Computed Tomography Protocol for the
Pre-operative Evaluation of Bronchogenic
Carcinoma
Helical CT has become the standard for
evaluation of the chest in patients with bronchogenic carcinoma. Continuous acquisition of
volumetric data during a single breath-hold is a major advance from conventional CT which
suffered from respiratory inconsistencies. The volumetric data collected during helical
acquisition can be constructed with overlapping transverse sections in order to minimize
the partial volume effects that limited evaluation of structures straddling two
conventional CT slices.
General guidelines for the acquisition of a spiral CT scan
of the chest include scan parameters of kV 120-135 and mAs of 140-220
(adjusted for specific scanner and patient weight) [54,103]. The scan should
start above the superior margins of the clavicles above the lung apices. The
inferior extent of the scan should include the adrenal glands [54]. Imaging
though the remainder of the liver is performed at some centers [60]. Slice
thickness on multidetector scanners should be 5 mm or less.. Some centers also perform additional thin slice images (1.5 mm) through
the lesion.
Prior to initiation of the study, the technologist
determines how long the patient is capable of holding their breath. Attempts are made to
perform the scan during a single breath-hold if the patient can comply. If not, the exam is performed in sections with overlaping margins to avoid gaps in the anatomic information collected. A high-resolution
algorithm is recommended for reconstruction of lung windows [54], with a
standard reconstruction algorithm for mediastinal, hilar, and soft tissue
evaluation.
Intravenous contrast is used for vascular opacification by
many insititutions [60], except in patients who have had an adverse reaction to contrast
material, or in those patients with an elevated creatinine (greater than 1.8).
Contrast is administered through an antecubital vein cannulated with an 18 to 20 gauge
intravenous cathether, and the use of a power injector at a rate of 2.0 to 2.5 ml/sec, for
a total volume of 60-120 ml. Varying scan delays between 25 to 40 seconds have been used.
Alternatively, a smart prep program can be used to determine the optimal time to initiate
scanning. Perivenous artifacts due to marked differences in density between the contrast
agent and surrounding structures can sometimes be a problem-particularly in evaluation of
structures in close proximity to the superior vena cava or brachiocephalic vein (when a
left arm injection is performed). Some authors advocate using contrast which has been
diluted to produce a concentration of 150-mg I/ml in order to decrease these perivenous
artifacts [95]. Some authors advocate that a non-contrast exam of the adrenals be
performed prior to the contrast study so that adrenal masses detected during the exam can
be better characterized and avoid the need for follow-up CT or chemical shift MR imaging
[109].
The use of intravenous contrast material is not universal.
Although the use of contrast does aid in evaluation of the hila for adenopathy, some
authors feel that hilar nodes are unimportant because the presence of hilar adenopathy
does not preclude resection in the majority of cases [30]. It is generally agreed that the
use of contrast material does not contribute much to the assessment of mediastinal nodes,
although a recent article suggested improved detection of enlarged mediastinal nodes with
the use of I.V. contrast [104]. Unfortunately, pathologic analysis of the enlarged
nodes was not preformed in this study, and no change in patient stage was demonstrated. In
another recent article, when compared with a non-contrast examination, contrast CT
resulted in a change in stage in 5% of patients, however, in no case was patient
management altered [114].
At our institution, images are viewed at the workstation
in a cine or stacked format which has been shown to improve nodule detection [96]. We find
that the cine viewing is also superior for differentiating lymph nodes from volume
averaging through vascular structures. All images are viewed in soft tissue, lung, and
bone windows. Where appropriate, liver windows are also viewed. |
