V. MAGNANOSANLIO, E.M. DIMAGGIO
Introduction and Technical Principles
In 1988 Siemens Medical Systems introduced the first single-section helical (spiral) computed tomography (SSCT). In 1992, Elscint introduced a dual- section helical scanner that is considered to be the first multisection CT. This technology was improved in 1998 with the advent of quad-section technolo- gy. This technology was up to four times faster than conventional SSCT. The next step was to increase the rotation speed to two revolutions per second.
This enabled multi-detector-row CT scanners to operate up to eight times faster than SSCT. Nowadays the principal equipment manufacturers are able to produce multi-detector-row CT scanners (MDCT) carrying out 32 or 64 slices per second. These two improvements have combined to increase scan- ning speed by a factor of 30 or 60 over most conventional SSCT scanners [1].
The benefits of MDCT in comparison with SSCT are evident: (a) the volume scan can be performed much faster, resulting in improved temporal resolu- tion (time to cover the entire volume) and consequently in reduced motion artefacts caused by voluntary and involuntary movement; (b) breath-holding times are reduced; (c) volume coverage is increased, permitting imaging of the entire aorta or the aortoiliac arteries and the peripheral vessels of the lower extremity within one acquisition [2]; and (d) spatial resolution along the longitudinal axis of the patient (z-axis) is improved, as is guaranteed by the thinner sections (1 mm). All these factors concur to improve the tempo- ral, spatial, and contrast resolution of the images and significantly increase the diagnostic accuracy of the examination. MDCT technique is applied to a wide spectrum of clinical indications which require long coverage and or multiphase studies. Another important application of MDCT is the multipla-
Struttura Complessa di Radiologia Diagnostica ed Interventistica, Azienda Ospedaliera Garibaldi-Nesima, Catania, Italy
nar reformation (postprocessing) in selected patholog y and vir tual endoscopy. Cardiac imaging is currently considered the most technologically advanced application of MDCT. With modern MDCT it is possible to obtain a near-isotropic resolution along the z-axis. These significant improvements in z-axis resolution are obtained on condition that the sections are acquired at a fine thickness and reconstructed at fine intervals (less than the section). A true isotropic spatial resolution obtained by MDCT means that we can obtain a cubic voxel, so that the image has equal resolution in any plane of the scanned volume. This development has facilitated three-dimensional (3D) image reconstruction [3]. Shaded surface display (SSD), maximum intensity projection (MIP), volume rendering (VR), and virtual angioscopy (Fig. 1) are the most commonly used 3D reconstruction techniques. In vascular imaging, MIP and VR obtained from MDCT angiography can enable visualisation that is equal or superior to that obtained w ith catheter ang iog raphy.
Postprocessing can be performed quickly and easily to obtain a two-dimen- sional MIP image. A 3D effect can be achieved with stepwise rotation of the object. VR has a high diagnostic accuracy and is the method most often used for reconstruction of CT angiographic image data. These techniques therefore are becoming the initial or the only imaging methods used for surgical plan- ning in emergency vascular conditions. CT angiography, which was limited to a small volume with SSCT, can be applied extensively to all areas with MDCT.
The two-dimensional multiplanar reconstruction (MPR) images and 3D image quality are improved with reduced image artefacts.
a b
Fig. 1. aStanford type B aorta dissection. Axial scan at the level of aortic arch shows the intimal flap tear that characterises communicating dissections. b Virtual angioscopy in Stan- ford type B aorta dissection.Virtual angioscopy shows the true lumen on the left side and the false lumen on the right, separated by the intimal flap that shows a large tear
Clinical Applications
SSCT has been used for evaluation of the aortic aneurysms since its debut.
This technique provides all row data during a single breathhold, minimising motion artefacts due to respiratory activity and reducing those due to car- diac activity. Moreover, all images are obtained during optimal contrast enhancement. In large prospective studies regarding the evaluation of aortic dissections, which are considered the most difficult diagnosis within acute aortic pathology, an overall sensitivity better than 95% has been reported, with an overall accuracy of 96% [4]. However, SSCT has some limitations: it cannot cover a large volume in a short time and has a lower longitudinal spa- tial resolution [5]. This problem is particularly felt in the case of aortic dis- section in the assessment of the real extent of the dissection (supra-aortic branches, iliac arteries). These limitations have been overcome with the introduction of MDCT, which has the capability of rapidly scanning large longitudinal volume with high z-axis resolution. MDCT angiography can be performed more efficiently because scanning is done more quickly and vas- cular contrast enhancement is improved. The diagnostic approach for patients with suspected thoracic or abdominal aortic aneurysms has changed substantially in the last few years. Digital subtraction angiography for the evaluation of the thoracic and abdominal aorta has given way to sonography, MR angiography, and CT angiography. Both preoperative plan- ning and postoperative evaluation can be performed with a non-invasive technique like MDCT.
Thoracic Aortic Dissection
Thoracic aortic dissection (AD) is the most frequent cause of aortic emer- gency and requires immediate diagnosis and treatment. It may be described as acute or chronic, depending on the timing of its clinical manifestation. In addition, dissection is classified according to the extent of involvement of the thoracic aorta, determined using the Stanford system into type A, involving the ascending aorta or the aortic arch, or type B, involving the descending aorta distal to the left subclavian artery. Acute type A dissection should be repaired immediately, as any delay in diagnosis and treatment can be fatal. In contrast, type B dissection is normally treated medically, except in the case of abdominal organ ischaemia, which can require surgery. About one-third of patients with aortic dissection show signs indicative of systemic involvement [6]. In this group of patients it is important to evaluate the entire aorta so as to determine the distal extent of the dissection.
Ischaemic complications related to the main abdominal arterial branches are best detected with arterial phase imaging.
SSCT and MDCT permit differentiation between proximal aortic dissec- tion (type A) and distal aortic dissection (type B) with a sensitivity and specificity of nearly 100% [4]. Limited to the thoracic aorta, the examination begins with an unenhanced CT scan, useful for diagnosing intramural haematoma, displacement of intimal calcifications, or acute haemorrhage.
The intravenous contrast CT scan is performed to include an area from the supra-aortic branches to iliac arteries. With the new generation of 16-section MDCT it is possible to cover the entire volume with a collimation of 1 mm, in a single continuous acquisition and in a single breathhold. A classic diag- nosis of aortic dissection is based on the detection of an intimal flap in the thoracic aorta due to displacement of the intima (Fig. 2); however, atypical dissections caused by intramural haematoma and penetrating atherosclerot- ic ulcer may manifest similar signs.
The most important diagnostic gain deriving from the clinical introduc- tion of MDCT is the high quality of MPR (Fig 3). This is particularly true in the presence of aortic dissection type B, or in extension along the descend- ing aorta of a type A dissection. In these cases only a high quality, sagittal oblique reformatted image of the thoracic aorta obtained with a MDCT scanner can identify the origin of the intimal flap, proximal or distal to the left subclavian artery (Fig. 4). Batra et al. [7] have described a variety of pit- falls and artefacts that mimic aortic dissection at SSCT angiography, poten-
Fig. 2.Stanford type A aortic dissection. MDCT shows the intimal flap in the ascending aorta
tially leading to confusion with flap dissection. They conclude that CT angiography of the thoracic aorta remains the imaging technique of choice for evaluating patients with suspected or known aortic dissection. One of the causes of artefacts in ascending thoracic aorta is related to the aortic wall motion. The recent introduction of prospective and retrospective electrocar- diographically (ECG) assisted imaging, normally used in cardiac imaging, has been used by Roos et al. [8] to determine the influence of ECG-assisted MDCT to reduce motion artefact of the ascending aorta. Their results showed a significant reduction of motion-related artefacts for the entire tho- racic aorta, especially the ascending portion.
Fig. 3.Stanford type A aortic dis- section. MPR shows aneurysm in the ascending aorta with aortic dissection. The intimal flap is proximal to the anony mous trunk
Fig. 4.Stanford type B aorta dissec- tion. MPR shows intimal flap between true and false lumen, involv- ing the descending aorta distal to the left subclavian artery
Abdominal Aortic Aneurysm
Aneurysmal dilatation of the infrarenal aorta is defined as a diameter of more than 29 mm. By this criterion, 9% of all people older than 65 have an abdominal aortic aneurysm (AAA) [9]. Rupture of an AAA is one of the most urgent vascular conditions and requires rapid intervention. In autopsy stud- ies reported by Lederle et al. [10], the 1-year incidence of AAA rupture according to initial diameter was 9.4% for diameters of 5.5–5.9 cm, 10.2%
for diameters of 6.0–6.9 cm, and 32.5% for diameters of 7.0 cm or more. A diagnosis of ruptured AAA may be made on the basis of a non-enhanced CT scan that shows an AAA with adjacent periaortic haemorrhage that extends into the perirenal and pararenal spaces of the retroperitoneum. Open surgi- cal repair is increasingly being replaced by endovascular repair, which is more efficient and has lower associated morbidity and mortality [11]. Lee et al. [12] found that patient eligibility for endovascular repair depends on the anatomy of the proximal aneurysm neck. Great effort has been put into the development of specialised software MDCT for planning endoluminal treat- ment of AAA. Most of the measurements required for determination of the optimal dimensions and type of aortic stent-graft are now obtained with MDCT image reconstruction.
Endovascular Stent-Graft Evaluation and Assessment of Complication
The current standard treatment for AAA is open surgical repair, which is associated with a low overall risk. The reported mortality rate associated with elective surgical repair ranges from 1.4% to 6.5% [13]. However, in high-risk patients with a comorbid medical condition such as severe cardio- vascular, pulmonary, or renal disease, the risk of death during surgical repair of AAA is considerably higher (5.7–31%) [14]. In an attempt to reduce risk in these patients, less invasive methods of repair have been considered.
Treatment of AAA with transfemoral placement of an endovascular stent- graft is increasingly being used as an alternative to surgical repair [15]. In 1991, Parodi et al. [16] reported the transfemoral placement of non-bifurcat- ed stent-grafts for treatment of AAA in a series of human patients. Although conventional arteriography has long been considered the modality of choice for arterial imaging, there are several reasons why helical CT angiography may be superior in the assessment of the abdominal arteries. First, the acqui- sition of volumetric data with helical CT allows clear delineation of the tor- tuous aorta and branch vessels and of adjacent aneurysms and pseudo- aneurysms. Second, blood pool imaging with the intravenous administration of contrast material allows visualisation of true and false luminal flow chan-
nels, intramural haematomas communicating with the aortic lumen, and slow perigraft flow around aortic stent-grafts. Finally, the aortic wall and non-communicating intramural collections can be directly visualised with CT angiography. These advantages of SSCT with the fast and high quality of MDCT angiography images today confine conventional angiography to interventional procedures only. Pre-procedural assessment of endoluminal stent-graft therapy with MDCT of the length and tortuosity of the proximal neck of the aneurysm, mural thrombus irregularity, and inferior mesenteric artery patency may help predict complications such as endoleak, shower embolism, and colonic ischaemia. Use of high quality MIP images often clar- ifies the complex anatomy of the tortuous aorta and branch vessels by show- ing the precise intravascular position and configuration of the stent-graft, whilst axial source images accurately demonstrate leaks and the patency of the stent-graft. Various complications may occur after treatment with endovascular stent-graft, and MDCT is fast, minimally invasive, and is con- sidered the gold standard procedure for the assessment of such common (endoleak, graft thrombosis, graft kinking; Fig. 5) and rare complications (pseudo-aneurysm caused by graft infection, graft occlusion, shower embolism, perforation of mural thrombus by means of inadvertent penetra- tion of the delivery system, colon necrosis, aortic dissection, or haematoma at the arteriotomy site).
Fig. 5.Aortic aneurysm treat- ed by stent-graft. Sagittal MIP image 6 months after therapy shows severe angulation of the aortic stent-graft
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