Evaluation of Abnormal Isotope Retention in Left Kidney on Bone Scan
The next step in evaluating a woman with abnormal retention of isotope in the left kidney on a bone scan should be a renal ultrasound with color Doppler to assess for hydronephrosis and other structural abnormalities. 1
Rationale for Renal Ultrasound
- Ultrasound is the best initial imaging modality for evaluating abnormal renal findings as it can identify hydronephrosis, which is the most likely cause of abnormal isotope retention on bone scan 1, 2
- Renal ultrasound with color Doppler provides both anatomic and functional information without radiation exposure or contrast administration 3
- Ultrasound can effectively differentiate between obstructive and non-obstructive causes of hydronephrosis, which is crucial for determining subsequent management 1, 2
Key Features to Assess on Ultrasound
- Presence and grade of hydronephrosis (dilation of the renal pelvis and calyces) 1
- Kidney size and echogenicity to differentiate between acute and chronic kidney disease 2, 4
- Evaluation for urolithiasis, which could be causing obstruction and abnormal isotope retention 1
- Assessment of ureteral jets using color Doppler to evaluate for distal obstruction 1
- Identification of any renal masses or cystic lesions that might be causing obstruction 5
Advantages of Ultrasound as First-Line Imaging
- Non-invasive and does not expose the patient to ionizing radiation 1
- Does not require contrast administration, avoiding potential nephrotoxicity 1, 6
- Highly sensitive for detecting hydronephrosis, with nearly 100% sensitivity for significant obstruction 1
- Can be performed rapidly and is widely available 2
- Cost-effective compared to other imaging modalities 2, 5
Follow-up Imaging Based on Ultrasound Findings
If ultrasound shows hydronephrosis:
- CT urography (CTU) would be the next appropriate step if renal function is adequate, as it provides detailed anatomical information about the entire urinary tract 1
- MR urography (MRU) should be considered if renal function is impaired, as it avoids iodinated contrast 1
If ultrasound is normal or inconclusive:
- Scintigraphic diuresis renography (DRG) should be considered to differentiate between non-obstructive hydronephrosis and true functional obstruction 1
- This uses nuclear medicine with diuretic administration to demonstrate flow during perfusion, extraction, and excretion phases 1
Common Pitfalls to Avoid
- Relying solely on bone scan findings without further imaging evaluation, as the abnormal isotope retention is nonspecific 1, 7
- Assuming hydronephrosis is always due to obstruction, as it can also occur in non-obstructive conditions such as vesicoureteral reflux 1
- Failing to correlate imaging findings with clinical and laboratory data, which is essential for accurate diagnosis 1
- Delaying evaluation, as progressive dilation of the upper urinary tract can lead to acute kidney injury and permanent nephron loss if not corrected 1