Radio Genitourinary & Renal Systems

Principles of Genitourinary and Renal CT Imaging

Image Orientation and Anatomy: * Radiological images are often presented in the coronal plane, which provides a vertical cut through the body, showing the kidneys bilaterally. * Distinguishing between organs (e.g., the liver and the right kidney) can be difficult due to masses or cysts that distort anatomical margins. * Radiologists use scrolling through image stacks to follow organs and establish continuity that is not apparent in a single static frame.
Radiodensity and Hounsfield Units (CT Density): * Fluid: Generally appears dark on CT. The simpler the fluid (e.g., pure water), the darker it appears. * Fat: Appears very dark on CT scans. * Blood: Appears lighter (hyperdense) relative to simple fluid. In trauma cases, high-density fluid around organs should trigger suspicion of active or recent bleeding.
Renal Cyst Classification: * Renal lesions and cysts are categorized on a scale of $1$ to $5$ (though typically referencing the Bosniak classification system, which usually goes up to $4$ ). * Category 1 (Simple Cysts): Characterized by a thin wall, well-circumscribed margins, and water-density content. These are generally benign regardless of size. * Complex Cysts (Increasing Severity): Complexity is determined by features such as: * Septations: Internal thin lines or walls within a cyst (identified by arrows on clinical imaging). * Enhancement: Whether the internal structures or walls take up contrast. * Irregularity: A "honeycomb" appearance can indicate higher complexity or potential malignancy.

Renal Pathologies and Diagnostic Signs

Pyelonephritis and Striated Nephrograms: * Striated Nephrograms: A specific CT finding where the kidney appears to have stripes of varying density. This occurs because specific parts of the kidney are experiencing cell death or fluid release (edema), leading to dark streaks. * This sign is characteristic of acute pyelonephritis (infection of the kidney).
Abscess vs. Pyelonephritis: * Abscess: Appears as a focal, cyst-like collection but is visually "complex" rather than simple. * Rim Enhancement: This is the defining keyword for an abscess. The "shell" or outer rim of the collection enhances (becomes brighter) with contrast, while the center remains darker (purulent material). * Management: Often requires surgical drainage, though antibiotics may suffice in some cases.
Renal Trauma and Excretory Phase Scan: * In trauma cases, clinicians may use a delayed CT scan (waiting approximately $10$ minutes post-contrast injection). * During this phase, contrast is excreted by the kidneys into the ureters and collecting system. * If bright contrast is seen leaking out and collecting around the kidney (perirenal space), it indicates an injury to the ureter or the collecting system (e.g., a urinoma).
Hydronephrosis: * A condition involving the backup of urine into the kidney, causing dilation. * Often caused by obstruction from a stone. Everything proximal to the stone will be filled with fluid contained within the collecting system.

Bladder and Ureteral Imaging

Contrast Enhancement in the Bladder: * On CT, urine normally looks dark. Once contrast is injected and excreted (after approximately $10$ to $15$ minutes), the urine becomes bright. * This is crucial for identifying bladder masses. A mass would remain dark against the bright, contrast-filled urine, whereas it would be invisible if the bladder were filled only with dark, non-contrast urine.
Fistulas: * Vesicovaginal Fistula: Occurs when an abnormal tract forms between the bladder and the vaginal canal, often due to infection or adjacent wall contact (e.g., with the colon). * It is identified by seeing contrast (which should be in the bladder) leaking into and opacifying the vagina.
Ureteral and Bladder Stones: * On a non-contrast CT, stones appear super bright (radio-opaque). * Stones often get stuck at narrow points, such as the distal ureter where it meets the bladder (Ureterovesical Junction).

Lower Urinary Tract and Prostate

Retrograde Urethrogram (RUG): * Involves the insertion of a catheter (e.g., a Foley) into the distal urethra (penis) to inject contrast directly. * Used to visualize the urethra and bladder without waiting for kidney excretion. * Clinical Application: Identifying urethral injury or strictures following trauma, such as a car crash.
Benign Prostatic Hyperplasia (BPH): * An enlarged prostate can cause "urinary hesitancy." * On CT, it appears as an incidental finding where a soft-tissue density pushes up into the bladder from the bottom. * Because CT cannot easily distinguish BPH from cancer, clinical correlations like a Prostate-Specific Antigen ( $PSA$ ) test, Digital Rectal Exam ( $DRE$ ), or biopsy are required.

Scrotal and Testicular Pathology

Color Doppler Ultrasound: * Uses red and blue color mapping to signify vascular fluid flow. * Testicular Torsion: Characterized by a complete absence of blood flow on Doppler due to the twisting of the spermatic cord. This is a surgical emergency. * Epididymitis: Characterized by increased blood flow (hyperemia), swelling, and a less homogenous appearance of the testis.
Hydrocele: A large collection of fluid surrounding the testis, clearly visible on ultrasound.

Obstetric and Gynecologic Imaging

Ovarian Cysts and Torsion: * Functional Cysts: Includes structures like the corpus luteum cyst. * Simple Ovarian Cysts: Have a regular wall and no internal septations. * Ovarian Torsion: The ovary becomes enlarged and swollen. While blood flow might sometimes be present, the primary diagnostic clue is a significant size difference compared to the contralateral ovary in a patient with acute pain.
Pregnancy and Ectopic Concerns: * At $6$ weeks of pregnancy, it is often too early to see a sac on ultrasound. If the patient has abdominal pain and a positive pregnancy test but no intrauterine sac, clinicians must perform serial $hCG$ tests and follow-up ultrasounds. * Ectopic Pregnancy: The presence of a sac-like structure outside of the uterus is a definitive diagnosis (a "slam dunk"), although they can be extremely difficult to locate depending on the location and patient pain levels.
Uterine Fibroids (Leiomyomas): * Size: Fibroids can grow large enough to fill the entire abdomen and displace all other organs. * Degeneration: Occurs when a fibroid outgrows its blood supply. The internal portions undergo necrosis and turn into fluid, which appears dark on imaging. This process is often very painful. * Management: Fibroids are typically removed via laparoscopy or abdominal surgery, not vaginally.

Questions & Discussion

Question: In complex cysts, are the septations what the yellow arrows are pointing to?
Response: Exactly. The thin lines inside represent those factors that lead to higher classification.
Question: When looking for metastatic disease, would you recommend an axial or coronal view?
Response: You get both, and both are useful. The key sign to look for is striated nephrograms.
Question: What is the oval-shaped thing above the kidney that you said could be blood?
Response: That is actually the gallbladder (in that specific image). Blood would be darker or differently positioned depending on the trauma.
Question: Are you worried about Contrast-Induced Acute Kidney Injury ( $AKI$ ) if the patient has a low Glomerular Filtration Rate ( $GFR$ )?
Response: This is a controversial topic for surgeons. In trauma (like a car crash), physicians often decide that the risk of missing an internal injury outweighs the small risk of contrast-induced kidney damage.
Question: Can you perform a Retrograde Urethrogram on someone who cannot use systemic contrast (e.g., due to kidney failure)?
Response: Yes, because the contrast is injected directly into the urethra/bladder and is not being filtered through the kidneys in a way that causes systemic toxicity.
Question: How are fibroids removed?
Response: Usually either laparoscopically or through an abdominal incision; they are not removed vaginally.