Sex: male
Age: 70 years
History
Non available.
Laboratory data
Creatinine value of 7 mg/dl and creatinine clearance of 7.8 ml/min.
Physical findings
None available.
Case text
Seventy year old male with a history of renal failure due to nephroangiosclerosis.
Image 1-2
Ultrasonography of the kidneys.
Image 3-6
MRI.
Four selected images obtained at time 0, 25 s, 45 s and 360 s following the injection of paramagnetic contrast.
Image 7-8
MRI, time intensity curves.
The time intensity curves are generated using the Region Of Interest (ROI) method at the level of the cortex, the medulla and the excretory system. A decrease of the intensity peak and a delay of the time to reach the peak of the signal intensity in the cortex is measured.
Image 1-2
1. What are the findings on ultrasound?
Ultrasound shows bilateral decrease of renal size with longitudinal diameter of 9 cm on the right side and 8.6 cm on the left side.
2. What is the most appropriate imaging modality to investigate the renal function in this type of patient?
Today nuclear medicine is the most appropriate method to evaluate the renal function.
However the evaluation of renal morphology is limited with nuclear medicine due to the limited resolution of this technique.
Nowadays magnetic resonance imaging is able to give both morphological and functional information of the kidneys and this capability is extremely interesting in the study of medical nephropaties.
3. Has the paramagnetic contrast agent used in the imaging of the kidney any influence on the renal parenchyma, mainly in patients with renal
failure?
Extensive data from the literature have documented that the paramagnetic contrast agents used for the imaging of the kidney have no side effects on the renal parenchyma, even in patients with impaired renal function. This is an important advantage in comparison with the iodinated contrast media which can not or have to be used with caution in patients with impaired renal function.
4. What are the imaging parameters to perform a morpho-functional examination of the kidney with a 1.5 T MR unit.
To obtain morphological and functional information of the kidney a turbo gradient echo sequence has to be performed. This sequence allows to obtain a series of images with a high temporal resolution for the time needed to evaluate the renal function. The technical parameters are the following:
- sequence: Turbo Fast Field Echo
- repetition time: 12 ms
- echo time: 5 ms
- flip angle: 25°
- number of averages: 2
- matrix size: 128 X 256
- temporal resolution: 9 s (or less)
- total scanning time: 6 min.
- bolus injection of 0.1 mmol/Kg of Gd-DTPA-BMA (Omniscan) at 2 ml/s
Image 3-6
5. What are the findings?
The images show the kidneys before the arrival of the contrast agent, in the cortical phase, in the medullary phase and in the excretory phase. Image 2 shows a faint corticomedullary differentiation due to low increase in signal intensity of the cortex. Image 4 shows high signal intensity in the calyces and pelvis (in normal patients the signal is low due to the T2 effect of the concentrated contrast agent).
Image 7-8
6. Should the signal intensity be measured only in the kidney or a measure of a reference tissue is recommended?
For clinical purposes the simple and immediate measurement with a ROI of the signal intensity in different sites of the kidney (cortex, medulla, excretory system) is generally sufficient. For more exact evaluation a reference measurement is recommended (for example the psoas muscle).
7. What are the expected results between the values of the cortical phase of this patient and a patient with normal renal function?
Nephropathic patients usually have decreased peak of cortical signal intensity and a variable degree of delay of time to peak in the cortex. Lack of inversion of the signal intensity in the inner medulla and the excretory system is invariably noted in patients with renal failure, also with mild renal insufficiency.
8. What is the difference between the cortical phase of this patient and that of a patient with normal renal function?
This patients has a decreased cortical peak. The maximum value of signal intensity is about 2000 units (normal patients have values above 2000 units); this value is reached with a great delay, about 200 s (normal patients have values of about 50 s). This date are consistent with a decreased perfusion of the kidneys. In fact, this patient is affected by nephroangiosclerosis.
9. Which are the signal intensity curves correlated with the renal perfusion? Is it possible to obtain an evaluation of the tubular function?
The measure of the peak and the time to peak of the contrast agent enable to evaluate the renal perfusion. The tubular function is more difficult to be evaluated. However, the changes of signal intensity in the inner medulla and the excretory system give an evaluation of the concentration capability of the kidneys.
10. Can a dynamic morphologic and functional MR examination avoid the renal biopsy in a nephropatic patient, with a high risk of complication?
Yes; the complete morphological and functional evaluation available with MRI restricts the need for renal biopsy.
Final diagnosis
Renal failure due to nephroangiosclerosis.
Differential diagnosis
None.
Discussion
Dynamic MRI is able to show both morphological and functional changes in patients with chronic renal failure.
Morphological changes include different degrees of decrease of renal size, cortical thickness and loss of cortico-medullary differentiation.
The morphology of the kidney does not allow to define the type of nephropathy but correlate to the degree of renal insufficiency.
Functional MRI enables us to study renal perfusion and excretion in patients with renal failure.
In our experience a significant correlation between serum creatinine, clearance of creatinine and cortical peak value and cortical peak/time to peak was found. A good correlation was also found between clearance of creatinine and the medullary peak, the medullary peak/time to peak.
The cortical time to peak value was significantly higher in vascular nephropaties compared to glomerular nephropathy. The pathological basis for this difference lies in the site of vascular involvement. In vascular nephropaties the pre-glomerular arteries are involved; this causes an increase in the vascular resistance in the renal cortex. In glomerular nephropathy the glomeruli are primarly involved without any increase (at least in the initial phases), in vascular resistances.
The delay of cortical peak time is indicative of altered renal perfusion and seem to be useful to differentiate vascular from glomerular nephropaties.
The lack of inversion of the signal in the medulla and the calyces indicates reduced renal excretion. However this parameter can not be correlated with the severity of renal insufficiency.
Dynamic MRI can have clinical and diagnostic outcome and improves the physiopathological interpretation of medical chronic nephropaties.