7 The Kidneys and Upper Urinary Tract
M. Taupitz and R. A. Kubik-Huch

Introduction

Indications

• Characterization of incidentally detected renal tumors if ultrasound or CT findings cannot exclude malignancy.
  
• Comprehensive evaluation of surgical renal lesions (extent, lymphadenopathy, intra-abdominal metastases) for surgical planning, which includes assessment of renal vascular anatomy and of the upper urinary tract.
  
• Identification of the cause of urinary obstruction, e. g., vascular compression of the ureteropelvic junction.
  
• Evaluation for renal artery stenosis as a cause of hypertension (discussed in Chapter 15).
  
• Evaluation of potential living kidney donors.
  
• Post-transplant evaluation for suspected vascular complications, urinary obstruction, parenchymal perfusion defects, or tumor.

Imaging Technique

Renal MRI is performed with the patient in a comfortable supine position; a knee support can be offered to improve comfort. As renal MRI regularly comprises sequences in the coronal plane with a left-to-right phase-encoding direction, the patient’s arms may cause wraparound artifacts. Such artifacts can be avoided by placing cushions underneath the arms to elevate them above kidney level.¹ The arms must not be placed on the patient’s abdomen as this will cause wraparound on axial images. Patients are carefully instructed to breathe shallowly and regularly during acquisition of non-breath-hold sequences to avoid excessive breathing motion of the abdominal wall. Careful breathing instructions are also necessary if respiratory-triggered acquisition is planned. Finally, it is important that the breathing commands to be given for breath-hold imaging are explained beforehand. For contrast-enhanced imaging, a flexible cannula should be placed, ideally in an antecubital vein, before positioning the patient in the magnet, and connected to a saline-filled syringe or, if available, an MR-compatible injection pump via extension tubing.

Coils

Body or torso phased-array coils for abdominal imaging are available from nearly all manufacturers and should be used to improve the signal-to-noise ratio (SNR), especially when fast pulse sequences are acquired². Phased-array coils (with 4– elements) are also needed to employ parallel imaging techniques such as sensitivity encoding (SENSE), which can be used to shorten scan time or to acquire higher-resolution images with both T1w and T2w sequences.3–5 However, this gain may come with a penalty in SNR.⁶

Pulse Sequences

The renal MRI protocol can be tailored to different clinical indications (Table 7.1). The basic protocol for soft-tissue evaluation comprises T2w and T1w sequences in multiple planes and should include a T1w sequence acquired with in-phase (IP) and opposed-phase (OP) echo times (TEs) (see Chapter 1). Static MRU of the renal collecting systems and ureters is performed by acquiring a heavily T2w sequence in the coronal plane (comparable to that employed for MRCP; see Chapter 2). The contrast-enhanced series is acquired using a coronal 3D MRA sequence and includes an arterial and venous phase for vascular assessment (see Chapter 15). The protocol is completed by acquiring a delayed axial T1w sequence identical to the corresponding precontrast sequence. To obtain an excretory MR urogram, the 3D MRA sequence is repeated 5 min and 10 min after intravenous contrast injection as well as at later times if contrast medium excretion is delayed. Details of the sequences used for the three main portions of the renal MRI protocol are summarized in Table 7.2.

Precontrast Imaging

At our institution, we acquire a localizer followed by a T2w single-shot TSE sequence (e. g., HASTE) in axial, sagittal, and coronal planes. The sagittal images are used to angle the coronal sequence to the long axis of the kidney. An additional respiratory-triggered, fat-suppressed high-resolution T2w TSE sequence should be acquired in patients with a small tumor⁷ (see “T2-Weighted Imaging” in Chapter 1) (Fig. 7.1). An axial 2D T1w GRE sequence (e. g., 2D FLASH) acquired with IP and OP TEs (see Table 1.3) is required for identification of lipid-containing renal tumors such as angiomyolipoma.¹ If available, IP and OP images can be acquired using a double-echo sequence. The 2D T1w GRE sequence can be replaced or supplemented by an axial or coronal fat-suppressed 3D GRE sequence (e. g., VIBE) (Fig. 7.1). The 3D GRE sequence improves spatial resolution because thinner slices can be acquired compared with its 2D counterpart; however, this is achieved at the expense of soft-tissue contrast on non-enhanced images.

Contrast-Enhanced Imaging

If contrast-enhanced MR images are primarily needed for optimal soft-tissue visualization, and vessel contrast is less important, an axial or coronal 3D GRE sequence emphasizing soft tissue (e. g., VIBE) should be used. Dynamic contrast-enhanced imaging is performed during the arterial/cortical phase (15 s), venous/corticomedullary phase (1 min), and early excretory phase (at ca. 3 min) (Fig. 7.1). If the contrast-enhanced series is performed for vascular evaluation, a coronal 3D MRA sequence is acquired with a field of view completely including the aorta and vena cava as well as the kidneys. The sequence is angled to the longitudinal axis of the kidney using the sagittal T2w precontrast sequence (see above). Following a timing run, the MRA sequence is acquired during the arterial phase and 1 min after bolus injection of the contrast medium (Fig. 7.2). The MRAdataset serves to generate multiplanar reformations (MPR) or maximum intensity projections (MIP) of the target vessels and at the same time provides detailed information on renal perfusion. Finally, the precontrast 2D or 3D GRE sequence is repeated to obtain delayed images emphasizing soft tissues. To obtain an excretory MR urogram, the 3D MRA sequence is repeated ca. 10 min after contrast injection (Fig. 7.2).

Fig. 7.1a–f Renal protocol in normal kidneys. Soft-tissue evaluation using unenhanced axial and coronal images and contrast-enhanced axial images. a Axial T2w TSE image acquired with respiratory triggering. b Coronal breath-hold T2w TSE image (HASTE). c–f Dynamic contrast-enhanced series comprising axial fat-suppressed breath-hold T1w 3D GRE images (VIBE) acquired before (c) and at different time points after IV injection of Gd-based contrast medium: at 15 s (d), 1 min (e), and 3 min (f). This renal protocol provides high-resolution postcontrast images for optimal soft tissue evaluation with excellent corticomedullary differentiation in the arterial phase (d) and characteristic tiger-stripe pattern of the spleen.

Fig. 7.2a–d Renal protocol with coronal contrast-enhanced MRA sequence (3D GRE sequence) instead of the soft-tissue sequence illustrated in Fig. 7.1. a, b MRA images were acquired 15 s (a) and 1 min (b) after IV contrast injection. c MIP reconstruction of a to generate an arteriogram. d MIP reconstruction of a sequence acquired 10 min after contrast injection to generate an excretory MRU. This variant of the renal protocol yields postcontrast images of the renal parenchyma, images for evaluation of renal vessels, and images of the renal collecting systems and ureters for evaluation of urinary outflow.

MR Urography

This can be performed in one of two ways:8,9

• T2w static MRU
  
• T1w excretory MRU.

T2w Static MR Urography. This technique involves the use of heavily T2-weighted TSE sequences initially developed for MRCP and yielding images on which stagnant fluid is depicted with a bright signal (hydrography). The slices are acquired in coronal planes angled to the kidneys and ureters using a sagittal localizer. One way to obtain a static MR urogram is to acquire thick slabs in multiple projections during short breath-holds of ca. 3–5 s using a single-shot T2w TSE sequence (e. g., HASTE, RARE). These have limited detail resolution and merely serve to gain a quick overview or as an additional planning scan. Better image quality is obtained by acquiring multiple thin slices during a 20-s breath-hold using a T2w single-shot TSE sequence. The thin-slice datasets can be postprocessed using MPR or MIP algorithms. Image quality can be further improved by respiratory-triggered acquisition of T2w 3D TSE sequences;¹⁰ however, the scan time is several minutes (Fig. 7.3). Static MR urograms provide no functional information but allow evaluation of the collecting system in patients with no or only little excretory function.

T1w Excretory MR Urography. This technique is based on the renal excretion of an intravenously injected MR contrast medium and its detection with a heavily T1-weighted coronal 3D GRE sequence identical to that employed for MRA. Images are acquired at similar time points as with conventional intravenous urography (Fig. 7.3). However, adequate contrast filling may be delayed in patients with severely dilated collecting systems and ureters or reduced excretory function. This limits the use of excretory MRU in the routine clinical setting as it may not be feasible to repeat imaging at later time points. Both techniques of MRU can be performed with administration of a diuretic (0.05–0.1 mg furosemide) for dilatation and improved visualization of the collecting systems and ureters. Furosemide improves overall image quality of excretory MRU by enhancing the elimination of the contrast medium and its mixing with the urine.⁸

Contrast Media

MRI Appearance of Normal Anatomy

The kidneys are paired organs located in the lumbar fossae in the retroperitoneum. Grossly, the renal parenchyma is composed of the renal medulla and the renal cortex. With MRI, the corticomedullary differentiation is best appreciated on unenhanced and early contrast-enhanced T1w images (Fig. 7.1). The kidney has a firm capsule, which is not usually seen on imaging, and an outer adipose capsule (or perirenal fat). The kidney, adrenal gland, and adipose capsule are enclosed by the renal fascia. The fascia is most conspicuous on MR images when it is thickened, e. g., due to inflammation.¹¹

Fig. 7.3a–d Illustration of normal collecting system on static MRU (hydrography) and excretory MRU. a MIP reconstruction of coronal heavily T2w multislice TSE sequence acquired with respiratory triggering. b–d Coronal 3D GRE images (MRA sequence) acquired 1 min (b), 7 min (c), and 15 min (d) after IV injection of Gd-based contrast medium (augmented by IV injection of 5 mg furosemide).

MRI Appearance of Pathologic Entities

Benign Conditions

A horseshoe kidney (Fig. 7.4) is the fusion of the inferior poles of the two kidneys by an isthmus of fibrous tissue or renal parenchyma. An ectopic or horseshoe kidney typically has normal function but is more susceptible to traumatic injury because it is less well protected than a kidney in normal location.¹²

Ectopic insertion of a ureter results from abnormal migration of a ureteral bud. An ectopic ureter opening into the bladder typically inserts lower than a normal ureter. Other ectopic insertions are the urethra, the vagina, and the uterus in females and the posterior urethra, the seminal vesicles, the ejaculatory duct, and the vas deferens in males. An ectopic ureter inserting below the continence mechanism causes urinary incontinence, which is the case in ca. 50% of ectopic ureters. About 70% of ectopic ureteral openings are associated with a duplex kidney and a bifid ureter.¹³ It is important to identify such anomalies as early as possible; however, the diagnosis is sometimes delayed until adulthood. MRI with the option of static T2w urography is the ideal imaging modality for this purpose because it will also depict a nonfunctioning or poorly functioning renal moiety (Fig. 7.5).¹⁴

Fig. 7.4a–d Horseshoe kidney without associated anomalies. a, b Axial T1w (a) and T2w (b) images through the renal hilum. c Axial fat-suppressed T2w TSE im age acquired through the parenchymal isthmus at a lower level. d MIP reconstruction of a contrastenhanced MRA showing the parenchymal vessels and beginning excretion of the contrast medium (from test bolus injection) into the renal caliceal system.

Fig. 7.5a, b Ectopic insertion of left ureter draining the upper moiety in duplex kidney. a Coronal T2w HASTE image. b Coronal heavily T2w thick-slab TSE image (projection technique). The rudimentary upper moiety shows hydronephrotic changes (arrow); the markedly dilated ureter draining the upper moiety inserts into the urethra (curved arrow), and there is high-grade stenosis of the ureteral orifice.

Abdominal trauma such as that associated with traffic accidents often causes renal contusion or renal vascular injury. Kidneys with prior damage (e. g., due to hydronephrosis) or in an ectopic position are more susceptible to traumatic injury. The role of MRI in emergencies is limited because monitoring of severely injured patients is generally not possible. Multislice CT is the method of choice in acute situations because it enables simultaneous evaluation of the renal parenchyma and renal vessels. Apart from accidents, intra- or perirenal hematoma can be caused by extracorporeal shockwave lithotripsy, diagnostic puncture, clotting disorders, arteriovenous malformations, renal artery aneurysms, renal cell carcinoma, or rupture of a renal cyst. In this setting, MRI will detect small hemorrhage and contributes to the identification of the underlying cause and is also useful for follow-up. The T1 and T2 signal intensities of hemorrhagic lesions vary with the age of the hemorrhage (Fig. 7.6).

Perirenal hematoma must be differentiated from urinoma, which—just like urine in the bladder—has low signal intensity on T1w images and high signal intensity on T2w images. If injury of the pelvicaliceal system is suspected in the acute setting, a postcontrast T1w series will demonstrate leakage of contrast medium and thereby of urine (see Figs. 7.33 and 7.34).

Inflammation/Abscess

Imaging modalities are typically used in patients with acute pyelonephritis to evaluate for complications such as pyonephrosis or abscess. Renal inflammation is seen on MRI as swelling of the kidney, altered signal intensities due to accompanying edema, blurring of renal contours, and loss of corticomedullary differentiation. There may be inflammatory stranding of perirenal fat and thickening of perirenal fasciae, best appreciated on unenhanced T1w images. Severe inflammation may be associated with focal perfusion defects on contrast-enhanced images. A full-blown abscess is a lesion with a central fluid collection of high T2 signal intensity surrounded by a wall that will enhance after contrast administration.¹⁵

Both solitary renal cysts and cysts in patients with polycstic kidney disease can become infected. An infected cyst resembles an abscess with a central fluid signal surrounded by a thickened wall showing intense enhancement on postcontrast T1w images (Fig. 7.7).¹⁶

Renal atrophy is usually due to chronic inflammation. MRI may be indicated for example if a tumor is suspected. Atrophied kidneys are only seen otherwise on MRI if the examination is performed for other purposes (Fig. 7.8).

Fig. 7.6a–c Older perirenal hematoma following puncture of the left kidney. Axial breath-hold T1w GRE image (a) and breath-hold T2w TSE images in axial (b) and sagittal (c) planes. Intralesional area of high SI on T1w image and low-SI margin on T2w sequence indicate different stages of hematoma organization with marginal hemosiderin deposits (arrow).

Fig. 7.7a–d Infected renal cyst in the right kidney in bilateral cystic kidney disease. a, b Axial (a) and coronal (b) breath-hold T2w TSE images. c Precontrast axial breath-hold T1w GRE image. d Fat-suppressed axial breath-hold T1w GRE image after IV contrast injection. The fluid in the infected cyst appears homogeneous on the unenhanced images and has a thickened wall, which enhances intensely on the postcontrast image.

Fig. 7.8a–d Pyelonephritic atrophy of both kidneys. a, b Breath-hold axial (a) and (b) coronal T2w TSE images. c, d Breath-hold axial T1w GRE images before (c) and 2 min after (d) IV contrast injection. There is marked atrophy of both kidneys with only a little perfused residual parenchyma.

Fig. 7.9 Small caliceal stone in the middle caliceal group of the left kidney. Respiratory-triggered, fat-suppressed T2w TSE image. The stone causes a signal void within the slightly dilated calix (arrow). Also present is a cyst in the lower pole of the right kidney.

Fig. 7.10 Urinary obstruction due to a stone in the right ureter. MIP image of excretory MRU performed with a coronal 3D GRE sequence ca. 10 min after IV contrast injection. Dilatation of the right collecting systemand ureter down to the level of the stone at the iliac vessel crossing (arrow).

Urinary Obstruction

Dilatation of the renal collecting systems and ureters occurs secondary to acute or chronic obstruction. The most common causes of acute obstruction are stones or blood clots, pregnancy, and ureteral edema developing after iatrogenic instrumentation, e. g., for stone extraction (Figs. 7.9 and 7.10). Chronic obstruction can be congenital or acquired. Causes of acquired obstruction are tumors of the urinary bladder, prostate, uterine cervix, or retroperitoneum, ureteral strictures, and benign prostatic hyperplasia. If the underlying cause cannot be identified by ultrasound, MRU performed as part of a renal soft tissue and vascular MRI protocol should be preferred to conventional intravenous urography or CT. The absence of radiation exposure is especially important in younger patients.

Renal Cysts

Simple cysts (Fig. 7.11), the most common renal lesions, are found in ca. 50 % of adults after the age of 50. They are typically benign and detected incidentally. On MRI they are seen as sharply demarcated lesions of low T1 signal intensity and high T2 signal intensity which do not enhance on T1w images following intravenous contrast medium administration.¹ Parenchymal, cortical, and parapelvic renal cysts are distinguished by location. Parapelvic cysts can mimic dilatation of the pelvicaliceal system on ultrasound and unenhanced T1w and T2w MR images. Unclear cases can be resolved by a contrast-enhanced T1w study including an excretory MR urogram (Fig. 7.12).

Complicated cysts

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