed on a 3.0 T scanner (MAGNETOM Prisma, Siemens Healthineers, Erlangen, Germany). The supine patient was covered with 18-element body coils on the abdomen and pelvis and 32-element spine coils. The following sequence parameters were used to implement the QISS pulse sequence for the acquisition of the image datasets: field of view (FOV) 400 × 260 mm, axial plane 2D, repetition time 5.75 seconds, acquisition time 5 minutes 51 seconds, flip angle 120°, and generalized autocalibrated partial parallel acquisition (GRAPPA) with an acceleration factor of 2.
Computed Tomography Angiography
All CTA examinations were performed on multidetector CT scanners with the patient in the supine position during a single breath-hold (Somatom Sensation 64; Siemens, Erlangen, Germany). The scanning parameters were as follows: slice thickness 0.6–0.625 mm, collimation 0.75–1.5, FOV ranging from 280 to 320 mm, gantry rotation time 0.42 seconds, tube voltage 120 kV, pitch 0.26. A bolus of 100 mL of iomeprol, 400 mg I/mL (Iomeron 400; Bracco, Courcouronne, France) was injected intravenously (4 mL/s). Multiplanar reconstruction images of the abdominal aorta were routinely produced in the axial, sagittal, and coronal planes with a 1-mm slice thickness.
Image Analysis
An experienced consultant vascular radiologist and an experienced consultant vascular surgeon analyzed each QISS-MRA and CTA dataset for image quality and this was controlled by another experienced consultant radiologist and consultant vascular surgeon, using a 4-point scale ranging from 0 to 3.
0 nondiagnostic,
1 moderate quality: AAA visible and measurement possible
2 good quality: AAA visible, measurement possible and thrombus volume visible
3 excellent quality: AAA visible, measurement possible, thrombus volume and calcification assessable.
For EVAR planning, the lumen diameter and length of 15 vessel segments was measured by an experienced vascular surgeon in 30 patients based on both examinations (CTA and QISS-MRA) (Figure 1). The QISS-MRA and CTA datasets were evaluated using conventional 3-dimensional (3D) reconstruction software (OsiriX MD, Pixmeo, Geneva, Swiss) for multiplanar reconstructions and maximum intensity projections to compare vascular measurements for EVAR. Fifteen segments were measured. Diameter measurements included a healthy aneurysm neck (D1), as well as left and right proximal and distal iliac arteries (D4, D5, D8, D9). Maximal AAA diameter (D2), aortic bifurcation (D3), and the respective smallest diameter of left and right external iliac arteries (D6 and D7) and common femoral artery (D10 and D11). Length measurements included the length of the proximal aortic neck (L1) and the length between the lowest renal artery and aortic bifurcation (L2), as well as the length of the left and right common iliac arteries (L3 and L4) (Figure 1).
Vessel segments measured for planning an endovascular aortic repair (EVAR).
Statistical Analysis
Statistical analysis was performed using commercially available software (SPSS, version 24; IBM Corporation, Armonk, NY, USA). The performance of QISS-MRA with CTA as reference was analyzed for the abdominal aorta and all regions in all 30 patients with a t test. A p value <0.05 indicated a significant difference. To assess interobserver variability, the agreement of image quality ratings between the readers was evaluated by using Cohen’s kappa values.
Results
The final dataset included the 30 patients who received QISS-MRA and CTA of diagnostic quality. QISS-MRA and CTA were assessed for subjective image quality and diagnostic value. All CTA exams were of good quality. Venous overlays were common in QISS-MRA but neither affected assessment of the arterial signal nor resulted in a reduction in subjective image quality. The average acquisition time for QISS-MRA, which depended on the heart rate, was 20 minutes (range 16–35 minutes) at a mean heart rate of 78 bpm. Highly accurate agreement between the 2 readers in regard to the diagnostic scores was achieved, with Cohen’s kappa values > 0.80 (Table 1), indicating that AAA can be diagnosed using QISS-MRA and thereby enable therapy planning. No significant differences between QISS-MRA and CTA were apparent for all measured parameters (Table 2).
Image Quality Rating and Agreement Between Readers (Kappa Coefficient).
Abbreviations: CT, computed tomography; CTA, computed tomography angiography; QISS-MRA, quiescent-interval slice selective magnetic resonance angiography.
Comparison Between Parameters Measured by CTA and QISS-MRA. a
Abbreviations: AAA, abdominal aortic aneurysm; CFA, common femoral artery; CIA, common iliac artery; CTA, computed tomography angiography; EIA, external iliac artery; QISS-MRA, quiescent-interval slice selective magnetic resonance angiography.
All results did not show any significance.
In 2 (6.6%) patients, the CTA as well as QISS-MRA showed angulation of the aortic neck >60° and the same patients also had common iliac angulation >90°. Intraluminal thrombus was visible in 25 (83.3%) patients imaged by QISS-MRA and in 30 (100%) by CTA. Based on both methods, 22 patients were deemed eligible for nonfenestrated EVAR. These 22 patients were treated with EVAR and the 8 patients were deemed unsuitable for EVAR (5 with aneurysm neck <1.5 cm and 2 with pelvic peripheral artery disease) were treated with open surgery (Table 3).
Comparison Between CTA and QISS-MRA: Number of Patients With Angulated Neck, Common Iliac Angulation, Visibility of Intraluminal Thrombus, and EVAR Feasibility.
Abbreviations: CTA, computed tomography angiography; EVAR, endovascular aortic repair; OR, open repair; QISS-MRA, quiescent-interval slice selective magnetic resonance angiography.
Discussion
QISS-MRA has been employed to diagnose peripheral arterial occlusive disease,4,9–14 to evaluate hemodialysis fistulas, 15 pulmonary arteries, 16 coronary arteries,2,5,17 and intracranial 18 and extracranial arteries. But the technique has not yet been applied to patients with AAAs. This study evaluates QISS-MRA for diagnosis and treatment planning of patients with infrarenal AAA (Figure 2).
Images of a patient with an abdominal aortic aneurysm using computed tomography angiography (CTA) and quiescent-interval single-shot magnetic resonance angiography (QISS-MRA). Displayed are axial images (A and B) and coronal images, showing measurement of the pelvic vessels (C and D).
Deciding whether AAA patients should be treated by EVAR or open surgery is based on preoperative imaging, whereby the CTA scan with 3D reconstruction is the commonly used imaging modality. Potentially limiting aspects of CTA are the use of iodine-containing contrast agents with attendant allergic and nephrotoxic complications and a relatively high radiation exposure of 7.5 to 13.7 mSv.19,20 Application of gadolinium-containing contrast agents used in MRAs significantly reduces the risk of contrast-induced nephropathy and the possibility of triggering anaphylactic reactions by the use of gadolinium. However, the occurrence of renal insufficiency in patients with impaired renal function resulting from the application of contrast agents and the occurrence of nephrogenic systemic fibrosis associated with gadolinium-based contrast agents group I and gadolinium deposition in the brain 3 has driven the development of non-contrast-enhanced MRA (NC-MRA) techniques in recent years. The QISS-MRA is a technique that allows vascular imaging without radiation exposure and contrast agents. Measured parameters for vessel sizing obtained through QISS-MRAs of diagnostic value showed no significant differences to those obtained by CTA. Therefore, QISS-MRA facilitates detection of abdominal aortic aneurysms and principally expedites the decision as to whether EVAR or open repair is the preferred treatment. Both methods have comparable cost. At our center, a CTA costs approximately 500 € and a QISS-MRA approximately 700 €. In a global cost comparison considering subsequent treatment of CTA associated side effects in some patients, use of the QISS-MRA is not significantly more expensive than for CTA.
There are some drawbacks to employing QISS-MRA. These include lack of information in regard to calcification and inaccurate imaging of intraluminal thrombus. To obtain this information an additional native CT scan would be helpful. QISS-MRA cannot be used for every patient. The contraindications are the same as in performing MRA in patients with pacemakers, cochlear implants, implanted insulin pumps, or ventricular assist devices. The patient must also be able to lie supine for approximately 20 minutes without movement, making compliance and physical fitness relevant. Another limitation of this method to be used for all patients is artifacts caused by possible inadequate electrocardiographic triggering, either as a result of movement due patient intolerance during data collection or due to arrhythmia (Figure 3).
Images of a patient with an abdominal aortic aneurysm of complex configuration using computed tomography angiography (CTA) and quiescent-interval single-shot magnetic resonance angiography (QISS-MRA): rated as nondiagnostic (A and B). Images of a patient with a fusiform abdominal aortic aneurysm using CTA and QISS-MRA coronal images (C and D).
The sensitivity and specificity of QISS-MRA for the primary diagnosis of AAA could not be evaluated in this pilot study since the presence of an AAA was already known. Further investigation is therefore necessary to evaluate the diagnostic potential of QISS-MRA. Here, screening, including non-AAA patients, would be useful.
Conclusion
QISS-MRA offers an alternative to CTA and CE-MRA to evaluate abdominal aortic aneurysms. QISS-MRA could be particularly useful when the administration of iodinated or gadolinium-based contrast agents is contraindicated. The method allows accurate vessel sizing and could be used in combination with ultrasonography or a non-contrast CT for EVAR planning. Thus, exact evaluation of calcification and thrombus not visualized by QISS-MRA would be possible. However, especially for treatment of complex juxarenal or suprarenal aneurysms, calcification detection is an important issue. Further studies will be initiated to determine whether QISS imaging can be used to plan such procedures. The usefulness of QISS-MRA with other non-contrast MRA techniques will also requires future evaluation.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
