MRI Atlas of Renal Pathology

1
MRI Atlas of Renal Pathology

• Mary Kitazono
• Patrick M Colletti MD

University of Southern California Keck School of
Medicine
Start
kitazono_at_usc.edu, colletti_at_usc.edu
2
Table of Contents

• Renal MRI Technical Considerations
• Indications for Renal MRI
• MRI Atlas of Normal Renal Anatomy
• MRI Atlas of Renal Pathology
• Vascular Disorders
• Diseases of the Renal Parenchyma
• Obstructive Uropathy
• References
• MRI Tutorial
• Proton MR Tutorial
• Pulse Sequences
• MR Angiography
• MRI Imaging Strategies

3
Renal MRI Technical Considerations

• Select a Pulse Sequence and Protocol
• Set Parameters
• TR (repetition time) determines T1 and
  T2-weighting
• TE (echo time) determines T2-weighting
• Flip angle determines T1-weighting in gradient
  echo sequences
• Maximize image quality
• Voxel size smaller improved resolution
• Acquisition time faster better temporal
  resolution
• Reduce Motion Artifacts
• Paramagnetic Contrast Enhancement
• Renal MR Angiography
• MR Urography

4
Basic Renal Imaging Protocol

• Localizer HASTE/ssFSE/ssTSE
• Coronal plane, 8 mm slices, breath held
• In-phase and opposed-phase T1 Dual echo spoiled
  gradient echo
• To characterize fat-containing lesions axial
  plane, 6-8 mm slices, breath held
• Include a precontrast fat-suppressed T1 to
  differentiate fat from blood
• Fat-suppressed T2 Fast/TurboSE or GE
• To characterize lesion, lymphadenopathy, and
  fluid axial plane, 6-8 mm slices, respiratory
  triggering
• Dynamic Gd-enhanced T1 2D or 3D spoiled GE
• Contrast enhanced MRA to characterize
  vascularity axial plane, 6-8 mm slices, breath
  held
• Non-contrast MRA
• To evaluate renal vein and IVC for patients with
  renal cell carcinoma
• MR Urography
• To evaluate the collecting system

5
Motion Artifact Reduction

• Causes of physiologic motion artifact
• respiration
• cardiovascular pulsation
• bowel peristalsis
• Due to the posterior location of the kidneys,
  there is much less artifact as compared to liver
  imaging, however various methods for reducing
  motion artifact can be employed.

6
Motion Artifact Reduction

• Ways to reduce motion artifact
• antero-posterior phase encoding for axial imaging
  (since artifacts develop in the direction of the
  phase encoding)
• cranio-caudal phase encoding combined with
  foldover suppression for coronal imaging
• breath-holding, with or without retrospective
  averaging of individual breath-holds
• single-shot and fast imaging pulse sequences
• fat-suppression techniques
• intermittent sampling of data using respiratory
  ordered-phase encoding, respiratory gating, or
  respiratory triggering (though not usually
  required)
• wrapping an elastic garment around the patients
  abdomen to minimize anatomic movement during
  respiration

7
Gadolinium

• Chelates of Gadolinium (e.g. Gd-DTPA)
• commonly used paramagnetic contrast agents
• completely filtered in the kidney, making it
  useful for examining excretory kinetics

• provides excellent corticomedullary delineation
  in the early perfusion phase (10-20 seconds after
  injection) and in the early excretory phase
  (50-90 seconds after injection)
• safe (approved by the FDA)
• MR imaging with Gd-DTPA is the appropriate choice
  for all children and women of childbearing age,
  patients with iodinated contrast allergy, renal
  insufficiency and solitary kidney, and for
  following the progression of disease over time.

8
Paramagnetic Contrast Enhancement

• How they work Paramagnetic contrast agents have
  unpaired electrons, which create small local
  magnetic fields in the surrounding tissue
• Main effect proton relaxation enhancement, or
  shortening of the T1 relaxation time in the
  surrounding tissues, increasing the signal
  intensity and tissue contrast
• Requirements blood flow and a compromised
  capillary basement membrane
• Use to assess the function of renal vessels and
  to enhance lesions for diagnosis, especially
  vascularized tumors

9
Renal MR Angiography

• Advantages of MRA over conventional angiography
• Gadolinium chelates are safe, even for patients
  with renal insufficiency, whereas iodinated
  contrast agent can cause allergic reactions and
  nephrotoxicity.
• MRA is a non-invasive procedure, not requiring
  local anesthesia or recovery time.
• 3D MRA allows a more accurate assessment of
  lesions, particularly atheromatous plaques.
• MRA can usually demonstrate accessory renal
  vessels.
• MRA can elucidate the significance of stenosis by
  depicting the presence of poststenotic
  dilatation, delayed renal enhancement, and/or
  reduced renal mass.
• Incidental renal pathology can be detected.
• Phase contrast flow MRA can provide a
  quantitative measurement of renal perfusion.

10
Renal MR Angiography

• Disadvantages of MRA
• MRA requires precise timing and calculation of
  the rate of contrast flow. Inaccuracies can
  result in signal loss if image acquisition begins
  too late, or venous enhancement, instead of
  arterial enhancement, if acquisition is begun too
  early.
• MRA requires the patient to hold his or her
  breath and remain motionless for a considerable
  amount of time.
• As with all forms of MR imaging, MRA can not be
  performed on patients with pace-makers or
  implanted ferromagnetic devices.

11
MR Urography

• Clinical Application
• to evaluate ureteral obstruction and congenital
  urinary tract abnormalities

12
MR Urography

• Static MR Urography
• Technique Heavily T2-weighted sequence similar
  to MRCP
• Works best for dilated collecting systems
• Can even be used in patients with severe renal
  insufficiency
• No imaging delay or IV contrast necessary, but
  overlapping fluid-filled structures can interfere
• Tip imaging nondilated ureters can be enhanced
  by hydrating the patient and administed a low
  dose of furosemide before the exam
• Excretory MR Urography
• Technique T1-weighted 3D gradient echo sequence
  after IV contrast administration
• Can demonstrate nondilated as well as dilated
  ureters
• No interference by overlapping fluid-filled
  structures
• Cannot be used in patients with severe renal
  insufficiency

13
Indication for Renal MRI

• Detection, localization, and characterization of
  renal masses (due to its multiplanar capability)
• Staging renal cell carcinoma
• Detection and differentiation between renal
  cysts, hematomas and angiomyolipomas
• Assessment of kidney function using dynamic
  imaging
• Visualization of the renal arteries using MR
  Angiography
• Preoperative planning and postoperative evaluation

14
Renal Cysts

• The Bosniak classification system, which is based
  on imaging findings, can be used to differentiate
  benign renal cysts from angiomyolipomas and renal
  cell carcinomas

15
Staging Renal Cell Carcinoma

• MRI is the imaging modality of choice for staging
  renal cell carcinoma
• multiplanar capability
• superior soft tissue contrast
• ability to depict tumor extension and invasion
• ability to detect enlarged lymph nodes

16
Renal Transplantation

• MRI is useful for both preoperative planning and
  post-transplant evaluation of the caliber of the
  collecting system and vasculature
• Peritransplant fluid collections, which may cause
  urinary tract obstruction, or regions of
  infarction (best seen with contrast) can be
  detected in the postoperative period

17
Utility of Dynamic Contrast-Enhanced MRI

• In addition to providing exceptional anatomical
  detail, dynamic contrast-enhanced MR can provide
  a wealth of information about renal function
  including perfusion and glomerular filtration
  rates, giving MR imaging a clear advantage over
  real time ultrasonography and radionuclide
  renography.

Excretory Function
Spatial Resolution
Blood Flow
Real time ultrasonography
with color doppler
Radionuclide renography
Dynamic contrast-enhanced functional MRI
18
Normal Renal Anatomy
T1-Weighted Image
Kidneys are surrounded by high-signal
retroperitoneal fat, which is contained by the
low-signal perirenal fascia
Liver
Spleen
T2-Weighted Image
19
Normal Renal Anatomy

• T1- weighted images allow corticomedullary
  differentiation between the intermediate-signal
  cortex and the low-signal medullary pyramids
• The absence of a distinct corticomedullary
  differentiation on T1-weighted images is a
  sensitive, though nonspecific, indication of
  renal parenchymal disease

20
Normal Renal Anatomy
T1-Weighted Image
T2-Weighted Image

• The renal pelvis, calyces, and infundibuli can
  normally be seen because of their water content,
  appearing as hypointense on T1-weighted images
  and hyperintense on T2-weighted images
• Low-signal ureters extend obliquely, inferiorly,
  and medially

21
Variant Anatomy Pancake kidney
Movie
22
Normal Renal Anatomy
a

• This MR angiogram demonstrates the renal arteries
  () extending from the abdominal aorta (a) in the
  axial oblique (top) and coronal views (bottom).

a
a

• Renal vessels are linear or tortuous areas of
  signal void
• Renal arteries extend horizontally from the aorta
  to the kidneys renal veins extend obliquely
  towards the IVC

23
Vascular Disorders that cause Renal Pathology

• Pre-renal vascular disorders
• Aortic dissection
• Arteritis (ie Takayasu Arteritis)
• Renal artery stenosis
• Renal artery aneurysm
• Renal vascular disorders
• Renal AVM
• Polyarteritis nodosa
• Hematomas
• Post-renal vascular disorders
• Renal vein or IVC thrombosis

24
Aortic Dissection
First Pass
Second Pass
Delayed
25
Takayasu Arteritis

• Takayasu arteritis is a granulomatous vasculitis
  of medium and large-sized arteries
• Classic clinical syndrome ocular disturbances
  and marked weakening of the pulse in the upper
  extremities (pulseless disease) due to fibrous
  thickening of the aortic arch and narrowing of
  the great vessels arising in the arch
• One third of cases also have involvement of the
  remainder of the aorta and its branches,
  including the renal arteries

26
Renal Artery Stenosis

• Best visualized with 3D contrast enhanced MRA
• Proximal lesions are the easiest to visualize
• Post-stenotic dilatation
• Small, smooth kidney
• Arterial collaterals occasionally noted

27
Renal Artery Stenosis
28
25 y/o, 8 weeks pregnant, BP 220/140
fetus
29
Renal Artery Aneurysm

• Classified morphologically as saccular or
  fusiform
• Annular calcification is common, especially in
  atherosclerotic aneurysms
• A large aneurysm may produce a concave deformity
  on the adjacent pelvocalyceal system
• Occasionally an aneurysm appears as a soft tissue
  mass accentuated by surrounding sinus fat
• Rapid contrast enhancement, with delayed washout
  of the aneurysm
• Look for pulsation artifact

Calcified Renal Artery Aneurysm
30
Renal AVM

• AVM usually occurs in the medulla and may cause a
  mass effect on the collecting system
• Peripheral, curvilinear calcification may be
  present in a cavernous AVM
• An enlarged feeding artery and draining vein may
  be seen
• The entire mass enhances with contrast

31
Renal AVM
46 y/o man with hematuria. There is a large left
upper pole AVM with massive IVC dilatation
32
Polyarteritis Nodosa

• Polyarteritis nodosa is a systemic vasculitis
  resulting from non-infectious transmural
  necrotizing inflammation of small or medium-sized
  muscular arteries
• Only the classic form, in which
  intermediate-sized arteries (the arcuate
  arteries) are affected, is seen in the kidneys
• It often results in irregular arterial dilation,
  nodularity, obstruction, and less commonly,
  infarction

33
Polyarteritis Nodosa
infarcts

• Multiple aneurysms in the arcuate arteries
• The aneurysms are 2 mm- 3mm and sharply defined
• Arterial involvement is focal, random, and
  episodic
• Infarcts are also common
• Because small vessels are spared,
  glomerulonephritis is not seen

34
Polyarteritis Nodosa

• Although the kidney is the organ that is most
  commonly involved, polyarteritis nodosa (PAN) is
  a systemic disease that can infect any organ or
  system, including the central nervous system as
  shown below

PAN in the brain (a)
a
PAN in the spinal cord (b, c, d)
b
c
d
35
Chronic Subcapsular Hematoma

• Homogenous collection of hypodense fluid (a) that
  is bright on T1 (b, c) and T2 images (d)

36
Renal Vein Tumor Thrombosis

• T1-weighted MR images or MRA are best for
  depicting renal vein thrombosis

37
Diseases of the Renal Parenchyma

• Introduction
• Glomerular Diseases
• Glomerulonephritis
• Glomerular lesions associated with systemic
  disease
• Tubular and Interstitial Diseases
• Acute Tubular Necrosis
• Tubulointerstitial Nephritis
• Acute and Chronic Pyelonephritis
• Cystic Diseases
• Neoplasms
• Angiomyolipoma
• Renal cell carcinoma
• Wilms Tumor

38
Imaging the Renal Parenchyma

• The renal parenchyma is often evaluated using a
  breath-hold fat suppressed T1-weighted spoiled
  GRE sequence in the axial plane, performed before
  and approximately 3-5 minutes after contrast
  administration.
• The coronal plane is particularly useful in
  evaluating lesions at the poles of the kidneys as
  well as lesions that extend outside the renal
  parenchyma.
• To characterize cystic lesions, a breath-hold
  T2-weighted half-Fourier single-shot turbo spin
  echo sequence in the coronal plane is often
  performed. Gadolinium enhancement and fat
  suppression methods can be used to further
  characterize the cystic components.

39
Irregular Renal Contour

• The renal capsule is normally smooth. There are
  many causes of an irregular renal contour

3. Papillary necrosis
40
Renal Scar

• Differential Diagnosis
• Unilateral
• Reflux nephropathy
• Previous renal surgery
• Bilateral
• With Normal Calyces
• Renal Infarcts
• With Abnormal Calyces
• Analgesic nephropathy
• Reflux nephropathy

Chronic pyelonephritis
41
Renal Scar

• Differential Diagnosis
• Unilateral
• Reflux nephropathy
• Previous renal surgery
• Bilateral
• With Normal Calyces
• Renal Infarcts
• With Abnormal Calyces
• Analgesic nephropathy
• Reflux nephropathy

c
c
Cortical infarcts
42
Glomerulonephritis

• Acute glomerulonephritis nonspecific, dense
  collection of inflammatory edema
• Chronic glomerulonephritis
• marked, symmetric reduction in the size of both
  kidneys
• smooth contours with normal calyces and papillae
• abundant peripelvic fat
• calcification of the renal cortex may be seen

Chronic Glomerulonephritis with iron loading
43
Siderotic Nodules

• Iron overload caused by genetic hemochromatosis
  or secondary hemochromatosis can result in the
  deposition of iron deposits in the kidneys and,
  if severe, the formation of siderotic nodules.

44
Myoglobinuria

• Coronal T1-weighted images in a patient with iron
  overload. Note the low signal renal parenchyma
  and loss of the corticomedullary distinction due
  to the very short longitudinal relaxation times
  of the iron in myoglobin.

(b) postcontrast administration note the
medullary enhancement
(a) precontrast
45
Acute Tubular Necrosis

• Acute, reversible renal failure due to severe
  ischemia or toxin exposure
• Clinical symptoms include oliguria (or anuria)
  and proteinuria
• Both kidneys are smooth and globally enlarged due
  to interstitial edema

46
Acute Tubular Necrosis

• Immediate contrast enhancement following an
  intravenous bolus injection of Gd-DTPA
• Contrast enhancement persists (often longer than
  24 hours), with little opacification of the
  pelvocalyceal system due to tubular blockage by
  debris

A T1-weighted GE sequence depicts persistent
contrast enhancement characteristic of ATN
47
Pyelonephritis

• Acute Pyelonephritis
• Characteristic features renal enlargement,
  increased water content, fascial thickening, and
  striated wedge-shaped enhancement
• T1 loss of corticomedullary distinction and dark
  striations
• T2 high signal intensity perinephric fluid

• Emphysematous Pyelonephritis air within
  the renal parenchyma appears as tiny focal areas
  of markedly decreased signal on both T1- and T2-
  weighted images
• Chronic Pyelonephritis calyceal
  dilatation, focal loss, and cortical attenuation

Xanthogranulomatous pyelonephritis- note
low-signal intensity striations
48
Xanthogranulomatous Pyelonephritis

• Chronic suppurative bacterial infection in the
  collecting system due to a partial obstruction
• Calculi, especially staghorn calculi composed of
  struvite, are seen in the pelvis in 75
• Characteristic MR findings
• low-signal intensity striations
• renal fascia thickening
• scarred pelvis
• dilated calyces
• chronic inflammation
• Extra-renal extension of the inflammatory process
  is common

49
Xanthogranulomatous Pyelonephritis
a
50
Xanthogranulomatous Pyelonephritis
pre-contrast

• Marked enhancement is seen surrounding the
  dilated calyces following IV contrast
  administration (caused by the inflammatory
  infiltrate).
• (a) pre-contrast (b, c, d) post-contrast

a
b
c
d
51
Cystic Kidney Diseases

• Simple Renal Cysts
• thin, imperceptible wall that forms an acute
  angle with the rest of the renal parenchyma
  (beak sign)
• nonenhancing
• sharp margin
• calcification, if present, must be thin, linear,
  and confined to the wall (Bosniak Category I or
  II cysts only)
• Simple cysts have a homogenous signal of similar
  intensity as the nearby CSF (dark on T1-weighted
  images, and bright on T2-weighted images)
• Hemorrhagic cysts range from very dark to bright
  on both T1 and T2-weighted images
• Polycystic Kidney Disease
• Medullary Cystic Disease
• Acquired Cystic Disease
• Cystic Neoplasm

52
Simple Renal Cyst
B
A
A
(A) T1-weighted image (B) Proton density
image (C) T2-weighted image
C
C
53
Renal Cysts

• The Bosniak classification system, which is based
  on imaging findings, can be used to differentiate
  benign renal cysts from angiomyolipomas and renal
  cell carcinomas

54
Cystic Kidney Diseases

• Simple Renal Cysts
• Polycystic Kidney Disease
• multiple expanding cysts that may massively
  enlarge both kidneys
• autosomal dominant disorder seen in adults is
  most common autosomal recessive form seen in
  children is rare
• cysts can be serous or hemorrhagic
• expanding cysts ultimately destroy the renal
  parenchyma, resulting in chronic renal failure
• Medullary Cystic Disease
• Acquired Cystic Disease
• Cystic Neoplasm

55
Adult Polycystic Kidney Disease
Note the enormous kidneys filled with both serous
and hemorrhagic cysts
56
Polycystic Kidney Disease
Biliary obstructing stones Polycystic kidney and
liver cysts
57
Polycystic Kidney Disease
MRCP with biliary obstructing stones Polycystic
kidneys and liver cysts
Movie
58
Acquired Cystic Disease

• This patient has Acquired Cystic Disease
  related to prolonged dialysis

59
Cystic Kidney Diseases

• Simple Renal Cysts
• Polycystic Kidney Disease
• Medullary Cystic Disease
• Acquired Cystic Disease
• Cystic Neoplasm
• thickened walls
• nodular calcification
• irregular borders
• non-uniform signal

60
Multicystic Dysplastic Kidney

• Seven percent of patients with Acquired Cystic
  Disease develop renal cell carcinoma in the walls
  of these cysts after 10 years of dialysis

61
Benign Neoplasms

• Renal Papillary Adenoma
• small, incidental adenomas arising from the renal
  tubules
• Renal Fibroma
• small foci of collagenous tissue found within the
  pyramids
• Renal Oncocytoma
• appears as a well-encapsulated area of uniform
  signal (hypointense on T1, variable on T2) that
  may become very large (up to 12 cm)
• frequently shows a central star-shaped scar or
  area of necrosis
• Angiomyolipoma
• benign, unifocal, expanding mass composed of an
  abundance of fat, smooth muscle, and
  neovascularity with a tendency to form aneurysms
• occurs in a bilateral, multifocal form in
  patients with tuberous sclerosis

62
Angiomyolipoma

• most are clinically silent and found
  incidentally, but they can become large and
  project into the perirenal space, forming a
  palpable mass
• MRI is the imaging modality of choice in
  assessing these lesions due to the importance of
  tissue composition for diagnosis. A
  fat-containing renal tumor is highly specifc for
  angiomyolipoma.

63
Angiomyolipoma
CT w/contrast
T1-MR w/gad

• Methods for diagnosis
• Fat within the tumor is bright on
  non-fat-suppressed T1-weighted images and dark on
  fat-suppressed images. In contrast, focal
  hemorrhage appears bright regardless of fat
  suppression.
• use chemical-shift imaging techniques
  angiomyolipomas show the characteristic India ink
  artifact (a rim of low signal intensity at the
  interface between the tumor and renal parenchyma)
  on T1 -weighted out-of-phase images.
• Contrast enhancement is heterogenous and spares
  fatty areas.

64
Malignant Neoplasms

• Wilms Tumor
• occurs in children, usually between the ages of 2
  and 5
• usually appears as a large, solitary,
  well-circumscribed mass with occasional foci of
  hemorrhage, cyst formation, and necrosis
• Renal Cell Carcinoma
• most characteristic finding spherical mass of
  signal intensity different from adjacent normal
  renal parenchyma due to frequent hemorrhage and
  necrosis
• commonly arise in the poles of the kidney
• clear cell type is unifocal and large (3 to 15 cm
  in diameter)
• papillary type is usually bilateral and
  multifocal

65
Wilms Tumor

• this large tumor may markedly displace the entire
  kidney
• a mixture of attenuation values is seen, due to
  the combination of blastemic, stromal, and
  epithelial tissue with occasional foci of
  hemorrhage and necrosis
• usually demonstrates moderate vascularity

66
Renal Cell Carcinoma

• MRI features
• Spherical shape
• Fails criteria of a simple cyst (thickened,
  irregular walls with significant septation and
  non-peripheral calcification)
• Lacks internal fat
• Enhances with contrast

67
Renal Cell Carcinoma

• The diagnosis of renal cell carcinoma relies on
  contrast enhancement. Solid malignant tumors may
  immediately enhance in density to the level of
  the surrounding normal renal parenchyma, but the
  density quickly decreases. Foci of old
  hemorrhage, necrosis, or cyst formation do not
  enhance and maintain a low signal intensity.

68
Renal Cell Carcinoma
T2
T1

• T1-weighted images regions of necrosis have
  decreased signal while regions of recent
  hemorrhage have increased signal intensity due to
  the paramagnetic effect of methemoglobin
• T2-weighted images heterogeneous, hyperintense
  mass

69
Renal Cell Carcinoma
thick, irregular border
non-peripheral calcification

• all sequences show an alteration of renal contour
  with an irregular, thickened, or nodular border
• non-peripheral calcification (approximately 90
  of all masses containing calcium in a
  non-peripheral location are carcinomas, whereas a
  thin rim of peripheral calcification is
  characteristic of a cyst)
• ureteral notching, due to the development of
  collateral veins following tumor invasion of the
  main renal vein, may also be seen

70
Renal Cell Carcinoma
MRA
early
late

• Most renal cell carcinomas are very vascular,
  showing an enlarged renal artery, chaotically
  distributed intrarenal vessels, small aneurysms,
  arteriovenous fistulas, and lakes of contrast
  material that clear slowly
• dilated capsular arteries and veins may be seen
  within the perirenal fat

71
Tumor Renal Vein Invasion

• T1-weighted MR images or MRA are best for
  depicting renal vein thrombosis

72
Renal Cell Cancer IVC Invasion

• Both renal cell carcinoma and Wilms tumor have a
  tendency to invade the renal vein, the IVC, and
  even the right atrium

t
t tumor i IVC a aorta
i
t
a
i
t
73
Renal Cell Cancer Invasion of IVC Level 2

• filling defects in the renal vein or IVC can be
  either due to metastatic spread of the tumor or a
  propagated tumor thrombus
• linear striated capillary staining of tumor in
  the renal vein is occasionally seen

74
Renal Cell Cancer Invasion of IVC Level 3
Movie
75
Staging Renal Cell Carcinoma

• MRI is the imaging modality of choice for staging
  renal cell carcinoma
• multiplanar capability
• superior soft tissue contrast
• ability to depict tumor extension and invasion
• ability to detect enlarged lymph nodes

76
Obstructive Uropathy

• Common causes of urinary tract obstruction
• congenital anomalies
• calculi
• tumors
• urethral stricture
• inflammation sloughed papillae
• benign prostatic hypertrophy
• pregnancy

Cervical cancer
MR urogram
77
Hematuria
T1
T2
Bleeding diathesis
78
Hematuria
right
T2
left
Bleeding Diathesis (iatrogenic)
79
Pregnancy

• Common causes of urinary tract obstruction
• congenital anomalies
• calculi
• tumors
• urethral stricture
• inflammation sloughed papillae
• benign prostatic hypertrophy
• pregnancy

18 weeks gestation
80
Pregnancy
MRCP
81
Obstructive Uropathy

• Common causes of urinary tract obstruction
• congenital anomalies
• calculi
• tumors
• urethral stricture
• inflammation sloughed papillae
• benign prostatic hypertrophy
• pregnancy

82
Renal Stone

• Calculi appear as focal areas of decreased signal
  intensity on all sequences

83
Acute Ureteric Stone
84
Congenital Obstruction of the Ureteropelvic
Junction

• Characteristic MR findings of Obstructive
  Uropathy (best seen in the
  coronal plane)
• Hydronephrosis dilation of the renal pelvis,
  calyces, and infundibuli associated with
  progressive cortical loss and atrophy. Large
  calyces appear as focal areas of decreased signal
  intensity on both T1- and T2- weighted images.

85
Obstructed Duplication

• Congenital ureteropelvic duplication may cause
  focal hydronephrosis in children
• A duplicated obstructed ureter filled with urine
  (having the same density as water) may also be
  seen

86
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87
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• Israel, G. M. and Krinsky, G. A. MR imaging of
  the kidneys and adrenal glands. Radiologic
  Clinics of North America 41 145-159, 2003.
• Krestin, G. P. Morphologic and Functional MR of
  the Kidneys and Adrenal Glands. Field and Wood
  Medical Publishers, Inc., New York, 1991.

88
References (cont.)

• Leyendecker, J.R. and Brown, J.J. Practical
  Guide to Abdominal and Pelvic MRI. Lippincott
  Williams Williams, New York, 1994.
• Schild, H. H. MRI made easy (well almost).
  Berlex Laboratories, Inc., NJ 1992.
• Schoenberg, S. O., et al. Morphologic and
  functional magnetic resonance imaging of renal
  artery stenosis A multireader tricenter study.
  Journal of the American Society of Nephrology 13
  158-169, 2002.
• Sohaib, S. A., et al. Assessment of tumor
  invasion of the vena caval wall in renal cell
  carcinoma cases by magnetic resonance imaging.
  The Journal of Urology 167 1271-1275, 2002.
• Zagoria, R. J., Tung G. A. Genitourinary
  Radiology The Requisites. Mosby-Year Book, Inc.,
  1997.
• Zhang, H. and Prince, M.R. Magnetic Resonance
  Imaging Clinics of North America 12 (3),
  487-504, Aug 2004.
• Zhang, J., Pedrosa, I., and Rofsky, N.M. MR
  Techniques for Renal Imaging. Radiologic Clinics
  of North America 41 (5), 877-908, Sept 2003.

89
References (cont.)

• Semelka, R.C., Ascher, S.M., and Reinhold, C.
  General considerations for performing MR studies
  of the abdomen and pelvis. MRI of the Abdomen and
  Pelvis, Wiley-Liss, Inc., 1-18, 1997.
• Semelka, R.C. and Kelekis, N.L. Kidneys. MRI of
  the Abdomen and Pelvis, Wiley-Liss, Inc.,
  389-470, 1997.

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Thank You
91
MRI Tutorial

• Proton MR Tutorial
• Proton MR Basics
• Longitudinal Relaxation
• Transverse Relaxation
• MR Signal Characteristics of Different Tissues
• Pulse Sequences
• Conventional Spin Echo
• Fast Spin Echo
• Gradient Echo
• Fat Saturation and Chemical Shift
• Inversion Recovery
• MR Angiography
• MR Urography

92
Proton MR Tutorial

• Atoms whose nuclei contain an odd number of
  protons possess angular momentum and act as
  dipoles (tiny bar magnets)
• The most commonly used atom in MR imaging is the
  hydrogen atom, which has a single proton

N
S
Proton Spin
93
Proton MR Tutorial

• When placed in a strong magnetic field
• Protons align themselves along the longitudinal
  plane of the external magnetic field in either a
  parallel or an antiparallel way
• The parallel state, which requires less energy
  and is preferred, results in a small net magnetic
  moment in the direction of the main magnetic
  vector

Net
94
Proton MR Tutorial

• When placed in a strong magnetic field
• Protons also precess (wobble) at a specific
  frequency around the magnetic field lines, in the
  net direction and orientation of the magnetic
  field
• The stronger the magnetic field, the higher the
  precession frequency, according to the Larmor
  Equation

N
s
95
The Larmor equation ?0 ?B0

• ?0 is the precession frequency (in Hz or MHz)
• B0 is the strength of the magnetic field, in
  Tesla (T)
• ? is the gyro-magnetic ratio, which is specific
  for each atomic nucleus (the gyro-magnetic ratio
  for protons is 42.5 MHz/T)
• SUM The stronger the magnetic field, the higher
  the precession frequency of the proton

96
Proton MR Tutorial

• A radiofrequency (RF) pulse is applied by a loop
  of coils in the magnet, which displaces the net
  magnetic vector from its longitudinal (resting)
  state onto the transverse plane

97
Proton MR Tutorial

• The energy supplied by the RF pulse causes more
  protons to align in a higher energy antiparallel
  way, decreasing the longitudinal magnetization

98
Proton MR Tutorial

• The RF pulse also forces the majority of the
  protons to precess together, or in phase,
  temporarily establishing a new transverse
  magnetization

99
Proton MR Tutorial

• The time (in milliseconds) required for the net
  magnetic vector to recover 63 of its
  longitudinal magnetization is called the tissues
  T1 relaxation time

Longitudinal Magnetization
Signal
63
T1
Time
100
Proton MR Tutorial

• The time (in milliseconds) required for the
  tissues transverse magnetization to decrease to
  37 of its peak value after the RF pulse is
  called the tissues T2 relaxation time

101
Proton MR Tutorial

• T1 Relaxation Time
• T1 varies with the magnetic field strength it is
  longer in stronger magnetic fields. T1 also
  depends on the tissue structure, composition, and
  precession frequency.
• Water has a long T1 because the precession
  frequency of water molecules are faster than the
  Larmor frequency, which makes energy transfer
  slow.
• Fat has a short T1 because the carbon bonds at
  the ends of the fatty acids have frequencies near
  the Larmor frequency, facilitating energy
  transfer.

Longitudinal Relaxation
Signal
63
T1Fat
T1Water
Time
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Proton MR Tutorial

• T2 Relaxation Time
• T2 depends on the inhomogeneities of both the
  external magnetic field and the local magnetic
  fields within the tissues.
• Pure water is a homogenous mixture of
  rapidly-moving water molecules which create a
  relatively uniform internal magnetic field.
  Protons stay in phase longer and therefore have a
  long T2.
• Impure liquids contain larger molecules which
  move slowly, have larger differences in
  precession frequencies, and have greater
  variations in the local magnetic field. Protons
  get out of phase more easily, resulting in a
  short T2.

Transverse Relaxation
Signal
37
T2- impure liquids
T2 - pure water
Time
103
Proton MR Tutorial

• Different tissues in the body are recognized by
  their distinct signal characteristics on
  T1-weighted and T2-weighted sequences

104
Proton MR Tutorial

• By applying a weak magnetic field gradient
  superimposed on the uniform strong magnetic
  field, the precessional frequency of spins are
  increased or decreased in a linear fashion. This
  linear spectrum of precession frequencies can be
  used to select an imaging plane and to localize
  the protons position in space.
• The RF energy returned (the spin echo or
  gradient echo, depending on the technique) is
  received by a computer and transformed into a
  visual representation of the volume of interest.
  

105
Pulse Sequences

• Proton MR
• Conventional Spin Echo Pulse Sequence
• Fast Spin Echo Pulse Sequences
• Gradient Echo Pulse Sequence
• Fat Saturation and Chemical Shift Sequences
• Inversion Recovery

106
Conventional Spin Echo
The conventional spin echo technique 1) 90
degree RF pulse displaces the longitudinal
magnetization into the transverse plane 2) 180
degree RF pulse rephases the vectors in the
transverse plane and neutralizes the external
magnetic field inhomogeneities (RF
refocusing) 3) The RF energy returned, the spin
echo, is detected by a receiver coil TR
repetition time (in milliseconds), or the time
between 2 successive RF pulses TE echo time (in
milliseconds), or the time between the 90 degree
RF pulse and the time the spin echo is generated
107
Conventional Spin Echo

• T1 and T2 signal intensity curves can be combined
  in order to determine what TR and TE should be
  used to achieve a certain signal intensity for a
  tissue
• The pulse sequence timing can be adjusted to give
  T1-weighted, T2-weighted images, or proton
  density images

Longitudinal Magnetization
Signal
Transverse Magnetization
TE
TR
Time
RF Pulse
RF Pulse
108
Conventional Spin Echo

• T1-weighted sequences use a short TR (less than
  500 msec) and a short TE to exploit the
  difference in T1 longitudinal relaxation times
  found in different tissues. With a short TR, the
  time between successive RF pulses does not allow
  the tissues with slow T1 relaxation times to
  fully recover, yielding a variable spectrum of
  signal intensities for all tissues according to
  their unique T1 relaxation times.

Longitudinal Magnetization
Signal
Transverse Magnetization
TE
TR
Time
RF Pulse
RF Pulse
109
Conventional Spin Echo

• T2-weighted sequences use a long TE (greater than
  80 msec) and a long TR to exploit the difference
  in T2 transversal relaxation times found in
  different tissues. A long TE provides sufficient
  time for protons with fast T2 relaxation times to
  lose phase coherence, while protons with slow T2
  times will still be in phase, causing a
  pronounced difference in signal intensity and
  contrast.

110
Conventional Spin Echo

• Proton density images use a long TR (greater than
  1500 msec) and a short TE (less then 30 msec).
  The density of protons within a given tissue
  influences tissue contrast, with a greater proton
  density yielding a stronger, brighter signal.

Longitudinal Magnetization
Signal
Transverse Magnetization
TE
TR
Time
RF Pulse
RF Pulse
111
Fast Imaging Pulse Sequences

• There are a variety of newer fast scan
  techniques, including the Fast Spin Echo (FSE)
  and Gradient Echo techniques.
• The main method for increasing imaging speed is
  to shorten the TR, the most time-consuming
  parameter.
• The fast imaging techniques differ in the way
  they overcome the lower signal/noise ratio caused
  by a short TR and in the way they refocus the
  dephasing spins in a limited amount of time.

112
Fast Spin Echo Pulse Sequence

• The Fast Spin Echo (FSE) technique
• similar to the conventional spin echo technique,
  however instead of a single rephasing RF pulse, a
  series of 180 degree RF pulses rephase the
  vectors in the transverse plane
• Advantages of Fast Spin Echo over Conventional
  Spin Echo techniques
• -increased signal/noise ratio
• -a larger matrix conferring enhanced spatial
  resolution
• -fat suppression techniques can be utilized

113
Gradient Echo Pulse Sequence

• Technique
• 1) An RF pulse displaces the longitudinal
  magnetization vector into the transverse plane by
  an amount less than 90 degrees (an ? pulse). By
  using flip angles less than 90 degrees, there
  is a residual amount of longitudinal
  magnetization which can be tilted by the next
  pulse.
• 2) A magnetic field gradient (linear variations
  in magnetic field strength) are momentarily
  applied at specific times to rephase the
  magnetization vectors in the transverse plane
• 3) The signal generated, the gradient echo, is
  detected by the receiver coil.
• Flip Angle
• larger flip angles give more T1-weighting, while
  smaller flip angles give more T2-weighting

114
Gradient Echo (GE) Pulse Sequence

• Advantages of GE techniques over SE techniques
• greater imaging speed (due to shorter TR and
  TEs)
• less motion artifact (due to shorter TR and TEs)
• better visualization of vascular structures
  (flowing blood
• Is consistently bright)
• Disadvantages
• greater loss of signal from static magnetic
  field inhomogeneity
• greater magnetic susceptibility artifacts

115
Fat Saturation and Chemical Shift Sequences

• Technique
• 1) A low-amplitude, long-duration RF pulse
  centered on the frequency of lipid proton
  resonance is applied to saturate the fat protons
  (Aliphatic hydrogen protons in fat precess at a
  frequency of approximately 220 Hz lower than
  water protons in a 1.5 T magnetic field).
• 2) A spoiling gradient dephases the
  lipid-specific transverse magnetization
• 3) A spin echo sequence is performed with signal
  reception centered on the resonance frequency of
  water before the fat protons have had enough time
  to recover. A T1-weighted image is produced.

116
Inversion Recovery Sequence

• Technique
• 1) A 180 degree RF pulse is applied, which
  inverts the longitudinal magnetization so that it
  is oriented in the opposite direction. This nulls
  the fat signal, while maintaining the water and
  soft tissue signals, thus this sequence can be
  used for fat suppression.
• 2) A 90 degree RF pulse rephases the
  magnetization vectors in the transverse plane
• 3) The signal returned depends on T1, which
  determines how fast the longitudinal
  magnetization returns
• TI inversion time, or the time between the 180
  degree inversion pulse and the 90 degree
  rephasing pulse
• TR repetition time, or the time between two
  successive 180 degree pulses

117
MR Angiography

• Conventional Angiography A trans-femoral
  catheter is inserted and advanced through the
  aorta to the renal artery, where iodinated
  contrast media is injected. A fluoroscopic device
  detects the contrast, and radiographic films are
  made.
• MR Angiography IV injection of a paramagnetic
  contrast agent such as gadolinium, followed by MR
  imaging once the contrast has reached the renal
  vessels.
• Digital Subtraction Angiography (DSA)
  Computer-assisted subtraction of post-contrast
  images from the precontrast ones to enhance
  vessel visualization can be used with either
  conventional or MR angiography.

118
MR Angiography

• Two main methods of MR Angiography
• Time of flight (TOF) A continuous inflow of
  protons with unsaturated, fully relaxed spins
  yields a high signal when RF-stimulated in
  contrast to the saturated surrounding tissue. TOF
  is best for imaging vessels with fast flow, such
  as the renal veins and inferior vena cava.
• Phase Contrast (PC) Protons with moving spins
  acquire a difference in phase as they move along
  a magnetic field gradient, in relation to their
  flow direction and velocity. This allows for
  background suppression of stationary tissue so
  that the blood flow in vessels can be visualized.
  PC is best for imaging the renal arteries.

119
MR Angiography

• Utility of Renal MR Angiography
• Detection of renal artery stenosis or
  arteriopathy (vasculitis, aneurysm, fibromuscular
  dysplasia, arteriovenous malformation,
  arteriovenous fistula, or hemangioma)
• Preoperative planning for renal revascularization
  surgery
• Palliative ablation of a hypervascular neoplasm
• Evaluation of vascular function and morphology in
  potential transplant donors and recipients
• Postoperative assessment of perfusion status

120
MR Urography

• Clinical Application
• to evaluate ureteral obstruction and congenital
  urinary tract abnormalities

121
MR Urography

• Static MR Urography
• Technique Heavily T2-weighted sequence similar
  to MRCP
• Works best for dilated collecting systems
• Can even be used in patients with severe renal
  insufficiency
• No imaging delay or IV contrast necessary, but
  overlapping fluid-filled structures can interfere
• Tip imaging nondilated ureters can be enhanced
  by hydrating the patient and administed a low
  dose of furosemide before the exam
• Excretory MR Urography
• Technique T1-weighted 3D gradient echo sequence
  after IV contrast administration
• Can demonstrate nondilated as well as dilated
  ureters
• No interference by overlapping fluid-filled
  structures
• Cannot be used in patients with severe renal
  insufficiency

122
The End
Thank You