Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

tiation between renal cortex and medulla, occurs during the early phase. Whether a corticomedullary phase or a later nephrogram phase should be used for subtle tumor detection is debatable,although some evidence suggests that the nephrogram phase is superior. Both neoplasms and normal renal parenchyma enhance significantly more during the nephrogram phase than during the corticomedullary phase. In general, more tumors <3cm in diameter are detected on the nephrogram phase than on the corticomedullary phase. The onset of a nephrogram phase varies among patients and technique used; a faster injection rate results in an earlier onset—roughly 100 seconds at 2mL/sec and 90 seconds at 3mL/sec. The corticomedullary phase is more useful, however, for detection of such conditions as an aneurysm, arteriovenous malformation, or fistula, and in evaluating tumor vascularity. Also, the earlier phase is more advantageous if optimal liver and other abdominal structure visualization is required. Renal tumors tend to be detected with greater confidence on delayed images than on early-phase images. One solution is to obtain images during both phases (corticomedullary and nephrogram), but whether the extra complexity and cost justify such an approach for a limited gain is not clear; a decision based on individual indications appears reasonable.

Renal CT performed shortly after excretory urography—called CT urography—is a variation of delayed-phase CT combining high spacial resolution of conventional filming with high contrast resolution of CT. Even pyelovenous backflow can be identified on CT urography (2). Only about half of renal parenchymal tumors identified on CT are detected on the previous excretory urogram, but such a combination study tends to increase the clinicians’ confidence in some findings. A variant of this technique is to obtain delayed postcontrast coronal images or a delayed CT scout image. Validity of these various combined procedures in evaluating hematuria is yet to be established in larger studies.

Furosemide-enhanced CT urography, obtained 10 minutes after contrast agent injection, outlines pelvicaliceal structures and identifies calculi inside opacified urine and differentiates them from phleboliths (3).

Three-dimensional CT imaging techniques are useful both in evaluating suspected tumors

ADVANCED IMAGING OF THE ABDOMEN

and in planning a partial nephrectomy, such as orientation of blood vessels to a tumor or other structure. One should not rely only on 3D images, however, because to an experienced eye axial and coronal images provide more detailed information, especially about small vessels that tend to be overlooked on 3D images.

In a patient with microscopic hematuria and a normal excretory urogram, should CT or ultrasonography (US) be performed next? Although this topic generated considerable controversy in the 1990s, currently many investigators believe that CT detects more tumors overall, especially smaller ones. In fact, a more pertinent current question is whether CT or MR is indicated as a primary imaging modality in such a clinical setting.

Ultrasonography

Renal cortex is isoechoic to liver, and the centrally located renal sinus is hyperechoic to surrounding renal parenchyma.

Similar to other structures, use of an intravascular US contrast agent (such as Levovist; Schering AG, Berlin, Germany) enhances vascular signals and makes vascularity more evident. Diagnostic accuracy is improved, especially for hyperechoic tumors and complex cysts.

Doppler US provides data for the intrarenal arterial blood flow resistive index (RI) and pulsatility index (PI). These indices increase with age, acute obstructive uropathy, use of certain drugs, and in some nephropathies.

Endoluminal US using a high-frequency transducer housed in a catheter and advanced endoscopically into a ureter is moving from research into clinical practice. Potentially, the information obtained helps guide biopsy and laser therapy and defines vessels adjacent to a ureter.

Magnetic Resonance Imaging

Currently for most suspected renal conditions CT is performed rather than magnetic resonance imaging (MRI), but MRI is used in a setting of contrast allergy or renal failure, and for studying some complex masses. It is also useful in evaluating venous thrombosis in a setting of renal carcinoma. It provides both spatial resolution and information on renal

KIDNEYS AND URETERS

function and potentially is more useful than either CT or nuclear medicine. Magnetic resonance applications range from multiphase 3D magnetic resonance angiography (MRA) to evaluate renal artery stenosis and renal perfusion abnormalities, MR nephrography, and MR urography of the renal collecting systems (especially useful in posttransplant complications). Potentially, a single MR study evaluates renovascular disease, assesses renal function, detects renal tumors, and identifies urinary tract abnormalities, all without radiation exposure. More often, however, the specific MR sequences best suited for each application are selected.

Conventional

As with CT, several distinct phases—cortical, medullary, and pyelocaliceal—are evident after IV paramagnetic gadolinium contrast injection. Some investigators add an intermediate corticomedullary junction phase. Magnetic resonance signal intensity normally decreases in the pyelocaliceal phase due to contrast agent concentration. When searching for small renal tumors, use of a body phased-array coil in combination with fast low-angle shot (FLASH) and fat suppression preand postcontrast thinsection MR allow imaging in single breathholds. Sagittal and coronal plane images improve evaluation.

Both T1and T2-weighted sequences are useful with contrast-enhanced MRI to evaluate renal blood flow and renal function. During the early phase, a renal cortex signal increase in T1-weighted sequences is matched by a similar signal decrease in T2-weighted sequences; during later phases, however, T2-weighted sequence signal intensity in the medulla decreases markedly. Thus renal cortical blood flow can be evaluated with either sequence, but T2-weighted sequences appear more useful in evaluating renal medulla. Presumably increased amounts of contrast in renal tubules account for the medullary decreased signal intensity during later phase T2-weighted sequences.

Serial dynamic MR gradient echo imaging using a low contrast dose can be used to obtain an intensity-time curve, similar to radionuclide renography. This technique allows assessment of split renal function and urinary excretory status and is an alternative to radionuclide renography. Preliminary studies suggest that

good correlation exists between MR renography and radionuclide renography results (4).

A general disadvantage of MR in children, especially younger ones, is the need for sedation. A relative disadvantage in certain renal applications is its poor sensitivity in detecting calcifications.

Surprisingly, diagnostic accuracy in patients with a clinical suspicion for renal tumor was comparable when studies were performed using either a low field [0.2 tesla (T)] or a high field (1.5T) magnet, although the signal-to-noise and contrast-to-noise ratios were significantly worse at low field strength (5).

Gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) is an ionic agent. A nonionic version is also available in some countries. No significant differences exist in either signal intensity or function between these two agents.

Magnetic Resonance Urography

Magnetic resonance urography both without and with contrast agents is evolving into viable alternative studies in select patients. No consensus is yet apparent on which specific MR sequences constitute MR urography. A more basic question concerns the role of MR urography: Does it have any advantages over conventional or CT urography?

A heavily T2-weighted sequence consisting of rapid acquisition of images obtained in less than 30 seconds with a relaxation enhancement technique results in solid organs and flowing fluid being hypointense, and stationary fluid, such as urine in collecting systems and ureters, being hyperintense; the entire urinary tract is visualized on one image without use of contrast (this technique is also called MR pyelography). The urinary tract is depicted even with nonfunctioning kidneys. A reconstructed 3D image provides an overall view. Magnetic resonance spatial resolution is superior to that of US. A current limitation of MR urography is that with the present MR units small calculi are poorly imaged.

A breath-hold 1.5T MR half-Fourier acquisition single-shot turbo spin echo (HASTE) sequence visualizes renal collecting systems and ureters similarly to excretory urography. The HASTE sequences can be used to acquire images in the axial, sagittal, or coronal planes. These MR urography sequences outline non-

functioning urinary tracts that cannot be visualized with excretory urography and identifies level of obstruction without the use of contrast agents. This technique is advantageous in evaluating hydronephrosis during pregnancy, but future indications will undoubtedly expand.

Gadolinium chelates are filtered by renal glomeruli and excreted. One should keep in mind that dilute gadolinium is hyperintense on T1-weighted images, but when concentrated it induces a signal loss and becomes hypointense. Also, if one is performing CT shortly afterward, renal excretion of gadolinium can mimic a calculus on noncontrast CT (6).

Breath-hold 3D MR urography using fast gradient echo performed 5 to 10 minutes after injecting only several mL of Gd-DTPA visualizes the urinary tracts as hyperintense structures (7); urinary tract detection was superior to that obtained with a heavily T2-weighted sequence.

The ureters can thus be studied either with nonenhanced T2-weighted TSE imaging or gadolinium enhanced T1-weighted sequences. A preliminary study comparing these two sequences in infants and children found that although the ureters were more often better visualized with the gadolinium enhanced sequences, the two sequences are complementary (8). MR urography is feasible in young infants using only oral sedation. MR urography appears useful in patients with an ileal neobladder. Although the nonenhanced T2-weighted sequences can detect a point of ureteric obstruction, they do not readily establish an etiology—generally additional MR sequences are required.

Several paramagnetic contrast agents initially designed primarily for hepatobiliary applications, such as Gd-ethoxybenzyl (EOB)- DTPA and Gd–benzyloxypropionic-tetraacetate (BOPTA) initially function as extracellular agents and are then eliminated via biliary and renal pathways. Thus MR urography using Gd-BOPTA–enhanced breath-hold 3D FLASH sequences was comparable to conventional excretory urography (9); caliceal fornices were better visualized with conventional urography, but MR urography was superior in the distal ureters and the bladder, and in evaluating obstructive causes.

A further refinement is the use of a diuretic agent. Even a nondilated urinary tract is visualized.

ADVANCED IMAGING OF THE ABDOMEN

Several virtual endoscopy techniques are feasible. Using surface-rendering techniques, unenhanced MR urography data can be presented in a virtual endoscopy format (10). Virtual endoscopy of the upper urinary tract can be reconstructed from T1-weighted 3D gradient-echo sequences obtained after urinary tract enhancement by IV gadolinium. Such a technique provides an endoluminal view of the calices, ureters, and ureteral orifices.

Scintigraphy

With its low radiation burden, renal scintigraphy is commonly used for initial evaluation and follow-up in children. Its main advantage over other imaging modalities is that to some degree function can be quantified.

Plasma clearance methods estimate overall renal function. Scintigraphy provides an estimate of each kidney function and detects gross structural defects. Currently scintigraphy is used to evaluate renal vascular hypertension, renal function after transplantation, suspected infection, and, occasionally, obstructive nephropathy.

Radiopharmaceutical agents useful in renal scintigraphy include the following:

Technetium-99m (Tc-99m)-mercaptoacetyl- glycilglycilglycine (MAG3) is an allpurpose renal agent. It is cleared by tubular secretion and is useful in flow and function studies. Effective renal plasma flow can be calculated with this radiotracer. It has a role in diuresis renography in neonates and infants. This compound is excreted vicariously by the liver and is then detected in the gallbladder. In imaging renal allografts, Tc-99m-MAG3 results in better image quality than Tc-99m-DTPA.

Tc-99m–L,L-ethylenedicysteine (L,L-EC) is a newer renal tubular tracer used as an alternative to Tc-99m-MAG3. It has similar excretion characteristics but higher plasma clearance than MAG3. It is also used to obtain the effective renal plasma flow rate. In patients with chronic renal failure, image quality is similar to that of Tc-99m- MAG3.

Tc-99m-DTPA is filtered by glomeruli and the glomerular filtration rate can be calculated.

KIDNEYS AND URETERS

Scintigraphy outlines renal and collecting system anatomy.

Tc-99m–2,3-dimercaptosuccinic acid (DMSA) binds to proximal tubules and results in prolonged renal retention. Its use in planar imaging allows evaluation of focal functional disorders and gross parenchymal damage. In children, planar Tc-99m-DMSA is useful in estimating renal function. Single photon emission computed tomography (SPECT) Tc-99m- DMSA scintigraphy is more effective than planar in detecting renal cortical defects.

Tc-99m-glucoheptonate is also used as a renal imaging agent. Neither glucoheptonate nor DMSA is taken up by renal cysts or neoplasms, and these appear as defects.

Iodine-131–ortho-iodohippurate (OIH) is an older agent that has been replaced to a large degree by Tc-99m-MAG3.

Carbon-11 acetate positron emission tomography (PET) imaging reveals prompt renal uptake even in a setting of reduced renal function.

Biopsy

Image-guided percutaneous renal biopsy is useful in obtaining tissue from suspected small lesions, and at times such a biopsy replaces surgical exploration. Most biopsies differentiate between benign and malignant disease and between primary renal cell carcinoma and metastatic disease, or they confirm renal involvement by lymphoma. If needed, several biopsies are obtained at the same time. Computed tomography or US–guided 18-gauge needle biopsies provide adequate biopsy material for analysis in most patients. Negative percutaneous biopsy results in small (<3cm) and large (>6cm) tumors, however, should be viewed with caution (11). Renal biopsy with an automated device using a 14-gauge needle in renal allografts and native kidneys has a >95% tissue recovery rate, but is associated with a serious complication rate of about 3% (12), including death, loss of renal allograft, major hemorrhage (requiring blood transfusion) and creation of an arteriovenous fistula.

In patients at high risk, a transfemoral vein biopsy using flexible forceps is an alternative approach.

Tissue cores adequate for histopathologic diagnosis were obtained in 98% of both transjugular renal biopsies and percutaneous renal biopsies (13); transjugular renal biopsies are an option in patients with contraindications or failure of percutaneous biopsy and in those requiring multiorgan biopsies.

Percutaneous

Nephrostomy/Stenting

Percutaneous nephrostomy is commonly performed for emergency decompression of an obstructed urinary system. It is an integral part of several interventional procedures such as percutaneous nephrolithotomy, ureteral stenting, and dilation. It is feasible in children and adolescents on an outpatient basis; one study excluded outpatient nephrostomies in a setting of infection, stone therapy, solitary kidney with renal failure, and similar reasons (14). A puncture success rate of almost 100% is common. At times a nephrostomy is necessary with a nondilated system, such as a ureteral leak or fistula; fluoroscopy or US-guided injection of air or carbon dioxide into nondependent calyces aids catheter insertion (15).

Double-J stents are commonly used to treat ureteral obstruction. These stents are inserted retrograde by an urologist or antegrade by an interventional radiologist. At times a guidewire passes through an obstruction but the catheter fails to do so; a combined retrograde and antegrade approach may then be successful.

Both self-expandable and balloon expandable metallic stents are used for palliation of malignant ureteral obstruction.

Complications of percutaneous nephrostomy tube placement include hemorrhage requiring transfusion,a complication minimized by maintaining a platelet count above 100,000/mm3. Inadvertent enteric puncture occurs occasionally. Long-term obstructions include urothelial hyperplastic reaction and tumor ingrowth or invasion of one stent end.

In a setting of unresectable pelvic cancer or radiation or both, a permanent percutaneous nephrostomy is often an option for a variety of conditions, including unresectable pelvic fistulas and incontinence. Many of these patients already have distal urinary tract obstruction. If not, ureteral occlusion is achieved using a

percutaneous approach and a permanent external diversion is then provided. Complications include nephrostomy catheter occlusion and retrograde coil migration into the renal pelvis.

Congenital Abnormalities

Screening for Congenital

Abnormalities

Ultrasonography screening for congenital genitourinary abnormalities is a simple procedure but it is rarely performed during a routine health check-up. Major anomalies can be expected in under 1% of infants, including unilateral renal agenesis, vesiculoureteral reflux, hydronephrosis or megaureter, horseshoe kidney and ureterocele.

Fetal MRI can evaluate for oligohydramnios or anhydramnios and congenital genitourinary anomalies (16). It is currently underused.

Renal Agenesis

Renal malformation includes the absence of renal tissue (aplasia) or the presence of undifferentiated renal tissue (dysplasia). Bilateral renal agenesis is incompatible with life; many of these babies are stillborn and tend to have a characteristic oligohydramnios-induced Potter facies and numerous other anomalies (Potter sequence). Incidentally, Potter sequence is also seen with bilateral renal hypodysplasia, severe obstructive uropathy, and polycystic kidney disease, but an occasional such baby survives. Although sporadic, agenesis is more common in males, and a familial pattern is identified in about one third of patients. In a broader context, renal agenesis is lumped together with dysgenesis and it is called hereditary renal adysplasia (multicystic dysplasia is discussed in a later section).

Unilateral renal agenesis is relatively common (about one in 1000 births), with the single kidney compensating through hypertrophy. In some individuals renal agenesis is the end result of an involuted multicystic dysplastic kidney. Renal agenesis is associated with an ipsilateral seminal vesicle cyst and ipsilateral ovarian dysplasia. Bladder duplication is also occasionally found with renal agenesis.

ADVANCED IMAGING OF THE ABDOMEN

The rare unipapillary kidney has a solitary calyx and papilla. It is more common on the left side. Almost always other abnormalities are present, including megaureter, ectopic ureter with vesicoureteral reflux, and renal agenesis on the contralateral side.

Imaging of renal agenesis reveals an empty renal fossa; keep in mind that an ectopic kidney also presents with an empty renal fossa. Prevalence of ipsilateral adrenal gland absence is slightly greater with renal agenesis than in the general population; when present, the ipsilateral adrenal gland tends to be more elongated than usual. On the left side the colon splenic flexure tends to fill an empty renal fossa.

A congenitally solitary kidney is about 1.8 times heavier than a normal kidney. The diameter of its glomeruli and convoluted tubules are similar to that of a control, but a solitary kidney contains twice as many glomeruli. The congenital solitary kidney is thus hyperplastic and not hypertrophic.

The imaging approach in a child with a suspected single kidney is not clear; a point can be made for initial US followed by MR urography. Children with unilateral agenesis are prone to vesicoureteral reflux on the contralateral side, with reflux being detected in over one third. A voiding cystourethrogram is thus recommended even if neither infection nor hydronephrosis is evident.

Hypoplasia

Simple hypoplasia implies a small but otherwise normal kidney. It is a rare anomaly. The number of calyces tends to be reduced. Focal hypoplasia entails a reduction in the number of calyces and associated renal parenchyma. Some patients have a small kidney that also has segmental parenchymal thinning and an associated dilated collecting system in this region; the appearance mimics that of chronic pyelonephritis.

In the rare oligonephronic hypoplasia, the kidneys are small and contain only one or two calyces.

Duplicated Collecting System

Duplication anomalies range from a bifid collecting system to duplication of both kidney and ureter. Most duplications are unilateral; if bilat-