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CT
of the Kidney
1.
I am in a large practice with multiple locations and multiple protocols
for different scanners. In some locations, we do our enhanced abd/pelvic
scans (for misc. indication such as abd. pain) in one shot and often
get the kidneys in the corticomedullary phase, known to have a lower
sensitivity for renal lesions. In other locations, we routinely do delayed
scans through the kidneys. There is an effort to standardize our practice
and do all abd/pelvic CTs in one shot without delays. I am worried we
are doing a disservice to our pts and referrers by degrading the quality
of the interpretation of the kidneys. Do you have any thoughts on this
or any advice?
| Answer:
With new MDCT scanners, it is not uncommon to get images in the
CM phase. Although, one can miss small lesions, they are usually
benign cysts. A key to reviewing CM phase images is a wide window/level
such as 550/50. We also will get selected delays in cases with suspected
renal pathology, or if something is notes on the CM phase. Please
note that in cases of suspected renal disease, we also do 4 minute
delays. |
2.
In patients with renal mass (as opposed to hematuria) what is the value
of delayed images in the diagnostic/staging CT examinations? Isn't a
3 phase scan (unenhanced, corticomedullary and nephrographic) sufficient?
| Answer:
The value of late phase imaging in this case is less than in hematuria,
but may help define the presence of calyceal invasion. If you have
an obvious tumor, then staging is done with arterial and venous
phase as you note. |
3.
In your article "Current Concepts in the Diagnosis and Management
of Renal Cell Carcinoma: Role of MDCT and 3D CT" (in Radiographics
2001; 21: S237-254) you state "the nephrographic phase is the most
valuable for detecting renal masses and characterizing indeterminate
lesions…This phase is best imaged after a scanning delay of at
least 80 seconds and lasts up to 180 seconds after the start of injection.
The renal parenchyma enhances homogeneously, allowing the best opportunity
for discrimination between the normal renal medulla and masses."
However, in you Siemens Sensation 16 protocols for kidney (mass and
hematuria) you image in the arterial and excretory phase. Shouldn't
the nephrographic phase be added or replace the arterial phase?
| Answer:
We like the arterial phase because we are able to get vascular mapping
and look at vascularity of tumors. For staging renal tumors we also
routinely get nephrographic phase images at 60 seconds. Excretory
phase is necessary at 4 to 5 minutes to look at the renal pelvis
and calyces, as well as finding small hypovascular tumors, or in
cases of inflammatory disease. |
4.
If urology needs to r/o renal mass, suspected as being a tumor:
a. should we do a noncontrast sequence. Your protocol says not, but
urology has specifically asked me to revisit this.
b. do you see it as important to see increased vascularity in the arterial
phase as well? Should we add a 3rd sequence at 25-30 seconds?
| Answer:
A non-contrast CT is indeed needed to determine the presence of
enhancement and r/o a high density renal cyst. They seem to be more
common lately. Arterial phase is best for neovascularity, and venous
phase best for renal vein. Late phase is best to r/o TCC, as you
know. I will repost the kidney protocols and make it clearer. Please
note that if a prior CT or US shows a definite tumor, then in those
cases, a non-contrast study is not needed. |
References
RH Cohan, LS Sherman, M Korobkin, JC Bass and IR Francis
Renal masses: assessment of corticomedullary-phase and nephrographic-
phase CT scans Radiology 1995; 196: 445-451.
- Summary:
Helical CT was used compare corticomedullary phase (CMP- 40 seconds
after initiating contrast infusion) and nephrographic phase (NP- following
CMP) for detection and characterization of renal masses in 33 patients.
While 111 lesions in the medulla were identified on NP images, only
25 lesions were identified on CMP images. Addition of NP images decreased
false negatives and false positives.
Birnbaum
BA, Jacobs JE, Langlotz CP, Ramchandani P. Assessment of a bolus-tracking
technique in helical renal CT to optimize nephrographic phase imaging.
Radiology 1999; 211:87-94.
- Summary:
Bolus tracking was used during CT of the kidneys in 75 patients, and
images were reviewed to determine the onset of the nephrographic phase
by identifying homogeneous enhancement of the kidney parenchyma. Contrast
(150 ml of 60%) was infused at 2 cc/second for patients with 2 kidneys
and 3 cc/second in those with a previous nephrectomy (100 cc). The
results revealed that the onset of the nephrographic phase ranged
from 60 to 136 seconds, with a mean of 89 seconds. Parameters which
impact the onset of nephrographic phase include patient age, volume
of contrast material and injection rate.
Catalano
C, Frailoi F, Laghi A et al. High-resolution MDCT in the preoperative
evaluation of patients with renal cell carcinoma. AJR 2003; 180; 1271-1277.
- Summary:
Forty patients with suspected renal cell carcinoma (RCC) were evaluated
with MDCT, all of whom had RCC confirmed. The protocol was as follows:
Four slice MDCT was conducted with 4 x 2.5 mm unenhanced sequence
(5 mm slice thickness and reconstruction interval). The contrast enhanced
acquisitions included 4 x 1 mm collimation, 1.25 mm slice thickness
with 1 mm reconstruction interval. Patients were administered 140
ml of 350 mgI/ml contrast material at 4 mL/second. Arterial phase
imaging was initiated at 22 seconds and parenchymal/venous phase at
50-60 seconds, with excretory phase imaging at 5 minutes.
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