HARVARD MEDICAL SCHOOL

Stone Doses and ER

Dushyant Sahani MD

Disclosures

• Grant support from GE Health Care

Clinical Perspective

• Urolithiasis has a life time risk of 10-15%

• It has a high relapse rate (50% in 5-10 yrs and 75% in 20 yrs)• Affects Men more than women

• Common clinical presentation – Acute flank pain and hematuria

Prevalenceof

Urolithiasis3.2%

5.2%

1970s 1990s1980s

Rising Prevalence

Stone Types

Calcium oxalate monohydrate (COM) & dehydrate (COD)

40-60% Radio-opaque Radio-opaque

Hydroxyapatite(Calcium phosphate)

20-60% Radio-opaque Radio-opaque

Radio-opaque Radio-opaque 2- 4%Brushite

Uric Acid

Struvite

Cystine

Radio-opaque Radio- lucent 5-10%

5-15% Radio-opaque Radio-opaque

Radio-opaque 1- 2.5% Mildly Opaque

Composition On KUB On CTOccurrence

Radioopacity

Factors Influencing Treatment Decision

Presence of obstruction is not the primary factor

considered for urologic intervention

Urologic intervention is influenced by 3 crucial factors

Stone size

Stone Composition

Patient Symptoms

MDCT in Urolithiasis

• Unenhanced CT - Initial investigation of choice in suspected urolithiasis

• Highly accurate test (Sensitivity = 95-98% & Specificity =96-100%)

• Allows accurate determination of stone size and composition

• Detects other causes of acute flank pain

• Reveals associated abnormalities like congenital abnormalities,

infections and neoplasms

MDCT Technique

• Scan Coverage - Upper pole of kidneys to the base of the bladder

• Patient preparation -Bladder distension to visualize stones within the distal ureter

Coronal Reformations (2.5 - 3mm)

Slice thickness – 2-5mmPitch - 1 - 1.6

Urolithiasis – CT Diagnostic Signs

Primary sign of Ureterolithiasis

Stone in the ureter with proximal hydroureter

Virtually all stones are

radio-opaque on CT (>200HU)

Stones radiolucent on CT - Pure matrix stones and stones made of pure Indinavir(Indinavir - Protease inhibitor used in the treatment of HIV)

• Improved detection of stones unrecognized on axial images

• Improved detection of small ureteral and renal calculi at poles

• Enhances radiologist confidence.

• Differentiates phleboliths and calcified vascular plaques from urinary

stones.

• Benefits the practicing urologists who are accustomed to visualizing the

urinary tract in coronal view

1 2

Value of Coronal Reformations

8mm 6 mm

• Accurate stone size measurement is paramount to plan treatment options

• The ideal method for accurate measurement on CT is to measure using

bone window settings (1250 X 250) and magnification

Stone Size

Soft tissue window Bone window with Magnification

• Stone burden (stone size and volume) determines the type of procedure

• ESWL or Ureteroscopy is performed for stones <1cm

•PCNL for stones >1.5cm

Stone Burden Assessment

Stones > 6mm & <15mm or smaller stone not responding to

conservative treatment

Stone < 5mm (98% for stones < or = 4mm

pass spontaneously)

Medical Expulsive therapy(Alpha blockers)

Intervene for unremitting pain, nausea, fever, failure

to passage on medical therapy

• Extracorporeal Shockwave lithotripsy (ESWL)

• Ureteroscopic lithotripsy

Stone >15mm or Staghorn Calculi

PercutaneousNephrolithotomy

(PCNL)

Stone Size & Treatment Decisions

• Linear measurement not suitable in irregularly contoured stones like

stag horn calculi

• Measuring the stone volume eliminates this problem

• Total stone volume is an appropriate measure of stone burden

22 cc

Stone Volumetry

Threshold based CAD Algorithms or manual semi-automated methods

<400 HU

Uric Acid Stone

Medical ManagementAllopurinol

Treatment of Hyperuricemia

<1000 HU(Struvite)

ESWL

>1000HU (Brushite, Cystine, COM)

UreteroscopyPCNL

Stone Composition & Treatment Decisions

> 500 HU

MDCT and Stone CompositionStone composition is determined either using HU values on MDCT or

on Dual-energy CT

Stone Composition Attenuation value at 120kVp

Uric Acid 200-450 HU

Struvite 600-900 HU

Cystine 600-1100 HU

Calcium Phosphate 1200-1600 HU

COM and Brushite 1700-2800 HU

• Stone composition also effects the efficacy of ESWL (Brushite, cystine and

COM stones are hard and resistant, while struvite stones usually fragment easily)

• Dual Source CT can discriminate uric

acid stone from other stone types

•Dual source CT is a 64 slice CT with two

X-ray tubes and two detector assembly

mounted onto a gantry with an angle of 90

degrees.

•By operating the two tubes at different

energies (80 & 140 kVp) it is effective

in tissue material characterization

1

2

2

1

Spectral Imaging for Stone Composition :Dual Source CT

Uric Acid Stone Non uric acid stone

Element composition Light Elements(H, C, N, O)

Heavy Elements(P, Ca, S)

Attenuation at 80 kVp Lower HU Higher HU

Attenuation at 140 kVp Higher HU Lower HU

80kV

Calcium710 HU Uric Acid

290 HU

140kV

Calcium480 HU

Uric Acid 315 HU

Dual Source CT – Uric Acid vs Non Uric acid

• The post processing software algorithm assumes a mixture

of water, calcium and uric acid for every voxel

• Those voxels with dual energy (DE) behavior are color

coded

• Voxels with DE similar to Ca++ are coded Blue

• Voxels with DE similar to uric acid are coded Red

• Voxels with linear density at both kV remain Grey

Uric Acid

Calcium

Color Coding for Stones on Dual Source CT

A- MD Water

C- MD Water

B-MD Iodine

D- MD Iodine

Uric acid6.92

Cystine 11.07

Struvite 100%9.72

Brushite14.12

Calcium oxalate 14.37

UANon-UA

UA Non-UA

UA stonesNon-UA UA stonesNon-UA stones

Potatoes

Bovine kidney Bovine kidney

E- Effective-Z vs HU graph to differentiate stones based on Z value

IMPROVED CHARACTERIZATIONIMPROVED CHARACTERIZATIONRenal stonesRenal stones

Stone composition with SSDE CT

Figure 1 Figure 2

Figure 3

Role of dual energy CT in determination of stone composition

Z=14.73

Clinical history

• 36 yo female with a history of stones. Under went cystoscopy, right retrograde, right ureteroscopy, right laser lithotripsy, right ureteral stent placement, stone retrieval (In Jan 2008).

• Past Surgical History: Open pyelolithotomy - right side 10 years ago, ureteroscopy x2.

Stone analysis (Jan 2007): calcium oxalate monohydrate 100%

Spectral attenuation curve of different stone fragments

Spectral CT: Technique

Step II: Non contrast low dose

Stone protocol MDCT using a

single source

Step III: Localization of the stone and focal dual energy acquisition of the anatomical region containing the stone

Step I: Scout film

Calcium

140 kV 80 kV

Step IV: Post processing of dual energy data using 3 material decomposition algorithm to obtain the color coded image

Color Coded Post processed Image

Spectral CT Technique

Stone Fragility in Guiding Treatment

• CT helps predict stone fragility and susceptibility to lithotripsy

• Stones which are heterogeneous are more fragile than homogenous

stones which are more resistant to ESWL

HeterogenousMore fragile toLithotripsy

HomogenousMore resistantto lithotripsy

Stone Precursor-Randall’s Plaque

Normal papillae

Stone bearing papillae

30 HU

58 HU

• Randall’s Plaque - Calcium salt deposits

in the tip of renal papilla of patients with

nephrolithiasis and are potential sites for

calculus formation

• These stone bearing papillae appear

denser on non-contrast CT and represent

Randall’s plaque on endoscopy

• MDCT can help select patients at high-

risk for stone formation, who may

undergo appropriate medical

management to halt the formation of

stones

Whitish deposits- Randall’s plaque

Calculi with attenuation > 300 HU are radio-opaque & are followed up by Abdominal Radiograph

Calculi with attenuation < 200 HU are radiolucent & hence followed up by CT

>300 HU Follow up Radiograph <200 HU Follow up CT

In patients managed medically,

CT attenuation is a good predictor of utility of radiography in routine follow up

Stone Follow up

Pre Treatment Post Treatment

Significant Stone Burden Residual Stone Burden though significantly reduced

Significant stone burden necessitates further treatment

Stone Follow up –Post Intervention

• Differentiation between stent and stone is vital in post surgical follow up

• Stents and stones have the same CT appearance on abdominal window.

• Bone window allows visual distinction between the stent and the stone

which is accounted for by differences in pixel density.

Stones

Stent

Bone windowAbdominal window

Stone vs Stent

MDCT in Planning Intervention

• Simple trigonometry on CT of the patients with complex stones could help

endourologists in planning renal access.

• CT also helps in planning surgical interventions by identifying the location of the

posterior calyx thus guiding fluoroscopic procedures like percutaneous

nephrolithotomy

• A key concern for repeated CT examinations in recurrent stone disease.

• The effective radiation dose during unenhanced CT

2.8 to 13.1 mSv for men / 4.5 to 18 mSv for women

• Techniques for Reduction of Radiation dose

- Limit scanning area (not typical Abd+Pelvis exam)

- Increase in axial slice thickness to 5mm from 1-3 mm

and include 2.5-3 mm coronal reconstructions

- Lower dose CT exam ( noise index, kVp, pitch)

MDCT and Radiation Dose

Dose Reduction by 15-20%

Strategies for Dose Reduction– Coverage Area

From top of diaphragm to lower border of pubic symphysis

From Top of kidneys to base of urinary bladder

Restrict Scanning Area

Targeted scans focused to area of interest can be performed for follow up CT exams

Increase Slice thickness from 1-3mm to 5mm and include 2.5-3mm Coronal Reformations

1-3mm 5mm

Strategies for Dose Reduction - Slice Thickness

Dose Reduction by 20-40%

Strategies for Dose ReductionRole of Low kVp - Exam based on Body Weight

140 kV (> 250 lbs)

120 kV (141-249 lbs)

Dose= kVp

100/80 kV (<140 lbs)

20% reduction

Total 40 % Dose Reduction

20% reduction

Strategies for Dose Reduction- Increase in Noise Index/ reference mAs (100-180)

Noise Index -15

Noise Index -20

Noise Index -30

15-20% reduction

Total 30-40 % Dose Reduction

15-20% reduction

Strategies for Dose Reduction

+ ++

LimitScanning area

Increase NoiseIndex

Low kVp

Increase slice thicknessInclude coronal reformations

Total Dose Reduction by 40-70% from Standard dose

DLP 200 mGy-cm

DLP 1000 mGy-cm

• MGH Experience (GE HD 750) 24 patients studied so far• Radiation dose reduction achieved (38-81%)

Adaptive Statistical Iterative Reconstruction (ASIR)

• Noise reduction reconstruction method to improve the

signal-to-noise

• Relies on the accurate modeling of the distribution of

noise in the acquired data

Kole et al 2006 Phys Med Biol

DLP 1382.07mSv

DLP 74411.16mSv 81%

July 2 2007 Feb 26 2009

263 lbs

DLP 3885.82

DLP 1842.7653%

18 DEC 2007 22 DEC 2008

Current Imaging Strategy

The scope of imaging has extended beyond the mere

detection of stone and its location

The current strategy is to determine the stone composition,

its fragility and quantification which has great implications in

treatment planning

Summary Points• MDCT with multiplanar reformations is accurate in stone assessment

• The qualitative assessment by CT influences management by

dictating treatment options like ESWL.

• Spectral imaging and CAD is emerging for stone

composition/Quantitation

• Various stratergies for radiation dose reduction in imaging of

urolithiasis achieving accurate diagnosis and reduced radiation dose

delivery.

• New reconstruction algorithms (ASIR) is promising in dose reduction

Dushyant V Sahani, MDDivision of abdominal imaging and intervention

Department of RadiologyMassachusetts General Hospital

White 270, 55 Fruit streetBoston, MA-02114

Email:dsahani@partners.org

Ph (o): 617-726-8396

Thank You

1. Curhan, G.C., Epidemiology of stone disease. Urol Clin North Am, 2007. 34(3): p. 287-93.2. Sandhu, C., K.M. Anson, and U. Patel, Urinary tract stones--Part I: role of radiological imaging

in diagnosis and treatment planning. Clin Radiol, 2003. 58(6): p. 415-21.3. Ege, G., et al., Acute ureterolithiasis: incidence of secondary signs on unenhanced helical CT

and influence on patient management. Clin Radiol, 2003. 58(12): p. 990-4.4. Heneghan, J.P., et al., Helical CT for nephrolithiasis and ureterolithiasis: comparison of

conventional and reduced radiation-dose techniques. Radiology, 2003. 229(2): p. 575-80.5. Boulay, I., et al., Ureteral calculi: diagnostic efficacy of helical CT and implications for treatment

of patients. AJR Am J Roentgenol, 1999. 172(6): p. 1485-90.6. Fielding, J.R., et al., Unenhanced helical CT of ureteral stones: a replacement for excretory

urography in planning treatment. AJR Am J Roentgenol, 1998. 171(4): p. 1051-3.7. Lin WC, Uppot RN, Li CS, Hahn PF, Sahani DV. Value of automated coronal reformations from

64-section multidetector row computerized tomography in the diagnosis of urinary stone disease. J Urol 2007; 178:907-911; discussion 911.

8. Smith, R.C., et al., Diagnosis of acute flank pain: value of unenhanced helical CT. AJR Am J Roentgenol, 1996. 166(1): p. 97-101.

• 9. Eisner BH, Iqbal A, Namasivayam S, Catalano O, Kambadakone A, Dretler SP, Sahani DV. Differences in Computed Tomography Density of the Renal Papillae of stone formers and Non-stone formers: A pilot study. J Endourol 2008 Oct 2.

• Bilen CY, Kocak B, Kiticci G, Danaci M, Sarikaya S. Simple trigonometry on computed tomography helps in planning renal access. Urology. 2007 Aug;70(2):242-5; discussion 245

References