Introduction
Computed tomography (CT) has dramatically changed the diagnostic approach to suspected pul- monary embolism in the last decade. Pulmonary computed tomographic angiography (CTA) is to- day an accurate test for the detection of pul- monary embolism (PE) and is gaining increased acceptance as a first-line study for diagnosing acute PE (Fig. 1) [1].
In Search of a Gold Standard
Although pulmonary angiography has traditional- ly been regarded as the gold standard for the diag- nosis of PE (Fig. 2), it is undoubtedly a flawed gold standard [2]. In the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study, the rate of non-diagnostic pulmonary angiogra- phy was 3% [3]. Isolated subsegmental pulmonary
emboli were seen in 6% of patients, and interob- server variability in the diagnosis of emboli at this level was 66%. There is accumulating evidence il- lustrating the limitations of this technique for the unequivocal diagnosis of isolated peripheral pul- monary emboli. Two recent analyses of the inter- observer agreement rates for the detection of sub- segmental emboli with selective pulmonary an- giography ranged from only 45% to 66% [4]. Given such limitations, use of this method as an objective and readily reproducible tool for the verification of findings from competing imaging modalities re- garding the presence of PE seems questionable, and the status of pulmonary angiography as the standard of reference for diagnosis of PE is in doubt.
The use of nuclear medicine imaging, once the first study in the diagnostic algorithm for PE, is al- so in decline [5], owing to poor interobserver cor- relation [6] and the high percentage of indetermi- nate studies (73% of all studies performed [3]).
III.4
MDCT for Diagnosis of Pulmonary Embolism:
Have We Reached Our Goal?
Andreas F. Kopp, Axel Küttner, Stephen Schröder, Martin Heuschmid and Claus D. Claussen
Fig. 1. Clinical algorithm for patients with clinical suspicion of pulmonary embolism (PE). CTA, computed tomographic angiography;
MDCTA, multi detector CTA
Newer technologies in nuclear medicine, such as single-photon emission CT [7], and revised crite- ria for the interpretation of ventilation-perfusion scans [8] might help decrease the rate of indeter- minate scintigraphic studies. Interobserver agree- ment for CT is considerably better than that for scintigraphy. Blachere et al. [6] found an interob- server agreement for the diagnosis of PE of 0.72 for CTA and only moderate agreement for ventila- tion-perfusion lung scanning (0.22).
Furthermore, with the advent of 16-slice multi- detector-row CT (MDCT), traditional technical limitations of CT in the diagnosis of PE appear to have been successfully overcome (Fig. 3) [9, 10].
Therefore, in more recent studies, clinical follow- up with CT has been chosen as the gold standard over pulmonary angiography [4, 11]. It is accepted that PE occurring within 3 months of a CT pul- monary angiogram that showed no signs of PE most likely indicates an initial false-negative ex- amination [12, 13]. This approach may miss small emboli that do not lead to the presentation of symptoms, but such emboli are unlikely to be clin- ically significant if no evidence of recurrent thromboembolic disease has been noted 3 months or longer after initial presentation [9, 14, 15].
Contrast Medium Injection
The advent of MDCT has necessitated an exten- sive revision of injection protocols for contrast material (Tables 1 and 2). Faster scanning times allow acquisition during maximal contrast en- hancement of pulmonary vessels but pose an in- creased challenge for the precise timing of the contrast material bolus. Strategies that have the potential to improve the delivery of contrast ma- terial for high and consistent vascular enhance- ment during pulmonary CTA currently include use of high-concentration contrast media (400 mg of iodine/ml, e.g., Iomeron 400) together with au- tomated bolus-triggering techniques [15]. It is es- sential to achieve a high level of iodine concen- tration rapidly. This is particularly important us- ing scanners that have a large number of detector rows. A given volume of high-concentration con- trast medium will produce higher attenuation in the arteries than the same volume of standard contrast medium (300 mg of iodine/ml). The use of high-concentration contrast medium has the advantages of facilitating post-processing, allow- ing depiction of smaller vessels and reducing the overall amount of contrast medium required by approximately 30%. Saline chasing has been used for the effective utilization of contrast material and for the reduction of streak artifacts arising from the superior vena cava. Multiphasic injection protocols have proved to be beneficial for general CTA, but so far have not been scientifically evalu- ated for the diagnosis of PE.
Negative Predictive Value
Patient treatment after a CT pulmonary an- giogram that shows no signs of PE is controver- sial. In a recent study, Kavanagh et al. [16] reported Fig. 2. A 29-year-old male patient with acute chest pain. Pul-
monary CTA with low-dose settings was performed to rule out pul- monary embolism. Despite a voltage of only 80 kV and a tube cur- rent of 100 mA, the image quality is still diagnostic (collimation 16 ×0.75 mm, 80 ml Iomeron 400)
Fig. 3. A 50-year-old male patient with chest pain: transverse con-
trast-enhanced 16-row multidetector scan with submillimeter col-
limation (16 ×0.75 mm, 80 ml Iomeron 400) reveals an isolated sub-
segmental embolus (arrow)
a negative predictive value of 99% for MDCT pul- monary angiography for subsequent clinically sig- nificant PE. An MDCT pulmonary angiogram with no signs of PE correlates with a low risk of subse- quent clinically significant PE.
Outlook
Recently, a new generation of MDCT scanners has been introduced with 64-row technology. Together with shorter gantry rotation, this significantly fur- ther increases the volume speed of MDCT. The SIEMENS Sensation 64 scanner with a rotation time of 0.33 s also uses oversampling in the z-di- rection for better spatial resolution (Fig. 4). This
allows isotropic 0.4-mm spatial resolution even at maximum table speed (64×0.6 mm collimation), i.e., the scan of the entire thorax at submillimeter resolution takes less than 5 s. To date, no study has been published for 64-row pulmonary MDCT an- giography. However, preliminary results demon- strate that the increased spatial resolution facili- tates delineation of even the smallest branches in the periphery of the lung (Fig. 5). Together with the very short scan time due to the better temporal resolution, image quality and robustness of image quality of CTA will be improved. This will also af- fect the sensitivity and negative predictive value of CTA. In the future, 64-row pulmonary CTA might very well become the new gold standard for the de- tection of PE.
Table 1. Protocols for suspected acute pulmonary embolism
4-slice 1 6-slice scanner
kV/effective mA/rotation time (s) 140/100/0.5 80–120/130/0.5
Detector collimation (mm) 2.5 0.75
Feed/rotation (mm) 15 (ca-cr) 15.0 (ca-cr)
Slice thickness (mm) 3 1.5
Increment 2 1
Kernel B30f B30f
IV contrast volume
(400 mg I/mg)/saline 120/30 ml 80/30 ml
Injection rate
3 ml/s 3–4 ml/s
Scan delay (s) 25–30 s (bolus) 25–30 s (bolus)
Table 2. Protocols for pulmonary embolism (PE) with a 4-row detector scanner. With four-row technology two separate protocols for evaluation of PE are recommended. A high-speed protocol for dyspneic patients with suspicion of acute PE and a high-resolution proto- col with the thinnest collimation, allowing the most detailed display of the pulmonary arteries
Acute PE Chronic PE
kV/effective mA/rotation time (s) 120/100/0.5 120/130/0.5
Detector collimation (mm) 2.5 1
Feed/rotation (mm) 15 (ca-cr) 6.0 (ca-cr)
Slice thickness (mm) 3 1.5
Increment 2 1
Kernel B30f B30f
IV contrast volume
(400 mg I/mg)/ saline 120/30 ml 120/30 ml
Injection rate
3 ml/s 3–4 ml/s
Scan delay (s) 25–30 s (bolus) 35–50 s (bolus)
Conclusions
With 16-32-, and 64-row MDCT technology, past limitations of CT for the diagnosis of PE have been effectively overcome. CT is now an attractive means for establishing a safe, highly accurate, and cost-effective diagnosis of PE.
versampling 0,6 mm
Z
32 Slice Detector 64 Slice DAS
0,6 mm O
