diff --git a/Modules/Pharmacokinetics/src/Models/mitkTwoStepLinearModel.cpp b/Modules/Pharmacokinetics/src/Models/mitkTwoStepLinearModel.cpp
index 6846ce111c..3771a4885a 100644
--- a/Modules/Pharmacokinetics/src/Models/mitkTwoStepLinearModel.cpp
+++ b/Modules/Pharmacokinetics/src/Models/mitkTwoStepLinearModel.cpp
@@ -1,184 +1,193 @@
 /*============================================================================
 
 The Medical Imaging Interaction Toolkit (MITK)
 
 Copyright (c) German Cancer Research Center (DKFZ)
 All rights reserved.
 
 Use of this source code is governed by a 3-clause BSD license that can be
 found in the LICENSE file.
 
 ============================================================================*/
 
 #include "mitkTwoStepLinearModel.h"
 
 const std::string mitk::TwoStepLinearModel::MODELL_NAME = "Two Step Linear Model";
 
 const std::string mitk::TwoStepLinearModel::NAME_PARAMETER_y1 = "BaseValue";
 const std::string mitk::TwoStepLinearModel::NAME_PARAMETER_a1 = "Slope_1";
 const std::string mitk::TwoStepLinearModel::NAME_PARAMETER_t = "Change_Point";
 const std::string mitk::TwoStepLinearModel::NAME_PARAMETER_a2 = "Slope_2";
 
 const unsigned int mitk::TwoStepLinearModel::POSITION_PARAMETER_y1 = 0;
 const unsigned int mitk::TwoStepLinearModel::POSITION_PARAMETER_t  = 1;
 const unsigned int mitk::TwoStepLinearModel::POSITION_PARAMETER_a1 = 2;
 const unsigned int mitk::TwoStepLinearModel::POSITION_PARAMETER_a2 = 3;
 
 const unsigned int mitk::TwoStepLinearModel::NUMBER_OF_PARAMETERS = 4;
 
 std::string mitk::TwoStepLinearModel::GetModelDisplayName() const
 {
   return MODELL_NAME;
 };
 
 std::string mitk::TwoStepLinearModel::GetModelType() const
 {
   return "Generic";
 };
 
 mitk::TwoStepLinearModel::FunctionStringType mitk::TwoStepLinearModel::GetFunctionString() const
 {
   return "Slope_1*t+Y_intercept_1 if t<Change_Point; Slope_2*t+Y_intercept_2 if ChangePoint<=t";
 };
 
 std::string mitk::TwoStepLinearModel::GetXName() const
 {
   return "x";
 };
 
 mitk::TwoStepLinearModel::ParameterNamesType
 mitk::TwoStepLinearModel::GetParameterNames() const
 {
   ParameterNamesType result;
   result.push_back(NAME_PARAMETER_y1);
   result.push_back(NAME_PARAMETER_t);
   result.push_back(NAME_PARAMETER_a1);
   result.push_back(NAME_PARAMETER_a2);
 
 
   return result;
 };
 
 mitk::TwoStepLinearModel::ParametersSizeType
 mitk::TwoStepLinearModel::GetNumberOfParameters() const
 {
   return NUMBER_OF_PARAMETERS;
 };
 
 mitk::TwoStepLinearModel::ParameterNamesType
 mitk::TwoStepLinearModel::GetDerivedParameterNames() const
 {
   ParameterNamesType result;
   result.push_back("AUC");
   result.push_back("FinalUptake");
   result.push_back("Smax");
   result.push_back("y-intercept2");
 
   return result;
 };
 
 mitk::TwoStepLinearModel::ParametersSizeType
 mitk::TwoStepLinearModel::GetNumberOfDerivedParameters() const
 {
   return 4;
 };
 
 mitk::TwoStepLinearModel::ModelResultType
 mitk::TwoStepLinearModel::ComputeModelfunction(const ParametersType& parameters) const
 {
 
   //Model Parameters
   double     t = (double) parameters[POSITION_PARAMETER_t] ;
   double     a1 = (double) parameters[POSITION_PARAMETER_a1] ;
   double     a2 = (double) parameters[POSITION_PARAMETER_a2] ;
 
 
   double     b1 = (double) parameters[POSITION_PARAMETER_y1] ;
   double     b2 = (a1 - a2)*t + b1;
 
   ModelResultType signal(m_TimeGrid.GetSize());
 
   TimeGridType::const_iterator timeGridEnd = m_TimeGrid.end();
   ModelResultType::iterator signalPos = signal.begin();
 
   for (TimeGridType::const_iterator gridPos = m_TimeGrid.begin(); gridPos != timeGridEnd; ++gridPos, ++signalPos)
   {
       if((*gridPos) < t)
       {
           *signalPos = a1*(*gridPos)+b1;
       }
       else
       {
           *signalPos = a2*(*gridPos)+b2;
       }
   }
 
   return signal;
 };
 
 mitk::TwoStepLinearModel::ParameterNamesType mitk::TwoStepLinearModel::GetStaticParameterNames() const
 {
   ParameterNamesType result;
 
   return result;
 }
 
 mitk::TwoStepLinearModel::ParametersSizeType  mitk::TwoStepLinearModel::GetNumberOfStaticParameters() const
 {
   return 0;
 }
 
 void mitk::TwoStepLinearModel::SetStaticParameter(const ParameterNameType& /*name*/,
     const StaticParameterValuesType& /*values*/)
 {
   //do nothing
 };
 
 mitk::TwoStepLinearModel::StaticParameterValuesType mitk::TwoStepLinearModel::GetStaticParameterValue(
   const ParameterNameType& /*name*/) const
 {
   StaticParameterValuesType result;
 
   //do nothing
 
   return result;
 };
 
 mitk::ModelBase::DerivedParameterMapType mitk::TwoStepLinearModel::ComputeDerivedParameters(
   const mitk::ModelBase::ParametersType& parameters) const
 {
-    double     tmax = (double) parameters[POSITION_PARAMETER_t] ;
-    double     s1 = (double) parameters[POSITION_PARAMETER_a1] ;
-    double     s2 = (double) parameters[POSITION_PARAMETER_a2] ;
+    double     t = (double) parameters[POSITION_PARAMETER_t] ;
+    double     a1 = (double) parameters[POSITION_PARAMETER_a1] ;
+    double     a2 = (double) parameters[POSITION_PARAMETER_a2] ;
     double     b1 = (double) parameters[POSITION_PARAMETER_y1] ;
-
+    double     b2 = (a1 - a2) * t + b1;
 
     unsigned int timeSteps = m_TimeGrid.GetSize();
 
     double Taq = 0;
     Taq = m_TimeGrid.GetElement(timeSteps-1);
 
-    double Smax = s1*tmax+b1;
-    double b2 = Smax-s2*tmax;
-    double AUC = s1/2*(tmax*tmax)+b1*tmax
-                 +s2/2*(Taq*Taq-tmax*tmax)+b2*(Taq-tmax);
-    double Sfin = s2*Taq+b2;
+    double Sfin = a2 * Taq + b2;
+
+    double Smax;
+    if (a2 <= 0)
+    {
+      Smax = a1 * t + b1;
+    }
+    else
+    {
+      Smax = Sfin;
+    }
+
+    double AUC = a1/2*(t*t)+b1*t
+                 +a2/2*(Taq*Taq-t*t)+b2*(Taq-t);
 
     DerivedParameterMapType result;
 
     result.insert(std::make_pair("AUC", AUC));
     result.insert(std::make_pair("FinalUptake", Sfin));
     result.insert(std::make_pair("Smax", Smax));
     result.insert(std::make_pair("y-intercept2", b2));
 
     return result;
 };
 
 itk::LightObject::Pointer mitk::TwoStepLinearModel::InternalClone() const
 {
   TwoStepLinearModel::Pointer newClone = TwoStepLinearModel::New();
 
   newClone->SetTimeGrid(this->m_TimeGrid);
 
   return newClone.GetPointer();
 };