الفهرس 
Anatomy of the Dual Pulmonary CirculationOnly 14 pages are availabe for public view
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Abstract
Pulmonary embolism (PE) is a common diagnostic problem, particularly in hospitalized patients, with significant morbidity and mortality; the latter reaching 30% in untreated cases. (O’Neill et al, 2004) & (Ryu et al., 2001)
Objective testing for PE is crucial, because clinical assessment alone is unreliable and the consequences of misdiagnosis are serious. Failure to diagnose PE is associated with high mortality and incorrect diagnosis of the condition unnecessarily exposes patients to the risks of anticoagulant therapy. (Kearon, 2003)
There are several diagnostic imaging modalities available to aid the diagnosis of PE, each with its own advantages & disadvantages.
Chest x-ray may identify mimics of PE, such as pneumonia, pneumothorax or congestive heart failure. However, it may be normal despite PE and patients may have concomitant PE and pneumonia or PE and congestive heart failure. (Schoepf , Goldhaber & Costello, 2004)
In patients with co-existing clinical DVT, leg ultrasound
as the initial imaging test is often sufficient to confirm VTE. However, a single normal leg ultrasound should not be relied on for exclusion of sub clinical DVT.
Isotope lung scanning may be considered as the initial imaging investigation providing : facilities are available on site, the chest radiograph is normal, there is no significant symptomatic concurrent cardiopulmonary disease, and standardized reporting criteria are used. A non-diagnostic
result should always be followed by further imaging.
where isotope lung scanning is normal, PE is reliably
excluded but a significant minority of high probability
results are false positive.
(BTS guidelines for the management of suspected acute pulmonary embolism, 2003)
MRA avoids the use of iodinated contrast material & lacks ionizing radiation.However, it is expensive, not readily available & critically ill patients are difficult to monitor. MRI may not be possible in dyspneic and claustrophobic individuals & is contraindicated in selected patients who may be at risk for PE, including those with pacemakers.(Kohli, 2002) & (Webb & Higgins, 2005)
Pulmonary angiography was classically considered the gold standard technique for the diagnosis of PE. On the other hand it is invasive, labour intensive, costly & there is high interobserver variability for the detection of sub-segmental emboli. (Schoepf , Goldhaber & Costello, 2004)
CT is readily available at most institutions and is rapidly becoming the first-line imaging test for the assessment of patients with suspected acute PE. (Quiroz et al., 2005)
With spiral CT, thrombus is directly visualized, and both mediastinal and parenchymal structures are evaluated, which may provide important alternative or additional diagnoses. (Schoepf, Goldhaber & Costello, 2004)
However, conventional single-slice spiral CT has insufficient sensitivity for isolated peripheral PE.
(Quiroz et al., 2005)
The clinical importance of detecting and treating peripheral PE remains uncertain. (Goodman, 2005)
The use of multi–detector row CT significantly improves pulmonary arterial visualization in the middle and peripheral lung zones. Narrower collimation improves the quality of the transverse and multiplanar images, and faster acquisition times make the examination better tolerated
and improve contrast and spatial resolution, which improves detection of peripheral pulmonary emboli.
(Raptopoulos & Boiselle, 2001)
The most recent generation of multidetector-row spiral CT scanners appears to outperform competing imaging modalities for the accurate detection of central and peripheral PE. (Schoepf, Goldhaber &Costello, 2004)
The addition of CT venography to helical CT pulmonary angiography allows for assessment of venous thrombo- embolism in general in addition to PE.(Webb & Higgins, 2005)
CT venography has several advantages over ultrasonography; it is not operator dependent and it can be used to evaluate for clot in areas that are not accessible to ultrasound or difficult to evaluate. This includes the deep pelvic veins (a common source of thrombus that may embolize to the lungs) and the region around the adductor canal. Ultrasound can be of limited value in patients who are obese or who have had recent surgery.
(Pretorius & Solomon, 2006)
The clinical validity of using a CT scan to rule out PE is similar to that reported for conventional pulmonary angiography. Results suggest that withholding anti-coagulant therapy after a negative CT scan appears to be safe. Additional imaging for ruling out PE is not warranted. This strategy may minimize radiation exposure, invasive procedures, and health care costs. (Quiroz et al., 2005)
The major draw backs of spiral CT pulmonary angiography are the use of intravenous contrast material and radiation exposure.
Conclusion
Spiral CT pulmonary angiography has become an attractive means for a safe, highly accurate, cost-effective diagnosis of acute PE and may provide alternative diagnoses and explanations for symptoms in the absence of PE. Multidetector-row spiral CT technology has overcome past limitations of CT and has become the first-line modality for imaging in patients with suspected acute PE.
New-generation multidetector-row spiral CT scanners now challenge catheter pulmonary angiography, once the standard of reference, for the accurate detection of PE.