Dual-energy CT and composition differentiation
Classic unenhanced computed tomography excellently detects the presence of stones and obstruction complications (for example, a calculus in the middle third of the ureter on a coronal reconstruction predictably causes proximal hydronephrosis and hydroureter). However, choosing treatment tactics often requires knowing the chemical composition of the stone.
- Study method: The patient undergoes scanning at two different X-ray photon energies (most commonly 80 kV and 140 kV).
- Physical principle: X-ray attenuation coefficients and, accordingly, X-ray density in Hounsfield units (HU) will differ significantly at different energies depending on the atomic number of the substance.
- Data analysis: The tomograph software calculates the difference in attenuation and plots a special linear attenuation coefficient curve for the specific stone. This profile is then compared with standard curves of known materials (material decomposition).
- Clinical significance: The method demonstrates exceptionally high sensitivity and specificity in differentiating uric acid calculi (uric acid calculi) from non-uric acid ones, and also allows for the unambiguous identification of calcium-containing stones.
Nephrocalcinosis: Basic concepts
Nephrocalcinosis (nephrocalcinosis) is a pathological process characterized by the deposition of calcium salts directly within the renal parenchyma. In a broad medical sense, this term includes the accumulation of both calcium phosphate and calcium oxalate.
The literature also uses the term "oxalosis" (oxalosis). Some authors prefer to use it exclusively to denote calcium oxalate crystal deposits in tissues.
- Etiopathogenesis: The disease is based on systemic metabolic disorders. Any conditions provoking hypercalcaemia (hypercalcaemia), hyperphosphataemia (hyperphosphataemia), as well as increased urinary excretion of calcium, phosphate, or oxalate, lead to nephrocalcinosis.
- Clinical presentation: The process is asymptomatic for a long time and is most often an incidental finding (incidental finding) during imaging studies of the abdomen or retroperitoneal space prescribed for other reasons.
- Diagnostics: Plain radiography, ultrasound, and CT are used to detect calcium deposits. Moreover, ultrasound and computed tomography have significantly higher sensitivity and specificity compared to traditional radiography.
Classification: Medullary nephrocalcinosis
Depending on the affected zone of the renal parenchyma, nephrocalcinosis is subdivided into two main types: medullary and cortical.
Medullary nephrocalcinosis (Medullary nephrocalcinosis) is by far the most frequent form of the pathology. It accounts for about 97% of all detected cases.
- Pathogenesis: This type of calcinosis is usually caused by systemic metabolic disorders.
- Radiological presentation: Tomograms (in axial and coronal projections) clearly visualize multiple calcifications that are symmetrically deposited in the medulla (medullary regions) of both kidneys—in the renal pyramid region.
Main causes of development (etiological spectrum):
- Hyperparathyroidism (Hyperparathyroidism)
- Sarcoidosis (Sarcoidosis)
- Medullary sponge kidney disease (Medullary sponge kidney disease)
- Renal tubular acidosis (Renal tubular acidosis)
- Loop diuretic therapy (Loop diuretics)
- Beta-thalassaemia (Beta-thalassaemia)
- Inherited tubulopathies (Inherited tubulopathies)
- Oxalosis (Oxalosis)
- Excessive vitamin D therapy (Vitamin D therapy)
Classification: Cortical nephrocalcinosis
Cortical nephrocalcinosis (Cortical nephrocalcinosis) is an extremely rare form of the disease.
- Pathogenesis: Unlike the medullary form associated with metabolism, cortical calcinosis usually occurs against the background of severe, structural, and often irreversible damage to the renal cortex.
Main causes:
- Renal cortical necrosis (Renal cortical necrosis)
- Glomerulonephritis (Glomerulonephritis)
- Renal allograft rejection (Renal allograft rejection)
- Alport syndrome (Alport syndrome)
- Oxalosis (Oxalosis — capable of affecting both the medulla and the cortex)
Briefly: answers to topical questions
Which urinary tract calculi are radiolucent on computed tomography?
On computed tomography, radiolucent calculi are those with soft-tissue density, which makes them difficult to detect on unenhanced scans. Such exceptions include:
- Indinavir stones — formed in connection with the use of protease inhibitors (indinavir sulfate).
- Mucoid matrix stones — also have soft-tissue X-ray density.
At the same time, uric acid and xanthine stones (urate and xanthine) are described as radiolucent (radiolucent) in conventional radiography, but on CT most calculi are hyperdense.
What is the X-ray density of uric acid and oxalate stones in Hounsfield units (HU)?
Sources do not specify exact density values separately for uric acid and oxalate stones, but provide a general range. Most urinary calculi on unenhanced computed tomography are hyperattenuating (hyperattenuating). Their X-ray density ranges from 150 to 1000 Hounsfield units (HU).
For precise composition differentiation, CT at two different photon energies is used:
- Uric acid stones (uric acid calculi) — are differentiated from non-uric acid stones due to the difference in attenuation coefficients.
- Oxalate stones — belong to calcium-containing calculi, which are also successfully identified by this method.
Class and exam questions
What is the main advantage of dual-energy CT in urolithiasis?
Scanning at two different energies (80 and 140 kV) allows not only visualizing the stone but also assessing the change in its X-ray density. This makes it possible to accurately differentiate uric acid stones from non-uric acid and calcium stones in vivo.
What is oxalosis and how does it differ from general nephrocalcinosis?
Nephrocalcinosis is a collective term for the deposition of any calcium salts (phosphates or oxalates) in the renal parenchyma. Oxalosis is a narrower term used by some authors exclusively to describe calcium oxalate deposits.
Where are calcifications in the renal parenchyma most commonly located on CT?
In 97% of cases, medullary nephrocalcinosis is diagnosed. On CT, multiple calcium deposits are clearly visualized in the medulla (pyramids) of both kidneys.
What complications of an obstructive ureteral stone are visible on CT?
In ureteral obstruction by a calculus (for example, in the middle third), unenhanced tomography reveals marked dilation of the superior urinary tract sections — hydronephrosis and hydroureter.
What else to study on this topic
- Mathematical principles of plotting material decomposition curves in dual-energy CT.
- Etiopathogenesis and radiologic semiotics of medullary sponge kidney disease.
- Mechanisms of hypercalcemia formation in primary hyperparathyroidism and sarcoidosis.
- CT signs of renal cortical necrosis and chronic glomerulonephritis.
- Pathophysiology of renal tubular acidosis development and its association with calcinosis.
Master the entire topic in the "Diagnostic Radiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "CT of Urinary Calculi" and another 2,199 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 10 video lessons on the subject
Other topics in the section "Diagnostic Radiology of the Abdominal Organs"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.